Dissertationen zum Thema „Pagrus auratus“

Thesis (PhD--Marine Science)--University of Auckland, 2009.
" A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Marine Science." Includes bibliographical references (leaves 161-173).

The population dynamics of juvenile snapper, Pagrus auratus, were investigated in the Hauraki Gulf, north-eastern New Zealand, between 1982 and 1990. Attention focused on age and growth, temporal and spatial variation in abundance, and recruitment. Daily increment formation was validated in the sagittae of snapper up to about 160 days old. Increment width varied with time of year, and snapper age, and increments were not resolvable with a light microscope during winter. Increment counts inside a prominent metamorphic mark showed that larval duration was 18-32 days, and was inversely related to water temperature. Spawning dates were back-calculated from increment counts in settled juveniles, and ranged from September to March with a peak in November-January. The onset of spawning was temperature dependent. Fast-growing snapper had smaller sagittae than slow-growing snapper, indicating an uncoupling of otolith and somatic growth. Snapper gonads differentiated first as ovaries during the second year of life, and then some juveniles changed sex to become males during their third year. Sex change occurred before maturity, so snapper are functionally gonochoristic. Growth was slow during the larval phase, but increased rapidly after metamorphosis to about 0.6-0.9 mm.day-1. From the first winter, growth followed a well-defined annual cycle, with little or no growth during winter, and linear growth of 0.16-0.43 mm.day-1 during spring-autumn for 0+/1+ and 1+/2+ snapper. Snapper grew faster at higher temperatures. Trawl catch rates were affected by numerous gear and environmental factors, but probably provided reasonable estimates of snapper relative abundance. Recommendations are made for improving snapper trawl survey procedures. There was a strong annual abundance cycle in the Kawau region, peaking in spring, and declining to a minimum in winter. Snapper were patchily distributed at a spatial scale of 1-2 km, probably because of preference for specific micro-habitats. Year class strength of 1+ snapper varied 17-fold over seven years, and was strongly positively correlated with autumn sea surface temperature during the 0+ year. The strengths of the 1991 and 1992 year classes are predicted to be below average, and extremely weak, respectively.

Snapper, Pagrus auratus is a valuable coastal fish species in New Zealand and forms an important commercial and recreational fishing industry in the north-east of New Zealand. Previous studies revealed evidence that this carnivorous, primarily benthic feeder consumes a non-indigenous macrobenthic species. Many non-indigenous macrobenthic species have now become established in New Zealand waters. For example, in Rangitoto Channel, Hauraki Gulf, non-indigenous macrobenthic species are prolific, with three bivalve species in particular having thriving populations: Limaria orientalis, Musculista senhousia, and Theora lubrica. The role of these species in the diet of snapper, however, is unknown. To assess the availability of indigenous and non-indigenous prey species to snapper, benthic macrofaunal assemblages throughout Rangitoto Channel were surveyed. To do so, sediment samples were collected at 84 sites. At 24 of these sites sediment was also collected for grain size analysis and at 40 of these sites the seafloor was surveyed with video. To investigate the diet of snapper, fish were collected from four monitoring sites within the channel. Bimonthly monitoring of the diet of snapper as well as the benthic macrofauna was completed at these monitoring sites and trends in the abundance of three prey species, two of which were non-indigenous species, within the sediment and the diet of snapper were compared from June to December 2008. A detailed description of the benthic macrofaunal assemblages throughout Rangitoto Channel confirmed that three non-indigenous species are established throughout this area. The analyses revealed that the diet of snapper has shifted compared to previous studies. Snapper now consume large quantities of two non-indigenous species, M. senhousia and L. orientalis. Consumption of the former species apparently results from its dominance and biomass within the sediment. It is therefore not surprising that snapper consumed large amounts of this species. In contrast, L. orientalis occurred disproportionately in the diet of snapper compared to its abundance within the sediment. I suggest that the establishment of some non-indigenous species benefits snapper.

This thesis examined the use of an estuary by the sparid Pagrus auratus, commonly known as snapper. The density and distribution of snapper (juveniles through to adults) was quantified over multiple spatial and temporal scales and associated with habitat. Juveniles enter or are spawned within the Mahurangi Harbour over the warmer months, with densities highest in March. Ontogenetic shifts in fine-scale habitat occurred. Fine-scale analysis from the beam trawl showed juvenile snapper (< 10 cm) were mostly associated with horse mussels. Larger juveniles (> 4 cm) were also associated with bare areas. The 0+ fish (from the DUV) occupied fine-scale habitat comprised of muddy to sand substrata with structure of sponges and horse mussels with and without epifauna. The remaining year-classes occupied a coarser substratum, with shell hash the major secondary structure. An artificial reef experiment showed juvenile snapper were attracted to artificial horse mussels with and without epifauna rather than bare areas or controls. The 1+ year-class increased their habitat range, occupying areas with more uniform substrata. A growth shift through to the 2+ year-class was not observed, and this may be due to increasing mortality, (natural or predation), or emigration out of the harbour. Densities of the larger year-classes decreased over the cooler months but not all snapper leave permanently, with tagging showing up to 80% of fish to be resident. Ontogenetic shifts occurred in diet with growth. Juveniles < 2 cm consumed planktonic copepods, with > 2 cm consuming benthic copepods, mysid and caridean shrimps and polychaetes. Snapper > 10 cm consumed brachyuran crabs, caridean shrimps, bivalves, polychaetes and hermit crabs, with > 30 cm fish able to consume harder shelled molluscs and bivalves. The a priori habitats were equally productive in terms of prey, and this may be advantageous for juveniles who can then select a habitat for other qualities, i.e. protection from predation. Despite the potential of snapper to utilise any sort of structure as cover or for rest, most structure within the Mahurangi are biogenic and susceptible to anthropogenic effects, especially sedimentation. The loss or decline of these biogenic species may therefore have a significant impact on the way snapper utilise the Mahurangi. Overall, understanding the ontogeny of snapper within estuaries will contribute to better management strategies for snapper in general.
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This thesis describes research designed to increase our knowledge of the nutritional requirements of Australian snapper Pagrus auratus and provide information on the potential of Australian feed ingredients to reduce the level of fishmeal in diets for this species. The apparent digestibility of organic matter (OM), crude protein (CP), crude fat (CF) and gross energy (GE) from selected animal, cereal or oilseed meals incorporated at different inclusion levels was determined. Snapper were extremely efficient at digesting the CP, CF and GE from fishmeal and rendered animal meals (range 80-100%) with the exception of meat meal, where CP and GE digestibility were lower (62-65%). The CP from oilseeds was better digested (87-91%) than OM (57%) or GE (64-67%). Digestibility of nutrients and GE from animal meals and fish oil was not influenced by inclusion level. The CP from extruded wheat was highly digestible (100-105%), but, the OM, CF and GE digestibility of extruded wheat declined as inclusion levels increased. The interactive effects of inclusion level (150, 250, 350 or 450 g kg-1) and fish size (110 vs 375 g snapper) on the apparent digestibility of OM and GE from gelatinised wheat starch were investigated. The OM and GE digestibility of gelatinised wheat starch was high (89%) at low inclusion levels, but declined significantly in both fish sizes as the level of starch increased. There was no interaction between inclusion level and size of fish and the decline in GE digestibility could be predicted by the regression; GEADC = 104.97(±3.39) - 0.109(±0.010) x inclusion level (R2=0.86). Larger fish were more capable of digesting the GE from gelatinised starch than smaller fish. Regardless of fish size, short and longer-term changes in the physiology of snapper fed or injected with carbohydrates were recorded. Liver and tissue glycogen concentrations and the hepatosomatic index (HSI) of snapper fed gelatinised starch were significantly elevated. The plasma glucose concentrations of fish injected intra-peritoneally with D-glucose increased from resting levels (0.4-4.6 mM) to 18.9 mM approximately 3 hours after injection and fish displayed a hyperglycaemic response for nearly 18 hours. In contrast, the post-prandial response to the uptake of glucose from normally digested gelatinised starch was more regulated. A dose-response study to determine the effects of digestible energy (DE) content (15, 18 or 21 MJ kg-1) on the digestible protein (DP) requirements of juvenile snapper was assessed using a four parameter mathematical model for physiological responses (4-SKM). DP content of test diets ranged from 210 to 560 g kg-1. Weight gain and protein deposition was strongly dependent on the ratio of DP:DE. According to the fitted models, diets for snapper weighing between 30-90 g and reared at temperatures ranging from 20-25ºC should contain a minimum of 28 g DP MJ DE-1 to promote optimal weight gain and protein deposition. The effect of varying the absolute content of DP and DE on the weight gain and performance of snapper (100-300 g) fed diets formulated with an optimal ratio of DP:DE was investigated. In addition, non-protein sources of DE were varied by adjusting the ratio of fish oil to gelatinised wheat starch in order to determine if different ratios of these ingredients affected performance. High-energy diets (22-23 MJ DE kg-1) suppressed feed intake, but provided DP intake was not limited by feed intake, maximum weight gain was approached. Lower-energy, lower-protien diets (15-18 MJ DE & 315-390 DP) encouraged higher feed intake but DP intake was restricted, which reduced growth potential. Snapper performed best on high-energy, high-protein diets (490 DP & 21 MJ DE), provided a significant proportion of DE was supplied as DP. Fish oil and pregelatinised wheat starch could be interchanged according to their DE values without unduly affecting fish performance in diets providing 390-490 g DP kg-1. Two utilisation studies were undertaken to investigate the performance of snapper fed diets containing increasing levels of poultry offal meal, meat meal and soybean meal. All diets were formulated with similar DP and DE contents. Snapper readily accepted feeds containing high levels of poultry meal (360 g kg-1), meat meal (345 g kg-1) or soybean meal (420 g kg-1), before weight gain and performance was negatively affected. In combination, these feed ingredients were able to replace all but 160 g fishmeal kg-1 in commercially extruded test feeds for this species. The research described in this thesis has extended knowledge of the nutritional requirements of Australian snapper by providing important information on the digestibility of Australian feed ingredients. These coefficients have been integral in formulating both experimental and semi-commercial test diets for snapper and will increase both the accuracy and flexibility of commercial diet formulations for this species. High performance feeds for snapper will contain high levels of DP, but must provide a significant proportion of DE in the form of protein. These constraints can be satisfied by using alternative, well-digested protein and energy sources that have the potential to replace all but 160 g kg-1 fishmeal.

