Dissertations / Theses on the topic 'Glass eels'

Anguilla australis glass eels proved to be resilient and present strong aquaculture potential. General husbandry techniques, anaesthesia and prophylactic treatments were established for glass eels between 0.1 g and 2.0 g and elvers between 2.0 g and 8.0 g, caught in rivers and estuaries along the South East Coast of Victoria. The protozoan parasites Ichthyobodo and Trichodina were found to be present on arrival to the hatchery developed during different rearing treatments, and were successfully eradicated. A. australis glass eels accepted artificial food, but it was recommended first be fed a preweaning diet of minced fish flesh. A weaning regime from minced fish flesh to commercially available eel grower mash, over 15 days was established. Growth rate proved to be highly variable, both between and within groups. The highest growth rate of 2.71%/day was found when the natural diet of minced fish and Artemia was fed. The maximum growth rate when reared on an artificial diet of 1.63%/day was observed at 25°C. Growth was affected by the presence or absence of a preweaning diet, weaning diet, weaning period, temperature, but not by size or density. Once weaned, glass eels were found to perform better on commercially available grower mash than on the minced fish flesh, which was used to aid in weaning them to artificial diets. Of the water quality parameters measured stocking density was found to affect pH, Total Ammonia Nitrogen, Total Phosphorus, and Dissolved Oxygen, through not to an extent which affected growth.

Glass eels of the European eel migrate from coastal waters inland to freshwater as part of the catadromous lifecycle. The osmotic challenge faced at this time is augmented by their large surface area to volume ratio, and by the fact that the migration may only be completed after several attempts, due to the effects of tide and river flow. Glass eels and elvers developed normally when maintained in waters of differing salinity over a six month period. Drinking rates increased with environmental acclimation salinity (from 0.072 +/- 0.023 mul/g/h to 0.698 +/- 0.099 mul/g/h in FW and SW respectively), and freshwater acclimated fish exhibited a rapid drinking response upon contact with seawater. These accounts of dipsogenic behaviour are similar to those previously reported for adult eels. Results obtained from determinations of branchial Na+K+ATPase activities were more equivocal. Only after nearly five months were activities higher in SW (508.52 +/- 99.76 nmoles/Spairs gills/h) as compared to FW fish (151.65 +/- 8.9 nmoles/5pairs gills/h). Following the transfer of FW acclimated fish to SW there was a trend towards increased Na+K+ATPase activity after seven days post-transfer, which reached a significant peak after two months post-transfer. A transient increase in whole body cortisol content was noted following the transfer of fish from freshwater (388.02 + 90.38 pg/g) to seawater (6268.44 +/- 773.14 pg/g). However, it was not possible to ascertain that this was due to a direct effect of environmental salinity change. There were no clear changes in interrenal cell morphology between salinity groups, although the cells did appear reduced in size with time, regardless of environmental salinity. Total body Na+ content increased with time, and was higher in SW (58.66 +/- 1.66 mumoles/g) as compared to FW reared fish (44.85 +/-1.01 mumoles/g).