This thesis describes research designed to increase our knowledge of the nutritional requirements of Australian snapper Pagrus auratus and provide information on the potential of Australian feed ingredients to reduce the level of fishmeal in diets for this species. The apparent digestibility of organic matter (OM), crude protein (CP), crude fat (CF) and gross energy (GE) from selected animal, cereal or oilseed meals incorporated at different inclusion levels was determined. Snapper were extremely efficient at digesting the CP, CF and GE from fishmeal and rendered animal meals (range 80-100%) with the exception of meat meal, where CP and GE digestibility were lower (62-65%). The CP from oilseeds was better digested (87-91%) than OM (57%) or GE (64-67%). Digestibility of nutrients and GE from animal meals and fish oil was not influenced by inclusion level. The CP from extruded wheat was highly digestible (100-105%), but, the OM, CF and GE digestibility of extruded wheat declined as inclusion levels increased. The interactive effects of inclusion level (150, 250, 350 or 450 g kg-1) and fish size (110 vs 375 g snapper) on the apparent digestibility of OM and GE from gelatinised wheat starch were investigated. The OM and GE digestibility of gelatinised wheat starch was high (89%) at low inclusion levels, but declined significantly in both fish sizes as the level of starch increased. There was no interaction between inclusion level and size of fish and the decline in GE digestibility could be predicted by the regression; GEADC = 104.97(±3.39) - 0.109(±0.010) x inclusion level (R2=0.86). Larger fish were more capable of digesting the GE from gelatinised starch than smaller fish. Regardless of fish size, short and longer-term changes in the physiology of snapper fed or injected with carbohydrates were recorded. Liver and tissue glycogen concentrations and the hepatosomatic index (HSI) of snapper fed gelatinised starch were significantly elevated. The plasma glucose concentrations of fish injected intra-peritoneally with D-glucose increased from resting levels (0.4-4.6 mM) to 18.9 mM approximately 3 hours after injection and fish displayed a hyperglycaemic response for nearly 18 hours. In contrast, the post-prandial response to the uptake of glucose from normally digested gelatinised starch was more regulated. A dose-response study to determine the effects of digestible energy (DE) content (15, 18 or 21 MJ kg-1) on the digestible protein (DP) requirements of juvenile snapper was assessed using a four parameter mathematical model for physiological responses (4-SKM). DP content of test diets ranged from 210 to 560 g kg-1. Weight gain and protein deposition was strongly dependent on the ratio of DP:DE. According to the fitted models, diets for snapper weighing between 30-90 g and reared at temperatures ranging from 20-25ºC should contain a minimum of 28 g DP MJ DE-1 to promote optimal weight gain and protein deposition. The effect of varying the absolute content of DP and DE on the weight gain and performance of snapper (100-300 g) fed diets formulated with an optimal ratio of DP:DE was investigated. In addition, non-protein sources of DE were varied by adjusting the ratio of fish oil to gelatinised wheat starch in order to determine if different ratios of these ingredients affected performance. High-energy diets (22-23 MJ DE kg-1) suppressed feed intake, but provided DP intake was not limited by feed intake, maximum weight gain was approached. Lower-energy, lower-protien diets (15-18 MJ DE & 315-390 DP) encouraged higher feed intake but DP intake was restricted, which reduced growth potential. Snapper performed best on high-energy, high-protein diets (490 DP & 21 MJ DE), provided a significant proportion of DE was supplied as DP. Fish oil and pregelatinised wheat starch could be interchanged according to their DE values without unduly affecting fish performance in diets providing 390-490 g DP kg-1. Two utilisation studies were undertaken to investigate the performance of snapper fed diets containing increasing levels of poultry offal meal, meat meal and soybean meal. All diets were formulated with similar DP and DE contents. Snapper readily accepted feeds containing high levels of poultry meal (360 g kg-1), meat meal (345 g kg-1) or soybean meal (420 g kg-1), before weight gain and performance was negatively affected. In combination, these feed ingredients were able to replace all but 160 g fishmeal kg-1 in commercially extruded test feeds for this species. The research described in this thesis has extended knowledge of the nutritional requirements of Australian snapper by providing important information on the digestibility of Australian feed ingredients. These coefficients have been integral in formulating both experimental and semi-commercial test diets for snapper and will increase both the accuracy and flexibility of commercial diet formulations for this species. High performance feeds for snapper will contain high levels of DP, but must provide a significant proportion of DE in the form of protein. These constraints can be satisfied by using alternative, well-digested protein and energy sources that have the potential to replace all but 160 g kg-1 fishmeal.

This study focused on obtaining sound quantitative data on the reproductive biology, length and age compositions and growth of the snapper Pagrus auratus in the waters off Carnarvon at ca 25oS and Perth at ca 32oS on the west coast of Australia and at ca 34oS on the south coast of Western Australia. Sampling thus encompassed both sub-tropical and temperate waters and the geographical range within which this species is abundant in Western Australia. The resultant data were used to explore the ways in which the biological characteristics of P. auratus differ with latitude and thus water temperature. An intensive sampling regime for eggs and spawning individuals of P. auratus was conducted in Cockburn Sound, a large marine embayment in the Perth region at ca 32oS. The resultant data were used to elucidate where and when spawning occurs in this large marine embayment and to determine more precisely the factors that influence the timing of spawning. The implications of the results presented in this thesis for the management of P. auratus, a species that has been subjected to very heavy fishing pressure in recent years, are discussed. The time and duration of spawning of P. auratus in the subtropical waters of Carnarvon differed markedly from those recorded for this sparid in the temperate and cooler waters of the Perth and the south coast regions. Spawning at Carnarvon occurred predominantly in the five months between late autumn (May) and mid spring (September), whereas it took place mainly in the three months between mid spring (October) and early summer (December) in the Perth region. Spawning of P. auratus on the south coast occurred predominantly in October and November in 2003 and 2004 and scarcely at all in 2005. Gonadal recrudescence was thus initiated when water temperatures were close to their maximum but declining in Carnarvon, and close to their minima and rising in the Perth and south coast regions, respectively. The prevalence of fully mature and spawning females in all three regions was greatest in those months when water temperatures lay between 19 and 21oC. Collation of the data in this thesis and those provided in the literature for other populations showed that the spawning period was related to latitude, occurring far earlier in sub-tropical than temperate waters. The females and males attained maturity at a far smaller total length (L50) in the Carnarvon region, i.e. 378 and 353 mm, respectively, than in the Perth region, 585 and 566 mm, respectively, and also the south coast region, i.e. 600 and 586 mm. The trends exhibited by the age at maturity parallel those for length, with the A50s for the two sexes increasing from ca 4 years in Carnarvon to ca 5.6 years in the Perth region and nearly 7 years in the south coast region. The L50 and A50 at maturity thus both increased with increasing latitude. Marginal increment analysis demonstrated that, irrespective of the number of opaque zones in the otoliths of P. auratus, a single such opaque zone is laid down each year in these otoliths. Furthermore, the trends exhibited by the monthly marginal increments showed that the opaque zone is laid down predominantly in winter in the subtropical waters of Carnarvon, as opposed to mainly in spring in the temperate waters of the Perth and south coast regions. Thus, the timing of formation of the opaque zone in the otoliths of P. auratus along the Western Australian coast is not related to the trends exhibited by water temperature, but, in both the Carnvarvon and Perth regions, was coincident with the timing of spawning. The maximum total lengths recorded for females and males in the Carnarvon region, i.e. 864 and 840 mm, respectively, were considerably less than the corresponding values of 1051 and 1056 mm in the Perth region, and 1083 and 1099 mm in the south coast region. Growth in the Perth and south coast regions was greater than in Carnarvon, as is reflected in, for example, the respective lengths of 820, 720 and 610 mm for females at 10 years of age, as determined from the von Bertalanffy growth equations. The length and age compositions in the Carnarvon and south coast regions were essentially unimodal, whereas those in the Perth region were bimodal. However, the “mode” in the length-frequency distribution for the south coast region was located well to the right of that in the Carnarvon region, reflecting relatively lower contributions by individuals of the age cohorts of 3 to 6 years. The marked bimodality in the length-frequency distribution for P. auratus in the Perth region was due to the presence of a group of mainly smaller individuals caught outside Cockburn Sound and another of mainly larger individuals that were caught in Cockburn Sound and which formed part of a spawning aggregation in that embayment. The proportion of fish > 10 years old in the Carnarvon region declined markedly between 2003 and the following two years, presumably reflecting the effect of heavy fishing pressure. This contributed to the decision by fisheries managers to reduced the TAC in those waters after 2003. Age-frequency data demonstrated that annual recruitment success in Cockburn Sound varied greatly, with the 1991, 1992 and 1996 year classes being particularly strong, and recognizing that the relative numbers of the first two year classes did decline progressively between 2002 and 2004. Annual recruitment was particularly variable in the south coast region, with the catches of the 1996 year class dominating the samples. The relative number of early stage P. auratus eggs in ichthyoplankton samples collected from Cockburn sound on each of four new moons during the spawning seasons of four consecutive years peaked in November in three of those years, i.e. 2001, 2003 and 2004, and in November/December in the remaining year, i.e. 2002. This showed that spawning in this embayment peaked during these months, at which times the mean sea surface temperatures ranged only from 19 to 20oC. The prevalence of spawning fraction females in sequential samples demonstrated that spawning peaks at the new and, to a lesser extent, full moons. This helps account for the strong positive correlation between spawning fraction and tidal regime, with spawning being greatest when the tidal range is greatest. Spawning times, back-calculated from the ages of the eggs collected during ichthyoplankton surveys in Cockburn Sound on each of 16 new moons within the spawning periods of four successive years, demonstrated that, in this embayment, P. auratus spawns at night and within the first three hours of the onset of the ebb tide. The distribution of egg concentrations on the 16 new moons showed that, each year, spawning occurred firstly in the north-eastern area of Cockburn Sound and then in the middle and finally north-western areas of this embayment. In the Perth region, the marine embayments of Cockburn and Warnbro Sound act as nursery areas for P. auratus during the first two years of life. The majority of 2 to 5 year old fish and a large proportion of the 6 year old fish occupy the marine waters outside the embayments. The remaining 6 year old and almost all of the older fish begin to move in September into particularly Cockburn Sound, where they form relatively large spawning aggregations between October and December, before undergoing a massive emigration from this embayment in December/January. The limited returns from fish that were tagged in Cockburn Sound and were subsequently caught outside this embayment indicate that, following spawning, P. auratus does not tend to move in a particular direction. Pagrus auratus stocks are heavily exploited in offshore, oceanic waters and in embayments, such as Cockburn Sound, where they are particularly susceptible to capture because of the tendency of this species to form spawning aggregations in these same easily accessible locations each year. The data obtained during this thesis show that the L50 at maturity of females and males in temperate waters, i.e. nearly 600 mm, is far greater than the current minimum legal length (MLL) of 410 mm TL. There is thus a need to increase the MLL and/or reduce fishing pressure on immature individuals in open waters. However, the effectiveness of an increase in the MLL may be limited because there is evidence that P. auratus suffers from fishing-induced barotrauma. Closures of specific areas during the spawning season of P. auratus, such as those that have been applied in Cockburn Sound and Shark Bay, are potentially a very effective method for reducing the effects of heavy fishing on spawning individuals.