L'anguille européenne (Anguilla anguilla) présente un cycle de vie complexe avec un stade civelle qui remonte les estuaires pour atteindre les rivières et entamer une phase de croissance. Cependant, cette migration estuarienne est connue pour être facultative, certains individus s'installant en mer, en estuaire ou alternant les séjours en rivière et en estuaire. Les civelles ne se nourrissent pas ou peu pendant leur migration et leurs schémas migratoires pourraient donc être étroitement associés aux réserves énergétiques des individus. La migration facultative des civelles pourrait donc reposer sur une stratégie conditionnelle, les individus présentant d’importantes réserves d'énergie ayant une capacité migratoire élevée. Cependant, certaines études s’avèrent contradictoires et l’objectif de cette thèse était d'étudier la stratégie conditionnelle chez les civelles européennes en se basant non seulement sur les réserves d'énergie mais également sur la mobilisation de l'énergie (métabolisme et expression des gènes liés à l'énergie). L’essentiel des travaux a été mené à la fois sur des civelles d'automne et de printemps, car elles présentent d’importantes différences de réserves énergétiques. Nous avons tout d'abord caractérisé le statut énergétique individuel des civelles marines échantillonnées en automne et au printemps et l’avons relié à leur comportement migratoire évalué dans des installations expérimentales. Les civelles d'automne présentaient des réserves énergétiques plus élevées et une plus grande capacité à produire de l'énergie que les civelles de printemps. Ces résultats confirment que les civelles d'automne et de printemps présentent de fortes différences dans leur statut énergétique et qu'elles doivent être étudiées séparément. Nous émettons l'hypothèse qu'il pourrait exister un seuil énergétique en dessous duquel la migration pourrait être conditionnée par ce facteur. Afin de mieux comprendre les mécanismes de sédentarisation dans les estuaires, la relation entre le statut énergétique et le comportement migratoire des civelles marines et estuariennes a ensuite été étudié parallèlement en automne et au printemps. Nos résultats soutiennent l’hypothèse d’une stratégie conditionnelle basée sur l'énergie lorsque les réserves énergétiques deviennent un facteur limitant (civelles de printemps). La perte de poids, la mesure du métabolisme et l'expression de gènes liés au métabolisme suggèrent également que les civelles estuariennes étaient plus stressées et avaient une capacité de production d'énergie plus faible que les marines. Les civelles non synchronisées au courant (considérées comme ayant une faible probabilité de migration) présentaient une dépense énergétique plus élevée que les individus synchronisés, ce qui pourrait refléter un stress et/ou une vulnérabilité au stress plus élevés chez les premières. Afin de tester cette hypothèse, des civelles ont été exposées à un facteur de stress potentiel dans les estuaires, à savoir le méthylmercure (MeHg), et les effets de ce contaminant sur le comportement migratoire et le statut énergétique des individus a été étudié. Nos résultats mettent en évidence une diminution de l’activité de nage chez les civelles non synchronisées mais pas chez les synchronisés, suggérant que les premières pourraient être plus sensibles au MeHg. Puis, les civelles non synchronisées présentaient un niveau d'expression des gènes du métabolisme inférieur à celui de leurs homologues synchronisées dans la tête et pas dans le muscle ni dans les viscères. Les travaux réalisés dans le cadre de cette thèse suggèrent que le statut énergétique et la sensibilité au stress pourraient conditionner la migration estuarienne des civelles mais les mécanismes sous-jacents et les relations entre ces facteurs ainsi qu’avec les horloges endogènes qui contrôlent la migration restent à élucider
Résumé en anglais : The European eel (Anguilla anguilla) present a complex life cycle with a glass eel stage migrating up estuary to reach river for growth. However, this estuarine migration is known to be facultative, with some individuals settling at sea, in estuaries or alternating stays in rivers and estuaries. As glass eels feed little or not at all during their migration, their divergent migration patterns may be closely associated with individual’s body condition. To date, one major theory of conditional strategy proposed that the facultative migration in European glass eels is based on energy stores, the individuals with a high migratory capacity presenting high energy stores. However, this theory has been proved controversial and the aim of this thesis was to investigate the conditional strategy in European glass eels based on more comprehensive measures of energetic status, including not only energy stores but also energy mobilization (metabolism and energy-related genes expression). We also focused on both autumn and spring glass eels, which present dramatic difference in energy stores. We first characterized the individual energetic status of marine glass eels sampled in autumn and spring and related this status to their migration behavior assessed in experimental facilities. Autumn glass eels presented higher energy stores and a higher ability to produce energy than spring ones. This confirmed that autumn and spring glass eels present strong differences in their energetic status and that they have to be studied separately. We hypothesized that a potential threshold in energetic status may exist below which migration could be conditioned by energetics. Then, to unveil the underlying mechanisms of settlement processes in estuaries in relation to energy-based conditional strategy, we investigated the relationship between energetic status and migration behavior in both marine and estuarine glass eels. Estuarine individuals displayed lower weight than marine ones in autumn but not in spring supporting the idea that a conditional strategy based on energy may explain facultative migration when energy reserves become a limiting factor. Weight loss, standard metabolism and the expression of metabolism-related genes suggested that estuarine glass eels were more stressed and had a lower capacity of energy production than marine fish. The non-synchronized glass eels also presented a higher energy expenditure than synchronized individuals possibly reflecting a higher stress and/or vulnerability to stress in the former. We further exposed glass eels to a potential stressor in estuaries - methylmercury (MeHg) in order to investigate the effects of this contaminant on glass eels’ migratory behavior and energetic status. Our results first suggested that non-synchronized glass eels were more vulnerable to MeHg reflected by a decrease in swimming activity. MeHg also affected the relation between individual metabolism-related genes expression level and swimming activity, supporting our hypothesis that stress factors may influence the settlement processes in glass eels. Finally, it is noteworthy that non-synchronized glass eels displayed lower expression level of metabolism genes than their synchronized counterpart in the head but not in muscle nor in viscera. Altogether, these results provided evidences that the energetic status and sensitivity to stress may condition estuarine migration in glass eels but the underlying mechanisms and relationships between these factors but also with the endogenous clocks driving migration remain to be elucidated

Européenne, (Anguilla anguilla) a conduit à une diminution du nombre d’anguille jaune dans les bassins versants. En 2007, le règlement (CE) instaure que 60% des civelles pêchées (<12 cm de long) doivent être dédiés à des opérations de repeuplement. Cependant, pour migrer et coloniser un nouvel environnement, les civelles doivent avoir une bonne condition énergétique. Or, les civelles ne s’alimentent pas ou peu durant leur migration estuarienne et la vitesse d’utilisation de leurs réserves dépend de divers facteurs (température, contaminants etc.). Dans l’estuaire de l’Adour, différents travaux ont mis en évidence de fortes concentrations en methylmercure (MeHg) dans les sédiments et une bioaccumulation de ce composé chez l’anguille jaune. L’exposition au MeHg entraîne la mise en place de processus de détoxication qui pourraient être couteux en énergie ce qui, chez des organismes jeûnant comme les civelles, conduirait à une perte de poids pouvant affecter leurs capacités de migration. Pour étudier l’impact du méthylmercure sur le potentiel de migration des civelles, nous avons tout d’abord (1) effectuer un état des lieux des concentrations en MeHg des civelles de l’estuaire de l’Adour, (2) évaluer l’effet du MeHg sur deux synchroniseurs de la migration estuarienne: le signal crépusculaire et l’alternance des marées chez les civelles grâce à l’utilisation de traceurs isotopiques et (3) étudier dans le milieu naturel (Adour) le comportement de migration, les taux de contamination au mercure et les mécanismes de détoxication des civelles. Nos résultats montrent des fluctuations temporelles et saisonnières de la concentration en MeHg des civelles prélevées en entrée et en milieu de l’estuaire. Par ailleurs, l’exposition au MeHg, en milieu contrôlé, a augmenté l'activité des civelles mais pas leur comportement de migration. Cependant, suite à la contamination, la structure mitochondriale et le métabolisme ont été modifiés suggérant un plus fort stress oxydant et une activation des défenses antioxydantes notamment des civelles ayant un faible potentiel de migration (non migrantes).En complément, l’étude en milieu naturel a mis en évidence une plus forte demande énergétique pour les civelles non migrantes entrainant une augmentation du fonctionnement des mitochondries et pouvant conduire à un plus fort stress oxydant
Over the last three decades, the arrival of Anguilla anguilla glass eels has markedly declined which has lead to a decrease in the number of yellow eels in the watersheds. In 2007, the Regulation (EC) indicated that 60% of glass eels (<12cm long) caught by fisheries should be used for restocking. To migrate and colonize a new environment, glass eels must have a good energetic condition. However, during their estuarine migration, glass eels either feed very little or more likely not at all. The speed at which they expend their energy stores depends on various factors (temperature, contaminants etc.). In the Adour estuary, many studies have reported relatively high methylmercury (MeHg) concentrations in the sediments, and bioaccumulation in yellow eels. Exposure to MeHg requires energy for detoxification which, in starving organisms such as glass eels, might lead to weight loss, decreasing their ability to migrate up the estuary. To study the impact of MeHg on migratory behaviour in glass eels, we first conducted an inventory of MeHg concentrations of glass eels in the estuary of the Adour, (2) evaluate the effect of MeHg on two synchronizers of estuarine migration: dusk and tide on glass eels through the use of isotopic tracers and (3) to study the natural environment (Adour) migratory behavior, mercury contamination levels and detoxification mechanisms of glass eels. Our results show temporal and seasonal fluctuations in the concentration of MeHg glass eels collected input and middle of the estuary. Moreover, MeHg exposure, in a controlled environment, increased the activity of glass eels but not in response to migratory behavior. However, due to contamination, mitochondrial structure and metabolism have been modified suggesting a stronger oxidative stress and activation of antioxidant defences of glass eels, particularly those with low migration potential (non-migrant). In addition, the wild study showed a higher energy demand for non-migrating glass eels causing an increase in mitochondrial function which may lead to higher oxidative stress

Cette thèse en recherches fondamentale et appliquée a pour modèle biologique l’anguille d’eau douce qui a toujours fasciné le monde de la recherche par son cycle de vie unique et tous ses mystères non encore résolus pour certains. Dans le monde, 19 espèces d’anguilles existent dont certaines en déclins dans les zones tempérées ce qui peut justifier l’amplification des recherches sous les tropiques. En Polynésie française, cet animal mystérieux est sacré ; les légendes polynésiennes associent les anguilles à la création de la vie. Trois espèces y sont présentes : l’anguille marbrée « Anguilla marmorata », l’anguille de montagne « Anguilla megastoma » et l’anguille de vase « Anguilla obscura ». Elles n’ont pas la même distribution spatiale où la première s’adapte à n’importe quel milieu, la deuxième remonte les rivières jusqu’en haute vallée à plusieurs centaines de mètres d’altitude, tandis que la dernière reste en basse vallée préférant les eaux stagnantes. Grâce aux nombreuses rivières présentes sur Tahiti, cette île principale de Polynésie est en pleine transition énergétique et privilégie maintenant les énergies renouvelables ; particulièrement l’hydroélectricité. Cependant, les aménagements hydroélectriques connus pour provoquer des problèmes de continuité écologique de la biodiversité dulçaquicole, notamment pour les anguilles, vont nous intéresser dans ce travail de thèse en bourse Cifre. C’est dans ce contexte que s’inscrit cette thèse, dans la plus grande vallée de Tahiti, la Papenoo. Deux problématiques y sont étudiées : le recrutement des civelles et l’étude de la population adulte. Nos résultats sur les trois années de suivi de l’arrivée des juvéniles d’anguilles dans l’embouchure de la Papenoo révèlent un recrutement saisonnier des civelles de Novembre à Mars avec deux pics en Décembre et Janvier. Plus de 80% des civelles sont des Anguilla marmorata. Concernant les populations adultes, nous observons : 1/ plus de 90% de la population sont également de l’espèce Anguilla marmorata, 2/ les anguilles adultes présentent un gradient de densité de l’aval vers l’amont et 3/ le problème de continuité biologique ne vient pas des barrages ou captages en eux-mêmes, mais proviendrait des canaux de fuite en sortie de centrale où nous observons une sur-attractivité pour les anguiles. Cette étude s’est révélée très importante dans l’avancement théorique de nos connaissances biologiques et écologique sur l’anguille, dans l’appropriation par l’industriel de bonnes pratiques et par une solide communication sur la coexistence possible entre gestion et développement
This thesis in fundamental and applied research has for biological model the freshwater eel which has always fascinated the world of the research by its unique cycle of life and all its mysteries not solved for some. Worldwide, 19 species of eels exist, some of which are declining in temperate zones, which may justify the increase in research in the tropics. In French Polynesia, this mysterious animal is sacred; the Polynesian legends associate the eels with the creation of life. Three species are present: the marbled eel "Anguilla marmorata", the mountain eel "Anguilla megastoma" and the eel "Anguilla obscura". They do not have the same spatial distribution where the former adapts to any medium, the second goes up the rivers to the upper valley at several hundred meters of altitude, while the latter remains in the lower valley preferring the stagnant waters. Thanks to the numerous rivers on Tahiti, this main island of Polynesia is in the midst of an energy transition and now favors renewable energies; particularly hydroelectricity. However, hydroelectric developments known to cause problems of ecological continuity of freshwater biodiversity, especially for eels, will interest us in this thesis work on the Cifre grant. It is in this context that this thesis, in the largest valley of Tahiti, is the Papenoo. Two problems are studied: the recruitment of glass eel and the study of the adult population. Our results over the three years following the arrival of juvenile eels in the mouth of the Papenoo reveal a seasonal recruitment of elvers from November to March with two peaks in December and January. More than 80% of elvers are Anguilla marmorata. For adult populations, we observe: 1/ more than 90% of the population is also Anguilla marmorata, 2/ adult eels have a density gradient from downstream to upstream, and 3/ biological continuity does not come from dams or water removal in themselves, but comes from the tailrace leaving the hydropower plant where we observe an over-attractiveness for the eels. This study proved to be very important in the theoretical advancement of our biological and ecological knowledge about the eel, in the appropriation by industry of good practices and in a solid communication on the possible coexistence between management and development