The eggs and larvae of the New Zealand snapper Pagrus auratus are pelagic with early buoyancy provided by dilute body fluids. The swimbladder begins to develop on the third day after hatch from a dorsal evagination of the gut tube. Communication w1h the gut is lost on about the tenth day following pneumatic inflation at around day eight. At this age the gas gland system appears fully functional and capable of secreting gas. By the age of settlement at around 30 days the swimbladder is a fully functional replica of the adult form except for the lack of a resorbent capillary system which does not develop until later in juvenile life. The swimbladder of the adult is of the euphysoclist form with a dorsally located resorbent oval area and sits high in the pleural cavity. The ventral tunica externa is firmly attached to the connective tissue lining the pleural space. The adult swimbladder displaces 5.6% of the volume of the body and its volume is regulated to provide near neutral buoyancy. The connective tissue integument provides almost no restriction to volume changes brought about by vertical movements of the fish and the swimbladder obeys Boyle's Law for physiological pressure changes. The ability of the connective tissue of the tunica externa to accommodate large tissue strains is due to massive regular crimping of otherwise straight collagen fibrils allowing reversible extensions up to 130%. In all other respects however the tissue structure of the tunica externa is consistent with a tissue providing an active mechanical role. The fibrillar morphology and physicochemical properties of swimbladder collagen is consistent with the vertebrate type I form however there are interesting variations in collagen form distributed throughout the swimbladder. Fibrillar morphology of the highly extensible tunica interna is significantly different to that of the tunica externa and appears to play very little mechanical rote. The extensibilty of the tunica externa appears to be regulated by physiological stress and related to the past history of tissue strain.

Growth, development and behaviour were examined in artificially reared larval Pagrus auratus and Forsterygion varium, from the time of hatching. Yolk-sac larval P.auratus hatched at a small size (2.00mm SL), without functional eyes, mouth or digestive tract, and for three days spent long periods at rest. Growth was initially rapid but slowed by 3 days as yolk reserves neared depletion. By days 4-5, the mouth had opened, eyes were functional, yolk was depleted, and a rudimentary gut had formed. Larvae were now able to maintain a horizontal swimming mode and were actively searching for and attacking prey. First feeding was observed in some larvae. Growth was retarded during the transition from endogenous to exogenous nutrition and then increased as feeding proficiency improved. Yolk-sac F.varium hatched at a larger size (4.78mm SL), with functional eyes and jaws. Larvae were able to maintain a horizontal swimming mode from hatching. First feeding was observed from the first day after hatching. F.varium larvae grew steadily from the time of hatching. Ocular morphology was examined in larval, juvenile and adult P.auratus and F.varium. There was a 96 fold increase in eye size, from 0.23mm diameter in a 4 day old larval P.auratus (3.4mm SL) to a maximum diameter of 22mm in an adult of 333mm body length. F.varium displayed a 26 fold increase in eye size, from 0.28mm diameter in the smallest larva (5.00mm SL) to a maximum eye diameter of 7.2mm in a 11gmm long adult. Larval fish had pure cone retinae, however putative rod precursor cells were present from hatching in F.varium and from 18 days in P.auratus. Juvenile and adult fish had duplex retinae with cones arranged in a square mosaic in which 4 twin cones surround a central single cone. Hypertrophy of cone ellipsoids with increasing eye size, resulted in maintenance of a closely packed array in fishes of all sizes. The appearance of retinomotor movements was coincident with the development of a duplex retina in both species. Theoretical spatial acuity (calculated as a function of cone spacing and focal length of the lens) was poor in the smallest larval fish (2° 1' and 1° 8' minimum separable angle in 4 and 1 day old P.auratus and F.varium respectively) but improved to asymptotic values in adults (3'- 4', and 9' in P.auratus and F.varium respectively). Behavioural acuity (determined using the optokinetic response) of 4 day old larval P.auratus (37° 30') and 1 day old F.varium (29°) was very much lower than histological estimates. Behavioural acuity improved to 8° 8' in 16 day old P.auratus and 4° 18' in 14 day old F.varium, but did not attain theoretical estimates for fish of that size (55' and 54'). A rudimentary retractor lentis muscle was first apparent in larval fish 1 week after hatching, and was coincident with the formation of a posterior lental space. Presumably larval fish eyes were incapable of accomodative lens movements until this time. A relative measure of Matthiessen's ratio (distance from lens centre to boundary of the pigmented retinal epithelium/lens radius) measured histologically, decreased from 4.2 and 2.7 in 3 day old P.auratus and newly hatched F.varium, to 2.2 and 2.3 in larvae 22 and 16 days of age respectively. This suggests that growth of the retina and lens were not symmetrical in the eyes of very small larval fish. If Matthiessen's ratio holds for little eyes, then they will initially be strongly myopic. This may account in part for the mismatch between behavioural and theoretical acuity. Perceptive distances of first feeding larval P.auratus and F.varium, estimated for prey items equal in dimensions to maximum jaw widths, were very small (0.2mm and 0.4mm for prey 0.15mm and 0.2mm in size respectively), but increased with increasing body size to 2.1mm and 4.0mm for prey 0.3mm in size, at 16 and 14 days of age respectively. These data have implications for larval feeding in the wild.

The environmental health of aquatic ecosystems depends amongst others, on the chemical pollution coming from activities in the catchment's area. In the Swan River Estuary, Western Australia, the chemical pollutants of concern released into the river are petroleum hydrocarbons and sodium pentachlorophenate (NaPCP). Decreased water quality causes a loss of biotic diversity especially amongst fish populations. The health of aquatic ecosystems can be monitored by fish health, especially fish located at higher levels in the food chain. Pink snapper (Pagrus auratus), an endemic Western Australian fish species, was tested for its potential as a bioindicator of aquatic environmental health. This thesis presents data on the responsiveness of pink snapper to the contaminants of concern, using biomarkers such as serum sorbitol dehydrogenase (SDH), mixed function oxygenase (MFO), metabolic enzymes such as citrate synthase (CS), cytochrome C oxidase (CCO) and lactate dehydrogenase (LDH) and the histological alteration such as hepatic cell lesions (hyperplasia and hypertrophy), and glycogen and lipid droplets. The metabolic enzymes CCO and LDH as well as the hepatic MFO induction and histopathology were proven to be the most suitable biomarkers for use for routine monitoring of the Swan River Estuary using pink snapper as a bioindicator. However, CS activity and hepatic cell lesions (hyperplasia and hypertrophy) did not respond to exposure to contamination and are therefore not suited as biomarkers of effects in pink snapper. The first phase of the study aimed at investigating the responsiveness of juvenile pink snapper to an MFO inducer. Polychlorinated biphenyl isomer # 126 was selected as a model MFO inducer for this study. In the initial experiment, MFO activity was measured as a biomarker of exposure, and serum SDH activity was assessed as a biomarker of liver damage.MFO and SDH activities were of special interest as these biochemical tools have not previously been validated for any Western Australia fish species. Juvenile pink snapper were injected intraperitoneally (i.p.) with 0, 10, 100, 500, 1000 microgram PCB-126 per kilogram. Fish were sacrificed 10 days postinjection, and liver and blood were collected for MFO and SDH analysis, respectively. Doses of 10 and 100 microgram PCB-126 per kilogram caused the highest MFO induction, while doses of 0 and 1000 microgram PCB-126 per kilogram did not result in higher MFO activity relative to carrier-injected (peanut oil) control fish. SDH activities were not significantly different among treatments indicating that hepatocellular damage was not responsible for the reduced MFO activity at the highest dose. Metabolic enzymes in pink snapper exposed by NaPCP were studied in the second phase of the experiment. The aim of this second experiment was to test the responsiveness of pink snapper to contaminants known to cause metabolic perturbations in vertebrates. Juvenile pink snapper were intraperitoneally (i.p.) injected with 0, 5, 10, 20 mg per kilogram. Oxidative enzymes were assessed by measuring CS and CCO activities and glycolytic enzyme was assessed by measuring LDI-1 activity in liver and white muscle tissues. CS activity remained unchanged in both the white muscle and in the liver. CCO activity was significantly enhanced in liver in all treated fish relative to control fish, but not in the white muscle. LDH activity was also higher in liver in all treated fish as compared to control fish, while in white muscle, LDH activity significantly increased at the highest dose injected.The use of a suite of biochemical markers is useful in determining the effects of xenobiotic exposure of aquatic organisms, because it provides a holistic approach with biomarkers at different levels of biological organization. For the third and final phase of the study the suite of biomarkers selected were MFO, metabolic enzyme (CS, CCO and LDH) activities, and histological alternations in combination with physiological indices. The aim of this last experiment was to investigate if a modified liver metabolic activity would alter the MFO induction potential. To test if altered liver metabolism would influence liver detoxication capacities, juvenile pink snapper were i.p. injected with peanut oil (control), or pentachlorobiphenyl # 126 (PCB 126), with sodium pentachlorophenate (NaPCP), or combination of PCB 126+NaPCP. Relative to controls, ethoxyresorufin-O-deethylase (EROD) activity was induced in the PCB 126 and PCB 126+NaPCP fish, but not in the NaPCP group. In the liver, CCO activity was enhanced by the treatments while CS activity remained unchanged and LDH activity was increased in the NaPCP treatment only. In the white muscle, only the PCB 126+ NaPCP treatment enhanced CCO activity, with all other enzymatic activities remaining unchanged. Low serum sorbitol dehydrogenase (sSDH) activity and histopathology of the liver indicated no significant alteration of cellular structure, albeit the lipid droplet size was increased in the PCB 126 and in the PCB 126+NaPCP treatments.It is concluded that the hepatic metabolic changes correspond to histopathological observations, but an altered metabolic capacity does not influence the metabolism of xenobiotics by liver enzymes, as measured by EROD activity. These experiments answered the need to identify a suitable fish species for routine monitoring of the aquatic environment in Western Australia. It also identified the most suitable biochemical markers of exposure and effects, and the suitability of the pink snapper as a bioindicator. Finally, the experiments investigated interactions between biomarkers and provided new knowledge useful to scientists using MFO and/or metabolic enzymes in field or laboratory toxicology.