碩士
國立臺灣海洋大學
水產養殖學系
104
This study intended to control disease problems of glass eels and improve survival rate of juveniles of marbled eels (Anguilla marmorata), including the optimal salinity test, the disinfection of parasites, and the development of oral bacterial vaccines of Aeromonas hydrophila and Edwardsiella tarda. The optimal salinity in glass eel stage (12000/kg) to elver stage (2000/kg) was between 3 - 5 ‰, and after elver stage (500/kg), the optimal salinity was 0‰. The disinfection of blood worms by using 4 ppm ClO2 was most effective, while treated with 10 ppm formalin exhibited best palatability. Both of the treatments could help glass eels digest blood worms. In glass eel stage, the use of blood worms as food was found to be the best. When the parasite Dactylogyrus sp. hooked on the gills of eels, soaking with 10 ppm levamisole and 0.3 ppm copper sulfate were found to be effective for removing parasites and eggs by 7 days. The formalinized bacterial cells were sonicated for 1.5 hours and encapsulated with 5% of sodium alginate or 9% CAP exhibited the best amount of antigen-releasing in vaccines. Eels were i.p. challenged with A. hydrophila and E. tarda after oral vaccination. The results showed that the highest relative percent survival was obtained at the 12th week, with highest RPS reaching 100%, but the antibody titer decreased after the highest, it maintained steadily at high level. The results showed that oral administration with encapsulated vaccine could protect bacterial antigens against stomach acid, and might therefore help the antigens to induce immune responses in eel intestine. The sonicated bacterial antigens encapsulated with biomaterials, had a higher antibody titer and maintained longer time. Eels boosted with the same oral vaccine, could obviously exhibited higher antibody titer and maintain much longer time. Eels orally administrated with sodium alginate or formalin killed cells vaccine, could also improve its SOD responses. The present results revealed that some factors including fluctuation of water salinity, bacterial and parasitic infection, and drug resistance etc. were major problems leading to mass mortality during the culture of marbled eels at glass eel stage. To improve survival rate of glass eels and juveniles, adjustment of suitable salinity disinfection of live foods control of parasites and use of encapsulated oral vaccines and boosting were suggested for the farmers.

Shortfin eels (Anguilla australis) and longfin eels (A. reinhardtii) are true freshwater eels of the genus Anguilla. There are many mysteries still unsolved for the freshwater eel lifecycle, such as location of the spawning grounds, conditions that promote metamorphosis from the leptocephalid to glass eel phase, and the mechanisms that affect glass eel recruitment. In Australia, little is also known about the estuarine habitats of glass eels as they migrate towards freshwater, and the age at which these eels enter estuaries. Both species are of commercial importance in the estuary fishery where they are caught in eel traps for export. There is also a small, but potentially lucrative, aquaculture industry for ongrowing glass eels to market demand size. This thesis investigates the spatial and temporal recruitment of both species of glass eels to estuaries within NSW, the habitats that may be of importance to them as they continue their upstream migration, and the age at which these eels entered the estuaries. Firstly, a new sampling device needed to be developed since conventional methods to catch glass eels often required constant observation of gear, multiple operators, specific physical site characteristics, and/or were expensive. The artificial habitat collectors that were developed were then used to sample six estuaries in NSW monthly within one week of the new moon. Shortfins showed a more consistent and defined recruitment across all sites than longfins, where the peak shortfin recruitment season was from April - August. Longfins recruited primarily from January - May but often recruited outside of this period. Five year collections at one of these sites provided important recruitment information. It appeared that longfins failed to recruit to this site during 2000/01, which could affect commercial catches of this species when they enter the fishery. The East Australian Current (EAC) probably transports glass eels from spawning sites in the Coral Sea southward to the east coast of Australia but there was no predicted lag time in the recruitment of eels from northern to southern estuaries. Therefore, it was not possible to predict the timing of recruitment of glass eels in one estuary based on the timing of recruitment in another more northern estuary. When glass eels enter estuaries their upstream migration is assisted by the night flood tide. During the ebb tide, glass eels burrow into the substrate and resurface at the next night flood tide. The eels do not select particular habitats at this time, rather, their location is dictated by the tide. However, once glass eels reach the estuarine/freshwater interface, they may prefer more complex habitats such as seagrass/macrophytes or rocks/cobbles in which to hide during the day. At this interface, glass eels undergo a physiological change to adapt to a freshwater existence and this change may take up to a few weeks. During this time, glass eels commonly enter the water column during the night flood tide and may be able to locate more suitable habitats in which to hide during the day. The ages of shortfin and longfin glass eels caught in estuaries were examined both spatially and temporally. As the EAC travels north to south, glass eels recruiting to the southern sites were expected to be older. However, shortfins that recruited to the northern-most site in this thesis were older than at all other sites while there was no difference in the ages of longfins. Also, when the ages of longfins that recruited during the main recruitment period were compared to the ages of longfins that recruited outside of this period, there was no difference in ages. Therefore, the hypothesis that these later recruiting eels may have been caught in an eddy prior to their estuarine arrival has been disproved. The ages of shortfins that recruited in two separate years were significantly different from each other and may be due to shortfins' ability to detrain more easily from the weaker currents that exist at these recruitment periods. Conversely, there was no difference in the ages of longfins that recruited in the same month during three separate years. The estimated hatch dates for shortfins was estimated at October to January, while for longfins, estimated hatch time was July to September for eels that recruited during the peak recruitment period. For longfins that recruited outside of the main recruitment period, estimated hatch times were from December to February. It is unknown, however, whether longfins have an extended spawning period, or whether silver eels arrived at the spawning grounds later and thus produced later arriving longfins. Continuous monitoring of glass eel recruitment to estuaries is necessary to determine whether there are long term declines in the recruitment of Australian eels similar to the declines recently observed for eels in Europe and Asia.

碩士
國立臺灣大學
漁業科學研究所
107
Japanese eel (Anguilla japonica) is an important commercial freshwater fish in East Asia. However, eel aquaculture is a typical capture‐based aquaculture. Recruitment of Japanese eel has declined since 1980s, and large interannual fluctuations were also observed. The causes of decline or extremely poor recruitment events remain unclear. By using numerical modelling method to reproduce larval migration routes, previous studies showed that the inter-annual variability of ocean circulation may influence the migration success of Japanese eel. However, the effect of environmental factor on wild larvae is less discussed by biological points of view. In this study, a recent 8–yr total length data of Japanese glass eel was used as representative of eel larval condition to evaluate the effect of environment by generalized additive model. And the environment experienced by eel larvae was based on a coupled biological physical model which were the used to estimate the migration routes. The result of generalized additive model showed larger total length of Japanese glass eel were observed at 21-24.5°C for spawning area temperature, higher than 24°C for migration routes temperature, 0.03-0.07 mg/m3 for spawning area chlorophyll concentration, higher than 13 mg/m3 for accumulated migration routes chlorophyll concentration with increasing trend, 35.1-35.3 psu for spawning area salinity, and during El Niño year. The environment of spawning area has greater effect on glass eel total length. The results correspond to previous studies in laboratory experiment, except the effect of spawning area temperature. The synchronized spawning behavior in Japanese eel may cause the density-dependent effect which potentially influence the result of this study. This study provided a different perspective of environmental effect on Japanese larvae, and may help to understand the cause of recruitment fluctuation.