The environmental health of aquatic ecosystems depends amongst others, on the chemical pollution coming from activities in the catchment's area. In the Swan River Estuary, Western Australia, the chemical pollutants of concern released into the river are petroleum hydrocarbons and sodium pentachlorophenate (NaPCP). Decreased water quality causes a loss of biotic diversity especially amongst fish populations. The health of aquatic ecosystems can be monitored by fish health, especially fish located at higher levels in the food chain. Pink snapper (Pagrus auratus), an endemic Western Australian fish species, was tested for its potential as a bioindicator of aquatic environmental health. This thesis presents data on the responsiveness of pink snapper to the contaminants of concern, using biomarkers such as serum sorbitol dehydrogenase (SDH), mixed function oxygenase (MFO), metabolic enzymes such as citrate synthase (CS), cytochrome C oxidase (CCO) and lactate dehydrogenase (LDH) and the histological alteration such as hepatic cell lesions (hyperplasia and hypertrophy), and glycogen and lipid droplets. The metabolic enzymes CCO and LDH as well as the hepatic MFO induction and histopathology were proven to be the most suitable biomarkers for use for routine monitoring of the Swan River Estuary using pink snapper as a bioindicator. However, CS activity and hepatic cell lesions (hyperplasia and hypertrophy) did not respond to exposure to contamination and are therefore not suited as biomarkers of effects in pink snapper. The first phase of the study aimed at investigating the responsiveness of juvenile pink snapper to an MFO inducer. Polychlorinated biphenyl isomer # 126 was selected as a model MFO inducer for this study. In the initial experiment, MFO activity was measured as a biomarker of exposure, and serum SDH activity was assessed as a biomarker of liver damage.
MFO and SDH activities were of special interest as these biochemical tools have not previously been validated for any Western Australia fish species. Juvenile pink snapper were injected intraperitoneally (i.p.) with 0, 10, 100, 500, 1000 microgram PCB-126 per kilogram. Fish were sacrificed 10 days postinjection, and liver and blood were collected for MFO and SDH analysis, respectively. Doses of 10 and 100 microgram PCB-126 per kilogram caused the highest MFO induction, while doses of 0 and 1000 microgram PCB-126 per kilogram did not result in higher MFO activity relative to carrier-injected (peanut oil) control fish. SDH activities were not significantly different among treatments indicating that hepatocellular damage was not responsible for the reduced MFO activity at the highest dose. Metabolic enzymes in pink snapper exposed by NaPCP were studied in the second phase of the experiment. The aim of this second experiment was to test the responsiveness of pink snapper to contaminants known to cause metabolic perturbations in vertebrates. Juvenile pink snapper were intraperitoneally (i.p.) injected with 0, 5, 10, 20 mg per kilogram. Oxidative enzymes were assessed by measuring CS and CCO activities and glycolytic enzyme was assessed by measuring LDI-1 activity in liver and white muscle tissues. CS activity remained unchanged in both the white muscle and in the liver. CCO activity was significantly enhanced in liver in all treated fish relative to control fish, but not in the white muscle. LDH activity was also higher in liver in all treated fish as compared to control fish, while in white muscle, LDH activity significantly increased at the highest dose injected.
The use of a suite of biochemical markers is useful in determining the effects of xenobiotic exposure of aquatic organisms, because it provides a holistic approach with biomarkers at different levels of biological organization. For the third and final phase of the study the suite of biomarkers selected were MFO, metabolic enzyme (CS, CCO and LDH) activities, and histological alternations in combination with physiological indices. The aim of this last experiment was to investigate if a modified liver metabolic activity would alter the MFO induction potential. To test if altered liver metabolism would influence liver detoxication capacities, juvenile pink snapper were i.p. injected with peanut oil (control), or pentachlorobiphenyl # 126 (PCB 126), with sodium pentachlorophenate (NaPCP), or combination of PCB 126+NaPCP. Relative to controls, ethoxyresorufin-O-deethylase (EROD) activity was induced in the PCB 126 and PCB 126+NaPCP fish, but not in the NaPCP group. In the liver, CCO activity was enhanced by the treatments while CS activity remained unchanged and LDH activity was increased in the NaPCP treatment only. In the white muscle, only the PCB 126+ NaPCP treatment enhanced CCO activity, with all other enzymatic activities remaining unchanged. Low serum sorbitol dehydrogenase (sSDH) activity and histopathology of the liver indicated no significant alteration of cellular structure, albeit the lipid droplet size was increased in the PCB 126 and in the PCB 126+NaPCP treatments.
It is concluded that the hepatic metabolic changes correspond to histopathological observations, but an altered metabolic capacity does not influence the metabolism of xenobiotics by liver enzymes, as measured by EROD activity. These experiments answered the need to identify a suitable fish species for routine monitoring of the aquatic environment in Western Australia. It also identified the most suitable biochemical markers of exposure and effects, and the suitability of the pink snapper as a bioindicator. Finally, the experiments investigated interactions between biomarkers and provided new knowledge useful to scientists using MFO and/or metabolic enzymes in field or laboratory toxicology.

This study focused on obtaining sound quantitative data on the reproductive biology, length and age compositions and growth of the snapper Pagrus auratus in the waters off Carnarvon at ca 25oS and Perth at ca 32oS on the west coast of Australia and at ca 34oS on the south coast of Western Australia. Sampling thus encompassed both sub-tropical and temperate waters and the geographical range within which this species is abundant in Western Australia. The resultant data were used to explore the ways in which the biological characteristics of P. auratus differ with latitude and thus water temperature. An intensive sampling regime for eggs and spawning individuals of P. auratus was conducted in Cockburn Sound, a large marine embayment in the Perth region at ca 32oS. The resultant data were used to elucidate where and when spawning occurs in this large marine embayment and to determine more precisely the factors that influence the timing of spawning. The implications of the results presented in this thesis for the management of P. auratus, a species that has been subjected to very heavy fishing pressure in recent years, are discussed. The time and duration of spawning of P. auratus in the subtropical waters of Carnarvon differed markedly from those recorded for this sparid in the temperate and cooler waters of the Perth and the south coast regions. Spawning at Carnarvon occurred predominantly in the five months between late autumn (May) and mid spring (September), whereas it took place mainly in the three months between mid spring (October) and early summer (December) in the Perth region. Spawning of P. auratus on the south coast occurred predominantly in October and November in 2003 and 2004 and scarcely at all in 2005. Gonadal recrudescence was thus initiated when water temperatures were close to their maximum but declining in Carnarvon, and close to their minima and rising in the Perth and south coast regions, respectively. The prevalence of fully mature and spawning females in all three regions was greatest in those months when water temperatures lay between 19 and 21oC. Collation of the data in this thesis and those provided in the literature for other populations showed that the spawning period was related to latitude, occurring far earlier in sub-tropical than temperate waters. The females and males attained maturity at a far smaller total length (L50) in the Carnarvon region, i.e. 378 and 353 mm, respectively, than in the Perth region, 585 and 566 mm, respectively, and also the south coast region, i.e. 600 and 586 mm. The trends exhibited by the age at maturity parallel those for length, with the A50s for the two sexes increasing from ca 4 years in Carnarvon to ca 5.6 years in the Perth region and nearly 7 years in the south coast region. The L50 and A50 at maturity thus both increased with increasing latitude. Marginal increment analysis demonstrated that, irrespective of the number of opaque zones in the otoliths of P. auratus, a single such opaque zone is laid down each year in these otoliths. Furthermore, the trends exhibited by the monthly marginal increments showed that the opaque zone is laid down predominantly in winter in the subtropical waters of Carnarvon, as opposed to mainly in spring in the temperate waters of the Perth and south coast regions. Thus, the timing of formation of the opaque zone in the otoliths of P. auratus along the Western Australian coast is not related to the trends exhibited by water temperature, but, in both the Carnvarvon and Perth regions, was coincident with the timing of spawning. The maximum total lengths recorded for females and males in the Carnarvon region, i.e. 864 and 840 mm, respectively, were considerably less than the corresponding values of 1051 and 1056 mm in the Perth region, and 1083 and 1099 mm in the south coast region. Growth in the Perth and south coast regions was greater than in Carnarvon, as is reflected in, for example, the respective lengths of 820, 720 and 610 mm for females at 10 years of age, as determined from the von Bertalanffy growth equations. The length and age compositions in the Carnarvon and south coast regions were essentially unimodal, whereas those in the Perth region were bimodal. However, the 'mode' in the length-frequency distribution for the south coast region was located well to the right of that in the Carnarvon region, reflecting relatively lower contributions by individuals of the age cohorts of 3 to 6 years. The marked bimodality in the length-frequency distribution for P. auratus in the Perth region was due to the presence of a group of mainly smaller individuals caught outside Cockburn Sound and another of mainly larger individuals that were caught in Cockburn Sound and which formed part of a spawning aggregation in that embayment. The proportion of fish > 10 years old in the Carnarvon region declined markedly between 2003 and the following two years, presumably reflecting the effect of heavy fishing pressure. This contributed to the decision by fisheries managers to reduced the TAC in those waters after 2003. Age-frequency data demonstrated that annual recruitment success in Cockburn Sound varied greatly, with the 1991, 1992 and 1996 year classes being particularly strong, and recognizing that the relative numbers of the first two year classes did decline progressively between 2002 and 2004. Annual recruitment was particularly variable in the south coast region, with the catches of the 1996 year class dominating the samples. The relative number of early stage P. auratus eggs in ichthyoplankton samples collected from Cockburn sound on each of four new moons during the spawning seasons of four consecutive years peaked in November in three of those years, i.e. 2001, 2003 and 2004, and in November/December in the remaining year, i.e. 2002. This showed that spawning in this embayment peaked during these months, at which times the mean sea surface temperatures ranged only from 19 to 20oC. The prevalence of spawning fraction females in sequential samples demonstrated that spawning peaks at the new and, to a lesser extent, full moons. This helps account for the strong positive correlation between spawning fraction and tidal regime, with spawning being greatest when the tidal range is greatest. Spawning times, back-calculated from the ages of the eggs collected during ichthyoplankton surveys in Cockburn Sound on each of 16 new moons within the spawning periods of four successive years, demonstrated that, in this embayment, P. auratus spawns at night and within the first three hours of the onset of the ebb tide. The distribution of egg concentrations on the 16 new moons showed that, each year, spawning occurred firstly in the north-eastern area of Cockburn Sound and then in the middle and finally north-western areas of this embayment. In the Perth region, the marine embayments of Cockburn and Warnbro Sound act as nursery areas for P. auratus during the first two years of life. The majority of 2 to 5 year old fish and a large proportion of the 6 year old fish occupy the marine waters outside the embayments. The remaining 6 year old and almost all of the older fish begin to move in September into particularly Cockburn Sound, where they form relatively large spawning aggregations between October and December, before undergoing a massive emigration from this embayment in December/January. The limited returns from fish that were tagged in Cockburn Sound and were subsequently caught outside this embayment indicate that, following spawning, P. auratus does not tend to move in a particular direction. Pagrus auratus stocks are heavily exploited in offshore, oceanic waters and in embayments, such as Cockburn Sound, where they are particularly susceptible to capture because of the tendency of this species to form spawning aggregations in these same easily accessible locations each year. The data obtained during this thesis show that the L50 at maturity of females and males in temperate waters, i.e. nearly 600 mm, is far greater than the current minimum legal length (MLL) of 410 mm TL. There is thus a need to increase the MLL and/or reduce fishing pressure on immature individuals in open waters. However, the effectiveness of an increase in the MLL may be limited because there is evidence that P. auratus suffers from fishing-induced barotrauma. Closures of specific areas during the spawning season of P. auratus, such as those that have been applied in Cockburn Sound and Shark Bay, are potentially a very effective method for reducing the effects of heavy fishing on spawning individuals.

This study focused on obtaining sound quantitative data on the reproductive biology, length and age compositions and growth of the snapper Pagrus auratus in the waters off Carnarvon at ca 25oS and Perth at ca 32oS on the west coast of Australia and at ca 34oS on the south coast of Western Australia. Sampling thus encompassed both sub-tropical and temperate waters and the geographical range within which this species is abundant in Western Australia. The resultant data were used to explore the ways in which the biological characteristics of P. auratus differ with latitude and thus water temperature. An intensive sampling regime for eggs and spawning individuals of P. auratus was conducted in Cockburn Sound, a large marine embayment in the Perth region at ca 32oS. The resultant data were used to elucidate where and when spawning occurs in this large marine embayment and to determine more precisely the factors that influence the timing of spawning. The implications of the results presented in this thesis for the management of P. auratus, a species that has been subjected to very heavy fishing pressure in recent years, are discussed. The time and duration of spawning of P. auratus in the subtropical waters of Carnarvon differed markedly from those recorded for this sparid in the temperate and cooler waters of the Perth and the south coast regions. Spawning at Carnarvon occurred predominantly in the five months between late autumn (May) and mid spring (September), whereas it took place mainly in the three months between mid spring (October) and early summer (December) in the Perth region. Spawning of P. auratus on the south coast occurred predominantly in October and November in 2003 and 2004 and scarcely at all in 2005. Gonadal recrudescence was thus initiated when water temperatures were close to their maximum but declining in Carnarvon, and close to their minima and rising in the Perth and south coast regions, respectively. The prevalence of fully mature and spawning females in all three regions was greatest in those months when water temperatures lay between 19 and 21oC. Collation of the data in this thesis and those provided in the literature for other populations showed that the spawning period was related to latitude, occurring far earlier in sub-tropical than temperate waters. The females and males attained maturity at a far smaller total length (L50) in the Carnarvon region, i.e. 378 and 353 mm, respectively, than in the Perth region, 585 and 566 mm, respectively, and also the south coast region, i.e. 600 and 586 mm. The trends exhibited by the age at maturity parallel those for length, with the A50s for the two sexes increasing from ca 4 years in Carnarvon to ca 5.6 years in the Perth region and nearly 7 years in the south coast region. The L50 and A50 at maturity thus both increased with increasing latitude. Marginal increment analysis demonstrated that, irrespective of the number of opaque zones in the otoliths of P. auratus, a single such opaque zone is laid down each year in these otoliths. Furthermore, the trends exhibited by the monthly marginal increments showed that the opaque zone is laid down predominantly in winter in the subtropical waters of Carnarvon, as opposed to mainly in spring in the temperate waters of the Perth and south coast regions. Thus, the timing of formation of the opaque zone in the otoliths of P. auratus along the Western Australian coast is not related to the trends exhibited by water temperature, but, in both the Carnvarvon and Perth regions, was coincident with the timing of spawning. The maximum total lengths recorded for females and males in the Carnarvon region, i.e. 864 and 840 mm, respectively, were considerably less than the corresponding values of 1051 and 1056 mm in the Perth region, and 1083 and 1099 mm in the south coast region. Growth in the Perth and south coast regions was greater than in Carnarvon, as is reflected in, for example, the respective lengths of 820, 720 and 610 mm for females at 10 years of age, as determined from the von Bertalanffy growth equations. The length and age compositions in the Carnarvon and south coast regions were essentially unimodal, whereas those in the Perth region were bimodal. However, the 'mode' in the length-frequency distribution for the south coast region was located well to the right of that in the Carnarvon region, reflecting relatively lower contributions by individuals of the age cohorts of 3 to 6 years. The marked bimodality in the length-frequency distribution for P. auratus in the Perth region was due to the presence of a group of mainly smaller individuals caught outside Cockburn Sound and another of mainly larger individuals that were caught in Cockburn Sound and which formed part of a spawning aggregation in that embayment. The proportion of fish > 10 years old in the Carnarvon region declined markedly between 2003 and the following two years, presumably reflecting the effect of heavy fishing pressure. This contributed to the decision by fisheries managers to reduced the TAC in those waters after 2003. Age-frequency data demonstrated that annual recruitment success in Cockburn Sound varied greatly, with the 1991, 1992 and 1996 year classes being particularly strong, and recognizing that the relative numbers of the first two year classes did decline progressively between 2002 and 2004. Annual recruitment was particularly variable in the south coast region, with the catches of the 1996 year class dominating the samples. The relative number of early stage P. auratus eggs in ichthyoplankton samples collected from Cockburn sound on each of four new moons during the spawning seasons of four consecutive years peaked in November in three of those years, i.e. 2001, 2003 and 2004, and in November/December in the remaining year, i.e. 2002. This showed that spawning in this embayment peaked during these months, at which times the mean sea surface temperatures ranged only from 19 to 20oC. The prevalence of spawning fraction females in sequential samples demonstrated that spawning peaks at the new and, to a lesser extent, full moons. This helps account for the strong positive correlation between spawning fraction and tidal regime, with spawning being greatest when the tidal range is greatest. Spawning times, back-calculated from the ages of the eggs collected during ichthyoplankton surveys in Cockburn Sound on each of 16 new moons within the spawning periods of four successive years, demonstrated that, in this embayment, P. auratus spawns at night and within the first three hours of the onset of the ebb tide. The distribution of egg concentrations on the 16 new moons showed that, each year, spawning occurred firstly in the north-eastern area of Cockburn Sound and then in the middle and finally north-western areas of this embayment. In the Perth region, the marine embayments of Cockburn and Warnbro Sound act as nursery areas for P. auratus during the first two years of life. The majority of 2 to 5 year old fish and a large proportion of the 6 year old fish occupy the marine waters outside the embayments. The remaining 6 year old and almost all of the older fish begin to move in September into particularly Cockburn Sound, where they form relatively large spawning aggregations between October and December, before undergoing a massive emigration from this embayment in December/January. The limited returns from fish that were tagged in Cockburn Sound and were subsequently caught outside this embayment indicate that, following spawning, P. auratus does not tend to move in a particular direction. Pagrus auratus stocks are heavily exploited in offshore, oceanic waters and in embayments, such as Cockburn Sound, where they are particularly susceptible to capture because of the tendency of this species to form spawning aggregations in these same easily accessible locations each year. The data obtained during this thesis show that the L50 at maturity of females and males in temperate waters, i.e. nearly 600 mm, is far greater than the current minimum legal length (MLL) of 410 mm TL. There is thus a need to increase the MLL and/or reduce fishing pressure on immature individuals in open waters. However, the effectiveness of an increase in the MLL may be limited because there is evidence that P. auratus suffers from fishing-induced barotrauma. Closures of specific areas during the spawning season of P. auratus, such as those that have been applied in Cockburn Sound and Shark Bay, are potentially a very effective method for reducing the effects of heavy fishing on spawning individuals.