碩士
國立臺灣海洋大學
水產養殖學系
103
The present study investigated culture techniques and bacterial diseases control for juvenile marbled eels (Anguilla marmorata), the promotion for survival rate of glass eels also investigated. To develop oral divalent vaccine for marbled eel, two common bacterial pathogens, Edwardsiella tarda and Aeromonas hydrophila of the animal were used. To improve survival rates of glass eels, immunostimulants were fed to enhance non-specific immunity. The divalent vaccines encapsulated with agar, sodium alginate or CAP (Cellulose acetate phthalate) were prepared and oral feeding for the elvers was also conduced. The survival rate of glass eels (500/kg) were 72、75、82 and 63% fed With LZ8-S, sodium alginate, β-1,6-glucan and control, for 2 weeks, respectively after challenge with bacteria. The results revealed that immunostimulants could enhance immunity for glass eels leading to a better survival rate. To find an optimal salinity, glass eels were acclimatized to various salinity (10‰、5‰、3‰、0‰) and raised from glass eel stage (10000/kg) to elver stage (1000/kg). Higher survival rates of glass eels were obtained at weeks 4 raising in 5‰ and 10‰ salinity with survival rates of 84% and 80%, respectively. Lower survival rates were obtained at weeks 4 raising in 0‰ and 3‰ salinity with survival rates of 38% and 68%, respectively. Formalinized bacterial cells of A. hydrophila and E. tarda were encapsulated with agar, sodium alginate or cellulose acetate phthalate (CAP), separately, and used as oral vaccines to immunize glass eel (about 2g each). The relative percentage survival (RPS) of groups orally immunized with Et oral vaccine encapsulated with agar, sodium alginate and CAP was 70, 90, and 80%, respectively, after challenging with E. tarda. In addition, the RPS of groups orally immunized with Ah oral vaccine encapsulated with agar, sodium alginate and CAP was 70, 80, and 80%, respectively, after challenging with A.hydrophila. It reveals that the group fed with formalinized bacterial cells encapsulated with CAP exhibited best RPS higher than 80%. The highest antibody titlers of the eels orally immunized with the oral vaccine were all detected at week 12 after vaccination. In conclusion, the optimal salinities for rasing glass eel to elver stage and elver to yellow eel stage were 5-10‰ and 3-5‰, respectively. The immunostimulants used in this study exhibited enhancement of fish immunity. Good immunoprotection results for juvenile eels against A. hydrophila and E. tarda were obtained after orally immunized with formalinized bacterial cells of the two pathogens encapsulated with agar, sodium alginate or CAP. Further field study results may be useful for eel aquaculture in the future.

碩士
國立臺灣大學
漁業科學研究所
100
Eel is an important aquaculture species in Taiwan, Japan, Korea and China of East Asia, and the eel fry of aquaculture demand must completely rely on capture in the estuaries from fishermen. However, the glass eel resource has been declining rapidly from 1970-1980. In the Western North Pacific, Taiwan and northern Luzon Island of the Philippines are important producing regions for larvae of several Anguilla species. To maintain and manage eel aquaculture industry, the research on geographic distribution and seasonal species composition of glass eel is needed. According to the DNA molecular technique, four species of freshwater eel were identified in Taiwan and Luzon Island, namely Anguilla japonica, A. marmorata, A. luzonensis (syn. of A. huangi), and A. bicolor pacifica. Several studies have revealed the life history of A. japonica and A. marmorata, but the research about life history for A. luzonensis and A. bicolor pacifica are still rare. The study investigates geographical distribution and seasonal species composition of these four species of glass eel in Taiwan and northern Luzon Island of the Philippines. The results revealed that A. marmorata was the dominant species, occurring year round in Taiwan and northern Luzon Island. While sub-dominant species in northern Luzon Island was A. luzonensis, and presented in the second half of the year, with peak from July to September. Comparing with northern Luzon Island, the amount of A. luzonensis was fewer in Taiwan. Furthermore, the relative abundance (%) of A. bicolor pacifica was rare in Taiwan and northern Luzon Island, and appeared mainly between September and March. However, A. japonica was occurred in Taiwan in winter only, and the distinct geographic tilt was observed. The results revealed that the geographic distribution and seasonal composition of glass eels of four eel species were different, and this difference may be related to reproduction seasons and recruiting temperature preferences of the glass eel.

碩士
臺灣大學
漁業科學研究所
98
Japanese eel (Anguilla japonica) is an important aquaculture species in East Asia. Due to unavailable commercial production of artificially propagated glass eels, the abundant elvers are caught on the estuaries to supply the aquaculture demand, which causes serious overfishing and competition among East Asia countries for glass eel resources. Therefore, monitoring and managing the eel resources should be the key strategy for the sustainability of eel aquaculture industry. In this study, we had investigated and monitored the yearly glass eel resource of Taiwan. We tried to establish a reasonable managing strategy of glass eels in Taiwan. Results showed that regarding the recruitment of Taiwanese elver, there is a significant difference in statistics between the Taiwan Fisheries Yearbook and Japanese aquaculture news. The annual glass eel recruitment is highly fluctuated. The Taiwan Fisheries Yearbook shows that the recruitment is between October and April, and in Taiwan the catching area is the countries near the sea. The Elver Online Reporting System discerns the elver production information and establishes online reporting system of elver fishing in Taiwan on time. However, the data collection method should be improved to reflect the actual situation. The suggestion for the improvement of the Elver Online Reporting System is to monitor the correct glass eel recruitment timely with the participation of all counties where elvers are caught. For management and strategic studies, we have collected relevant laws and regulation of eel management from Japan, China, Korea, Europe, Taiwan and the United States of America. The regulation for glass eel catch is based on units of county in Japan, province in China, central government in Korea, European Union in Europe, and states in U.S.A. In Taiwan, glass eels export is banned in period between November and March. However, the early recruitment of glass eels is popular for eel farmers in Japan, leading to a high unit price and illegal export by Taiwanese local eel traders. This indicates the ineffective enforcement of the law in Taiwan. The government should promote the fisheries association of counties all over Taiwan to form managing units, in order to perform regulations of glass eels to maximize the profit of fishermen and eel farmers and protect the eel resource efficiently. For example: 1. catch license must be applied so that the government can track glass eel recruitment. 2. The illegal catch time can be set between November and February. 3. The Taiwan officials are recommended to start to talk with Japanese government. If Japan agrees to export glass eel to Taiwan prior to Feb or March, Taiwan will cancel the ban on export elvers. This result surely will be favored by both sides. Lastly, the Taiwan government is suggested to consult with other East Asian countries to maximize the benefit of the eel industry.

碩士
國立臺灣大學
漁業科學研究所
100
The Japanese eel Anguilla japonica is an important aquaculture species in Northeast Asia countries, however, all of the eel fry needed for aquaculture depends on the wild catch. That the Japanese eel stock has declined rapidly in recent decades has resulted in the decreasing glass eel catch and the eel aquaculture industry is facing serious problems. Therefore, understanding the recruitment dynamics of the Japanese eel can propose references for the utilization, preservation and management of the glass eels. The aim of this study is to investigate the spatial and temporal variation of the total length (TL) of the estuarine Japanese glass eels. This study uses a total of 19,059 Japanese glass eel specimens which had been collected from Taiwan, mainland China, North Korea and Japan for 28 years since 1983. After measuring TL and observing pigmentation stages, the results revealed that mean TL of the glass eels decreases gradually from the stage VA to stage VIA2, but from stage VIA3, there is an abruptly increase. Although both the monthly and annual mean TL varies considerably, the long term trend is not significant. The fluctuation of mean TL oscillation seems has some relationship with El Niño and La Niña events. In general, the TL of Japanese glass eel during El Niño period is above the regression line. Besides, the TL of Japanese glass eel during the La Niña period is below the regression line. In other words, the mean TL of Japanese glass eel in La Niña year was shorter than the other years; this may be due to the shorter larval duration. The spatial analysis of TL of Japanese glass eel reveals that the TL of most Japanese glass eels increases with latitude and this phenomenon may have some relationship with longer larval duration for northern glass eels. In conclusion, the resource of Japanese eel has been decreasing significantly for past 30 years and suffered from global warming. However, there is no significant impact on the TL trend of Japanese glass eels. The ENSO events and the transportation velocity will influence the larval duration, thus influence the size of the estuarine glass eels.

博士
國立臺灣師範大學
地球科學系
107
Satellite data and assimilation products are used to investigate fluctuations in the catch of Japanese glass eel (Anguilla japonica) in eastern Asian countries. It has been reported that the salinity front has extended farther south, which has shifted the eel’s spawning grounds to a lower latitude, resulting in lower glass eel catches in 1983, 1992, and 1998. Interannual variability in the glass eel catch is strongly correlated with the combination mode (C-mode), but not with the El Niño–Southern Oscillation or supper El Niño events. For example, the 2015/15 El Niño events is a super El Niño events, but it did not result in extremely poor recruitment of Japanese eel in East Asia. The spawning grounds accompanied by the salinity front extend farther south during the C-mode of climate variability, and eel larvae fail to join the nursery in the North Equatorial Current (NEC) and Kuroshio, resulting in poor recruitment in East Asia. We have proposed an appropriate sea surface temperature index to predict Japanese eel larval catch in the East Asia area.

碩士
國立臺灣大學
獸醫學研究所
107
Eel is one of the most economically important fish for aquaculture in East Asia. Owing to the shortage that the yield of artificial eel breeding cannot fulfill the requirement that commercial sale needs, we can only obtain glass eels by catching in the wild. The species of the glass eels that have ever been discovered in the coast of Taiwan are Anguilla japonica, A. marmorata, A. bicolor pacifica, A. luzonensis and A. celebesensis. Recently, the most popular species, A. japonica has been listed as an endangered species by the International Union on the Conservation of Nature due to a number of facts including overfishing and loss of habitat; however, since A. marmorata has been lifted from the conservation list of Taiwan in 2009, A. marmorata can be expected to be the substitute species for eel aquaculture in Taiwan. A. marmorata is a tropical species, compared with the A. japonica, the cultured conditions and the recruitments pattern are different. In recent years, there have been many studies on aquaculture techniques of A. marmorata. However, research about the A. marmorata recruitment pattern is quite limited. How to select the elvers with better growth potential have been issues that people engaging in aquaculture concern. According to fishermen saying, elvers recruited around the Tomb Sweeping Day, seems to have better growth performance. However, such a statement has not been confirmed by scientific methods. Besides, the dominant recruitment seasons of the glass eels of A. marmorata in Taiwan is not consistent as indicated by previous studies. Therefore, this study collected the A. marmorata glass eels in Yilan River of eastern Taiwan from 2011 to 2018 to investigate the detailed recruitment patterns. Besides, sampled the glass eels in June and December 2018 and cultured for twelve weeks respectively for comparing the growth performance of glass eels from different seasons. Results showed that the A. marmorata glass eels recruits to eastern Taiwan throughout the year and reach peaks roughly in May and September. The two recruitments may come from different geographic regions with different spawning time. We sample and culture the glass eels around these two peaks and found that the spring recruitments grow faster than those of autumn recruitments. The results could be used as an index to select elvers for aquaculture.