Worldwide, an increasing demand for fish and fisheries products, together with socioeconomic pressure for industry expansion, is placing considerable pressure on wild fish stocks – more than 80% of which are considered by the Food and Agriculture Organisation of the United Nations (FAO) to be either maximally- or over-exploited. Adding value to the existing catch and/or improving the sustainability of current wild capture methods may offer a means of providing industry growth while negating the need for increased landings. In particular, the peri-mortem condition of a fish plays an integral role in the condition of the tissues post-mortem and hence in product quality, with harvesting techniques that result in stress or fatigue yielding a lower quality product. An understanding of the physiology of the target species and its response to harvest is therefore essential to implementing targeted improvements in harvesting technologies. For species harvested using trawl-based technologies, this includes knowledge of their exercise physiology, in particular their swimming capacity, since this is a key determinant of the interaction between fish and trawl gears, and hence of the nature and severity of stress experienced and of the condition of fish at landing. This thesis describes a series of discrete studies relating to the exercise physiology of juvenile snapper, Pagrus auratus, an iconic New Zealand finfish that comprises important recreational and commercial fisheries. In particular, we sought to characterise the capacity of snapper for sustained swimming activity, including how performance may differ between fish of different size or with environmental temperature; to determine the consequences of exhaustive exercise for both subsequent swimming activity, an important determinant of survival in escaping or discarded catch, and for tissue biochemistry, which ultimately determines product quality in harvested fish; to validate the use of laboratory-based simulations for the study of capture-related stress by comparing the response of laboratory-exercised snapper with commercially caught fish; and to determine the tolerance of snapper to environmental hypoxia, and further, the possible consequences of hypoxia for swimming capacity and for recovery in fish retained for subsequent rested-type harvest. The capacity of snapper for sustained swimming activity was characterised through the use of incremental exercise tests to determine critical swimming speeds, Ucrit. Juvenile snapper (94-107 mm length, 16-157 g mass) demonstrated a strong swimming capacity, with individual fish attaining critical swimming speeds of up to 7.1 body lengths per second (bl s⁻¹). Swimming performance demonstrated an allometric association, with absolute critical speeds increasing with fish size, whilst relative performance favoured smaller fish. The relation was described by the function Ucrit (m s⁻¹) = 0.003412 [length (mm)] + 0.2669. Critical swimming performance also exhibited variation in response to environmental variables. Thermal performance curves were evident in snapper acclimated to 12, 18 and 24 °C, with the suggestion of optimal performance at acclimation temperatures between 18 and 24 °C. Critical swimming performance was also significantly reduced during exposure to ambient oxygen tensions below 80 mmHg; at 40 mmHg, snapper attained only 21% of the critical swimming speeds observed under normoxic (150 mmHg) conditions. In juvenile snapper (~75 g), exhaustive exercise resulted in severe metabolic, acid-base, haematological and hormonal perturbations, the nature of which were similar to those classically demonstrated in other strong-swimming fish species, especially salmonids. These included the depletion of glycogen from within the white muscle (WM) and the concomitant production of lactate, with a resultant lactacidosis of the plasma; recruitment of erythrocytes from the spleen; and the release of cortisol to the plasma. The recovery of these disturbances required 6 hours under laboratory conditions. As the stresses experienced by fish during commercial capture are often considered to be greater than those which can be induced during laboratory-based simulations, it was necessary to investigate whether the magnitude of the perturbations observed in laboratory-exercised snapper were an appropriate model of those of trawl-caught fish. In trawl-caught snapper (1100 g, 38 cm) obtained under commercially-relevant conditions (tow speed ~3.0 knots; duration 2.25-2.75 hours), the magnitude of the perturbations were greater than for laboratory-exercised fish. While the recovery of some metabolites was evident within the first 18 hours post-capture, their recovery was prolonged relative to laboratory-exercised fish; other metabolites, namely muscle glycogen and plasma cortisol, exhibited no signs of recovery. These observations suggest that the response of snapper to exhaustive exercise within the laboratory may underestimate the severity of the response induced by commercial harvest. This is further suggested by post-capture mortality rates of 14%, whereas no mortality was observed following fatigue at Ucrit. Exhaustive exercise also resulted in the impairment of subsequent critical swimming performance. Immediately following fatigue, snapper (85-160 g) were capable of sustained swimming activity at speeds of up to 60-70% Ucrit; however, critical swimming performance was reduced 30%, presumably due to limitations in WM function. There was no suggestion of the recovery of WM function within the first 30 minutes post-fatigue; thereafter, Ucrit was progressively restored, such that snapper were able to repeat their initial swimming performance in a second Ucrit test performed 2 hours after the conclusion of the first. Snapper were moderately tolerant of hypoxia, oxygen-regulating at reduced oxygen tensions (<100 mmHg) by virtue of increased ventilatory rate and stroke volume, with a distinct bradycardia developing at PO₂ below 60 mmHg. Larger snapper appeared to possess a greater hypoxia tolerance than did smaller fish, with Pcrit resolved to 77 in 20 g fish, and 50 mmHg in 150 and 230 g fish. Exposure to moderate hypoxia (60-80 mmHg) during recovery from an exhaustive exercise event constrained MO₂ max to 78% of that of normoxic fish, however did not appear to impede the return of MO₂ to routine levels. The present study is the first to examine in detail the swimming performance of snapper, and the consequences of exhaustive exercise for physiological condition. By understanding the swimming capacities of snapper, it may be possible to refine harvesting practices (i.e. tow speeds) or utilise technologies (i.e. net design) such that the water velocities through the trawl net are within the range at which the fish can swim sustainably, minimising the extent of stress and fatigue experienced by fish, and hence their effects on both quality and survival. The study also demonstrates that whilst snapper experience significant physiological disturbance during commercial harvesting, including significant mortality, some fish demonstrate the potential for metabolic recovery, which may permit their retention in an on-board tank facility for subsequent rested-type harvest. Finally, the present work highlights a number gaps in our understanding of the link between harvesting conditions and fish condition, and makes a number of suggestions for future studies or directions.

This study explored an important aspect of the basis for the highly complex population structure of pink snapper (Pagrus auratus) within the inner gulfs of Shark Bay, investigated how growth and reproduction differ among these closely-adjacent but separate stocks, and obtained biomass estimates for each stock that are essential for the sustainable management of the regionally-important recreational pink snapper fishery. Using ichthyoplankton data in combination with hydrodynamic modelling, P. auratus eggs and larvae were shown to be retained within localized meso-scale eddies that were coincident with the main inner gulf spawning areas. Such hydrodynamic retention, in conjunction with tagging and otolith chemistry data that indicates very limited movement of juvenile and adult fish, explains how separate pink snapper populations can exist in the adjacent waters of the Eastern Gulf, Denham Sound and Freycinet Estuary. The study found significant variation in maximum age, growth, maturity and spawning time at fine spatial scales. Such variation, unusual for a large, potentially mobile fish inhabiting a marine environment with no obvious physical barriers, is linked to the inner gulfs’ marked environmental heterogeneity, the low levels of mixing and historic differences in fishing pressure among the three areas. The daily egg production method (DEPM) was used, for the first time with this species in Western Australia, to provide estimates of spawning biomass of the three separate inner gulf P. auratus stocks. While relatively imprecise, mostly due to imprecision in estimation of daily egg production, these estimates demonstrated that these stocks are very small (measured in tens of tonnes) compared with P. auratus stocks elsewhere in Australia and New Zealand. Biological data and DEPM estimates obtained from this study were incorporated in age-based stock assessment models that have been used to determine the status of inner gulf pink snapper stocks since 2002.

This study explored an important aspect of the basis for the highly complex population structure of pink snapper (Pagrus auratus) within the inner gulfs of Shark Bay, investigated how growth and reproduction differ among these closely-adjacent but separate stocks, and obtained biomass estimates for each stock that are essential for the sustainable management of the regionally-important recreational pink snapper fishery. Using ichthyoplankton data in combination with hydrodynamic modelling, P. auratus eggs and larvae were shown to be retained within localized meso-scale eddies that were coincident with the main inner gulf spawning areas. Such hydrodynamic retention, in conjunction with tagging and otolith chemistry data that indicates very limited movement of juvenile and adult fish, explains how separate pink snapper populations can exist in the adjacent waters of the Eastern Gulf, Denham Sound and Freycinet Estuary. The study found significant variation in maximum age, growth, maturity and spawning time at fine spatial scales. Such variation, unusual for a large, potentially mobile fish inhabiting a marine environment with no obvious physical barriers, is linked to the inner gulfs' marked environmental heterogeneity, the low levels of mixing and historic differences in fishing pressure among the three areas. The daily egg production method (DEPM) was used, for the first time with this species in Western Australia, to provide estimates of spawning biomass of the three separate inner gulf P. auratus stocks. While relatively imprecise, mostly due to imprecision in estimation of daily egg production, these estimates demonstrated that these stocks are very small (measured in tens of tonnes) compared with P. auratus stocks elsewhere in Australia and New Zealand. Biological data and DEPM estimates obtained from this study were incorporated in age-based stock assessment models that have been used to determine the status of inner gulf pink snapper stocks since 2002.

This study explored an important aspect of the basis for the highly complex population structure of pink snapper (Pagrus auratus) within the inner gulfs of Shark Bay, investigated how growth and reproduction differ among these closely-adjacent but separate stocks, and obtained biomass estimates for each stock that are essential for the sustainable management of the regionally-important recreational pink snapper fishery. Using ichthyoplankton data in combination with hydrodynamic modelling, P. auratus eggs and larvae were shown to be retained within localized meso-scale eddies that were coincident with the main inner gulf spawning areas. Such hydrodynamic retention, in conjunction with tagging and otolith chemistry data that indicates very limited movement of juvenile and adult fish, explains how separate pink snapper populations can exist in the adjacent waters of the Eastern Gulf, Denham Sound and Freycinet Estuary. The study found significant variation in maximum age, growth, maturity and spawning time at fine spatial scales. Such variation, unusual for a large, potentially mobile fish inhabiting a marine environment with no obvious physical barriers, is linked to the inner gulfs' marked environmental heterogeneity, the low levels of mixing and historic differences in fishing pressure among the three areas. The daily egg production method (DEPM) was used, for the first time with this species in Western Australia, to provide estimates of spawning biomass of the three separate inner gulf P. auratus stocks. While relatively imprecise, mostly due to imprecision in estimation of daily egg production, these estimates demonstrated that these stocks are very small (measured in tens of tonnes) compared with P. auratus stocks elsewhere in Australia and New Zealand. Biological data and DEPM estimates obtained from this study were incorporated in age-based stock assessment models that have been used to determine the status of inner gulf pink snapper stocks since 2002.

Aspects of the population biology of juvenile Pagrus auratus in the western gulf of Shark Bay, Western Australia, were investigated to explore the causes of an observed size difference between 0+ fish (first year of life) in the northern and southern regions of that gulf. Five trawl surveys were conducted in each region of the western gulf, from November 2000 to December 2001, to collect juvenile P. auratus. The 0+ fish from the northern region were found to be consistently greater in length by between 10 and 20mm LCF, than those in the southern region, thereby confirming those observed size differences. Three hypotheses were developed and investigated to explain the confirmed size difference between fish in the northern and southern regions of the western gulf. Size differences could be explained by (I) different growth rates; (2) age differences associated with different spawning times; and/or (3) fish migrating from south to north. The first and second hypotheses were tested by determining the daily ages of 125 fish collected by trawling in both regions between November 2000 and December 2001. Length-at-age data allowed the growth patterns of fish aged from 100 to 357 days to be tested using ANCOVA, while back-calculating from these ages allowed spawning times to be estimated. Examination of the third hypothesis was attempted by tagging 3485 0+ P. auratus in the southern region, followed by a trawling program to recapture tagged fish. Length-at-age data derived from validated age estimates indicated that while growth rates of O+ P. auratus, 100-357 days old, were similar between regions, the 0+ fish in the northern region were 15-l6mm greater in length than fish in the southern region, at a similar age. Back-calculated birth dates of O+ P. auratus showed that spawning had predominantly occurred between June and mid-August in both regions. While no tagged fish were recaptured during the study, independent studies examining the spatial movement of P. auratus in the western gulf suggest that 0+ P. auratus are unlikely to migrate between regions. Hence, the difference in length offish between regions almost certainly resulted from differences in growth rates within the first few months of life (age). A laboratory-based aquarium experiment was carried out at water temperatures of 18, 22 and 26oC and salinities of 36, 39 and 42% o to determine whether temperature and/or salinity influences growth of O+ P. auratus. ANOVA showed that growth, in terms of length, of 0+ P. auratus increased significantly at temperatures of 22 and 26oC compared to 18oC. Furthermore, in terms of weight, growth increased significantly with each 4oC increase in temperature. In comparison, growth was higher at salinities of 39%o than at 36 or 42%o but only in terms of length. Sea surface temperatures (SSTs) derived from remote-sensing satellite data demonstrated that P. auratus experienced SSTs 2-3oC cooler in the southern region compared to the northern region of the western gulf. In light of this, and the results of the aquarium experiment, depressed growth of wild 0+ P. auratus in the southern region was most likely due to the cooler water temperatures occurring during their first few months of life. The growth rate differences between the 0+ P. auratus of the northern and southern regions are discussed with regard to their lack of significance to the application of minimum size limits to adult P. auratus in Shark Bay. Further implications of the knowledge gained from this study to management practices applied to the snapper fishery in Shark Bay are highlighted and discussed. In particular, a recommendation was made to modify the dates of the closed snapper-fishing season around spawning to July-August of each year, rather than mid-August to September, as is currently the case. Knowledge gained from the present study is also highly applicable to any future P. auratus restocking programs in Shark Bay and may enhance the effectiveness of such projects by identifying: optimal temperature and salinity conditions for artificial rearing of snapper; favourable locations, times of year and optimal fish size for releases; and providing valuable advice regarding tagging methods.

The nutritional effects of dietary selenium and lupin meal in snapper Pagrus auratus and cobia Rachycentron canadum were evaluated. Dietary selenium supplementation improved growth and physiological responses of cobia, but not in snapper. Dietary selenium supplementation improved growth, feed utilisation and physiological responses of cobia fed lupin-based diets. Both species showed symptoms of selenium toxicity after fed excessive selenium levels. Adverse effects were observed in snapper and cobia fed high inclusion levels of lupin.

Stress inhibits reproduction in most fish and can be exerted through normal hatchery practices in aquaculture. Previous research with wild New Zealand snapper, has shown inhibitory effects of stress on reproduction, however, there is no information on the effect of stress in captive or domesticated stocks. This study examines the role of stress in the reproduction of snapper, by determining the effect of capture and confinement on plasma steroid levels, gonadal condition, the ovulatory response to exogenous hormone treatment and in vitro ovarian steroidogenesis. The effects of stress were investigated in wild fish caught by trap, 5-year-old fish, caught as juveniles and on-grown in captivity and 2- and 3-year-old hatchery-reared snapper. Fish were held post capture for up to 168h. Blood was sampled at 0h and after the confinement period, the fish were sacrificed and the gonads preserved for histology. Plasma levels of cortisol and the gonadal steroids, 17β-estradiol (`E_2`), testosterone (T), 11-ketotestosterone (11KT) and 17,20β dihydroxy-4-pregnen-3-one (17,20βP) were determined by radioimmunoassay (RIA). Stress resulted in an increase in plasma cortisol and concomitant decreases in `E_2`, and T in females, and plasma T and 11 KT in males. In addition there was an increase in the incidence of ovarian atresia in females, and in the proportion of spermatozoa in the testes of males. These results confirm that snapper are highly susceptible to stress-induced impairment of reproduction, and this response is still present in hatchery-reared fish. Induced ovulation is a common requirement in aquaculture, but stress effects associated with handling may affect the efficacy of exogenous hormone treatments. This was tested by treating hatchery-reared female snapper, Oh (unstressed) or 24h after capture (stressed), with either luteinising hormone releasing hormone analogue (LHRHa), human chorionic gonadotropin (hCG), or 17α-hydroxyprogesterone (17P). Blood was sampled prior to treatment and again after 168 h and fish were checked periodically for ovulation. In unstressed fish, hCG gave the best ovulatory response, followed by LHRHa, in terms of numbers of ovulators, egg volumes, egg quality and percent fertilisation. A delay in injection resulted in significantly lower `E_2` and T at injection, smaller egg volumes, and poorer egg quality, confirming that treatment at first capture, yields a better ovulatory response than treatment after exposure to capture and handling stress. Studies on plasma levels of gonadal steroids suggest that stress effects of the type found in snapper might result from impairment of ovarian steroidogenesis. Accordingly 3-year-old female hatchery-reared snapper were stressed for up to 168h and isolated ovarian follicles were incubated with hCG, 17P, or hCG plus 17P. 17P was most effective at stimulating production of `E_2` and T, and there was no further benefit in co-stimulation with hCG. Stress markedly reduced the capacity of ovarian follicles to convert 17P to T. `E_2` production was unaffected, suggesting that aromatase-mediated conversion of T to `E_2` is not affected by stress. This indicates that decreases in E2 concentrations evident in the plasma following stress are possibly the result of stress-induced reduction in substrate availability and not a reduction in aromatase activity.

Snapper (Pagrus auratus) are part of a developing marine aquaculture industry in Australia. However, little is known about the immune system of this species. This study investigated :fundamental aspects of the acquired immune response of snapper. Initially, snapper lg was purified by Staphylococcal protein A (SpA)-agarose affmity chromatography and polyclonal and monoclonal antisera were produced against the purified product then characterised by indirect ELISA, Western blot and flow cytometry. The polyclonal anti-snapper lg antisera bound to both heavy (H) and light (L) chains of reduced snapper serum while three monoclonal anti-snapper lg antibodies bound to the mucosal and/or systemic lg H chain. Both monoclonal and polyclonal anti-snapper lg antisera bound to cells with flow cytometric light scattering profiles similar to lymphocytes as well as macrophages and granulocytes. It is hypothesised that the macrophages and granulocytes do not express mlg but are identified upon binding to systemic lg bound to putative Fe-like receptors (FcR). Monoclonal anti-snapper lg was also shown to mediate quantitative and qualitative changes in PBL intracellular protein tyrosine phosphorylation, indicative of B cell activation. Prominent phosphorylated proteins are hypothesised to be mlg accessory molecules as in murine B cells (CD79α and CD79β. Other leucocyte differentiation markers were sought and assessed by screening 54 anti-human and anti-murine monoclonal and polyclonal antibodies against snapper PBLs in flow cytometry. An anti-human CD3ε antibody bound to a population ofmlg\(^-\) PBLs and it is hypothesised that the anti-CDε antibody may have identified T cells, since the peptide immunogen used to produce the antibody is conserved in CD3ε amino acid sequences from humans to sterlet (a Chondrostean). Preliminary experiments indicated that snapper may utilise cellular communication as in higher vertebrates, although it was not determined if communication was in the form of cognate cellular interaction, soluble factor(s) or a combination of both. Activation with either B cell or T cell mitogens drove both mlg\(^+\) and mlg\(^-\) cells into cycle while activation with both mitogens initiated a synergistic proliferative effect, indicative of an interaction between directly or indirectly activated cell types. Finally, the apparent similarity between teleost B cells and murine B1 cells was assessed using techniques that were originally used to characterise B1 cell derived lg. Snapper lg bound to bromelain treated erythrocytes and binding was inhibited by liposomes of phosphatidylcholine, reminiscent of B1 derived lg. It is hypothesised that the binding to this autoantigen together with the binding of other teleost natural lg to autoantigens are a product of a teleost B1-cell like equivalent.

This thesis identified optimal physical rearing regimes for Australian snapper, Pagrus auratus larvae, the suitability of saline groundwater for snapper culture, and the effects of salinity and potassium-deficient saline groundwater on osmoregulation of snapper. The effects of photoperiod, salinity and temperature on growth, survival, onset of feeding, swimbladder inflation, presence of urinary calculi and tail flexion of first-feeding (3 days after hatching; dah) to pre-metamorphosis (21-32 dah) snapper larvae were determined in a series of factorial experiments conducted in specially designed, replicated, 100-1, cylindroconical tanks. The optimal photoperiod changed during larval ontogeny based on success of initial swimbladder inflation and subsequent growth. Snapper larvae tolerated a wide range of salinities from near-isoosmotic to hypersaline environments but optimal salinity for growth and development was from 20 to 35%o. Snapper larvae tolerated a relatively narrow range of water temperature from 15 to 24°C and larval growth increased as temperature was increased. The performance of snapper larvae from 4-33 dah under a "new" regime that combined optimal salinity (20-35%0), temperature (24°C) and photoperiod (12L:12D to swimbladder inflation, then 18L:06D), determined systematically in a series of experiments, was compared with a previous "best-practice" regime of salinity (35%0), temperature (21 °C) and photoperiod (14L:10D) in 2000-1 commercial-scale larval rearing tanks. Larvae reared in the "new" regime grew and developed more quickly than larvae in the previous "best-practice" and by 33 dah were fully weaned from live feeds to a pellet diet. Approximately eleven hatchery cycles per year are possible when larvae are reared under the "new" regime compared with seven hatchery cycles per year for the previous "best-practice" regime. The suitability of saline groundwater (SG; —20%0) from inland New South Wales for growth and survival of juvenile snapper was determined in a series of replicated 7-8 d survival bioassays and 42 d growth studies in tanks of 2-1 or 100-1, respectively. Raw SG was very deficient in potassium compared with similar salinity coastal seawater (CS) and snapper died within 2-3 d after transfer from CS to rawSG. However, growth and survival of snapper was the same in SG and CS, provided the [K+] in SG was increased to 60-100% of the [K+] in CS by adding KC1. It was not possible to acclimate snapper to rawSG by dilution of [K+] over time. The effects of rapid transfer of juvenile snapper from seawater (30%0) to near-isoosmotic (15%0) and hypersaline (45%0) seawater, and from seawater (30%0) to SG (30%0) fortified with 0 (rawSG), 40% (SG40) and 100% (SG 100) of [K] in seawater on serum osmolality, [Na+], [K+], [Cl-], blood haematocrit and branchial chloride cell morphology were assessed during 168 h after transfer. Changes in serum chemistry occurred rapidly in fish transferred to 15%0 and 45%0 but had returned to near initial levels after 168 h. Restoration of homeostasis was concomitant with changes in the number and size of immunoreactive chloride cells. Serum chemistry of fish transferred to SG40 and SG100 was similar, in general, to the initial levels. However, serum osmolality, [Cl-], [Na+] increased and serum [K+] decreased rapidly after transfer to rawSG. The morphology of chloride cells was unaffected by SG treatments. The research described in this thesis has provided an experimental base for new culture conditions to greatly improve survival and growth of snapper larvae through to fingerlings and survival and growth of snapper juveniles in inland saline groundwater. These new culture conditions represent major cost savings to hatchery operations and improve the aquaculture potential of snapper in Australia.

The population dynamics and fishery productivity of snapper, Chrysophrys auratus, in South Australia are strongly driven by inter-annual variation in recruitment. This variable recruitment produces the occasional strong year-class which, over a number of years, results in a cycle of increasing and then decreasing biomass. The aim of this study was to develop an understanding of the recruitment dynamics of snapper through a study of the reproductive biology of adults, as well as the biology and ecology of 0+ recruits. Field sampling was done through 2006 to 2008 in northern Spencer Gulf, the region that generally contributes the majority of South Australia’s snapper catch. The recruitment of 0+ snapper was measured from a study of inter-and intra- annual patterns of distribution and abundance. This was done using two independent trawl sampling regimes, one using a beam trawl and the other an otter trawl, at different times in the settlement season. There was considerable inter-annual variation in abundance of 0+ fish of up to two orders of magnitude and, in some years, almost no recruitment was observed. The spatial pattern of dispersion of recruits was clumped and consistent between years; one area, Western Shoal, always produced the highest catches indicating that it is an important nursery. The effects of the timing of spawning and water temperature on growth patterns of the 0+ snapper collected in the trawl sampling were studied to determine possible impacts on recruitment processes. Growth was studied from age/length regressions and by measuring the widths of microincrements in the sagittal otoliths. Sub-surface water temperature was also logged in the region. Growth rate varied inter- and intra-annually but was not limited by temperature in the pre-settlement period. However, post-settlement growth rate was significantly correlated with water temperature and fish exhibited a dramatic slowing of growth as temperature declined in autumn. Later spawned fish were considerably smaller than earlier spawned fish of the same age, which may have implications for post-settlement survival. The reproductive biology of snapper was studied to determine if annual variation in recruitment was related to egg production. Reproductive samples from northern Spencer Gulf were collected over three seasons (2005/06, 2006/07 and 2007/08) and were analysed macro- and microscopically. Spawning activity was determined by calculating estimates of spawning fraction and batch fecundities. The onset of spawning occurred in November but varied between years and corresponded with times when water temperature was between 18 and 20ºC. The length of the spawning season also differed between years. In each year the peak spawning activity occurred during December when fish spawned almost daily. Spawning frequency and relative batch size did not differ between the first two spawning seasons but, in the third season, batch size was considerably greater and spawning fraction lower. However, recruitment was considerably more variable than the annual differences in spawning output could explain. This indicates that mortality processes during the planktonic or early post-settlement period are important in the recruitment dynamics of snapper. The impact of water temperature, lunar cycle and tide on the timing and strength of recruitment was investigated. Pre-settlement duration, spawn dates and settlement dates were determined from the microstructure of the sagittal otoliths of 0+ snapper. The patterns of successful spawning and settlement were determined by back-calculating to the day on which individual recruits were spawned and settled. The frequency distributions of these dates were compared with water temperature, lunar periodicity and the temporal variation in spawning. There was considerable variation within a season in the timing and magnitude of successful recruitment. Strongest recruitment resulted from spawning during December and January on days when water temperatures were between 21 and 23ºC but spawning on days in this range did not necessarily result in recruitment. Pre-settlement duration was unaffected by water temperature. Some evidence of lunar periodicity was detected in both the spawn and settlement date frequencies. Importantly, the spawn date frequency distributions of successful 0+ recruits did not correspond with the measured spawning activity of adults as considerable portions of the spawning season in each year did not produce successful recruits. These results indicated that spawning output and water temperature cannot explain the observed magnitude in recruitment variation. Food availability for 0+ snapper has been implicated in their patterns of distribution and abundance in New Zealand and Japan. Stomach contents of 0+ snapper were described as an initial step in developing some understanding of the dispersion of 0+ recruits. In spite of their generalist feeding habit, in the area of highest abundance (Western Shoal), snapper took considerably more polychaetes than elsewhere in northern Spencer Gulf. If polychaetes are more abundant at Western Shoal, this could explain the higher density of 0+ snapper there but insufficient information was available on these animals for northern Spencer Gulf to address this hypothesis. Food availability and/or quality may influence the distribution of 0+ recruits. The multi-species collections from the beam trawls were described to develop an understanding of the spatial dispersion of recruits and their habitat associations. 0+ snapper co-occurred with an assemblage that was characterised by fish and invertebrate species that are associated with mud/soft bottom, but they never occurred with the assemblage of species associated with seagrass, even when recruitment was strongest. This association partly explains the observed distribution pattern, but not all areas of mud/soft bottom had 0+ recruits, even in strong recruitment years. In northern Spencer Gulf, seagrass areas could be excluded from future snapper recruitment surveys. The recruitment dynamics of snapper in northern Spencer Gulf were characterised by dramatic inter-annual variation but a consistent pattern of dispersion. 0+ snapper were concentrated in a few small areas in northern part of the study region. One of these areas, Western Shoal, appears to be very important as a nursery for snapper. Furthermore, the pattern of 0+ snapper dispersion was independent of recruitment strength. The potential magnitude of 0+ snapper recruitment, set by egg production, was altered by mortality during the early life history. Some of this mortality was related to temperature regimes at the time of spawning but this did not explain all the variation in the magnitude and timing of recruitment. Snapper spawning occurred at times with suitable temperature conditions but recruitment did not always result. This indicates the presence of other factor(s) that have substantial influences on mortality in the early life history.
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Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

Australia has an abundance of saline groundwater due to land clearing and irrigation within the Murray-Darling river system. Generally saline groundwater contains similar ions to coastal seawater with concentrations varying due to the source of the water. In this study Australian snapper (Pagrus auratus), mulloway (Argyrosomus japonicus) and yellowtail kingfish (Seriola lalandi) were chosen to examine their suitability for culture in saline groundwater. The growth and metabolic response of the aforementioned species were investigated to determine their suitability for culture in saline groundwater from the ‘outfall’ of the Stockyard Plains Disposal Basin (SPDB) in Waikerie, South Australia (potassium ~ 80 mg.L-1). In the first experiment snapper, mulloway and yellowtail kingfish were cultured in saline groundwater (salinity = 19 ± 1 g.L-1), half-strength coastal seawater (iso-osmotic; 19 ± 1 g.L-1) and oceanic seawater (except snapper; 37 ± 1 g.L-1) for 61, 45 and 21 days respectively, to determine survival, growth rate and food conversion ratio. At the end of each growth experiment, fish were placed in static box respirometers to record changes in oxygen consumption in the different water types. Changes in oxygen consumption were used as an indirect method to determine routine metabolic rate (Mo2rout), maximum metabolic rate (Mo2max) and metabolic scope (Mo2scope). In the second experiment, blood samples were taken from the fish 1 hour postexhaustion and analysed for blood plasma sodium, potassium, chloride, lactate and osmolality to determine if fish were able to cope with osmoregulatory changes.