碩士
臺灣大學
漁業科學研究所
98
Glass eel of Anguilla japonica migrating in the open ocean will encounter with different environmental factors such as temperature, salinity, and starvation, which might affect the glass eel’s status such as somatic growth, otolith growth, and pigmentation stage. This study aims to determine the effect of temperature, salinity, and feeding on somatic growth, otolith growth, and pigmentation of the glass eel. Fish were immersed with alizarin complexone (ALC) then reared at different salinities (35, 26, 17 and 0 psu) under feeding or unfed conditions. To test the effect of temperature, glass eels were reared at 20, 15, 10 and 5 ℃ under unfed conditions. The results of this study indicated salinity appears no strong effect on pigmentation and somatic growth. Because the pigmentation stage at different salinities under feeding or unfed conditions was more advanced compared to the initial group but there was higher percentages in stages VIA4 in fresh waters than in seawaters. TL and W was significantly growth under feeding conditions compared to the initial values but no significant difference was observed in somatic growth among various salinities. And with food supply, there were higher percentages in later stages VIA3 to VIA4 than the unfed groups. The mean otolith growth increment per days of glass eels reared under unfed conditions was significantly lower (0.7757 ± 0.0941 μm/d) than under feeding conditions (1.0703 ± 0.2454 μm/d) of glass eel reared at different temperatures ranges from 149.83 ± .08 μm to 196.21 ±12.46 μm and the mean otolith radius was increased as the temperature elevating compared to the initial value (133.02 ± 9.46 μm) after 1 and 2 months. The pigmentation stage was more advanced into late stage with the temperature was increased, but glass eels reared at 5 ℃, the pigmentation stage developed slowly and remained in early stages VA to VB after 1 or 2 months. According the result of this study, otolith growth and pigment formation of the glass eels were strong affected by temperature and feeding, but these were not affected by salinity. In the wild, when glass eels encounter starvation conditions and low temperature, their somatic growth, otolith growth and pigment formation are significantly slowed down, resulting in delayed the upstream migration time and underestimated the daily age of the glass eels.

Dissertação de mestrado em Ecologia, apresentada ao Departamento de Ciências da Vida da Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Nos estuários, a pesca ilegal é uma das principais atividades que afeta uma vasta gama de espécies conduzindo ao seu declínio, provocando danos nas suas populações e levando à sobre-exploração dos juvenis das espécies-alvo e não-alvo (pesca acessória). Nos estuários do Nordeste Atlântico, os juvenis de enguia-europeia (Anguilla anguilla) são um alvo preferencial durante a fase de enguia de vidro. Em Portugal, e apesar de a sua pesca não ser permitida (com a exceção do Rio Minho), uma quantidade substancial de pesca ilegal ocorre durante a estação migratória. Este caso de estudo teve como objetivo estudar os impactos da pesca ilegal da enguia de vidro no estuário do Mondego (Portugal) em espécies alvo e não-alvo. Com este trabalho, os objetivos específicos, e tendo em conta dois cenários diferentes em termos hidrológicos (ano regular vs chuvoso), foram: a) determinar a variabilidade sazonal e inter-anual das capturas de enguias de vidro e das espécies não-alvo, e b) estimar a quantidade total de capturas considerando vários cenários hidrológicos e de pressão de pesca. A amostragem foi realizada entre novembro de 2011 e março 2013. As capturas de enguia de vidro foram semelhantes entre os dois anos de estudo. No entanto, ocorreram diferenças entre as duas campanhas de pesca, com um maior número de capturas acessórias em 2012-2013 (ano chuvoso), especialmente no outono. Os resultados também indicam uma maior diversidade de espécies no ano chuvoso, durante os meses de outono, bem como o maior número de indivíduos. Capturas mais elevadas das espécies mais abundantes foram também observados durante o ano chuvoso, quando comparado com o ano regular, o que sugere que a magnitude do impacto destas práticas ilegais pode estar relacionada com os ciclos hidrológicos. O trabalho efectuado demonstra que práticas ilegais e não-seletivas tais como esta têm um impacto significativo nas espécies capturadas, e em última análise no funcionamento do ecossistema estuarino.
In estuaries, illegal fishing is one of main activities that affects a wide range of species leading to the decline of total abundances, damage in fish stocks and overexploitation of juveniles in both the target and non-target species (by-catch). In North-eastern Atlantic estuaries, the European eel (Anguilla anguilla) juveniles are particularly targeted during the glass eel stage. In Portugal, and despite that fishing eel juveniles is not allowed (with the exception of the Minho River), a substantial amount of illegal fishing takes place during the migratory season. Our case study aimed at studying the impacts of glass eel illegal fishing in the Mondego estuary (Portugal) in both target and non-target species. With this work, the specific objectives, considering two different hydrological scenarios (regular vs rainy year), were; a) to determine the seasonal and interannual variability in glass eels and non-target species catches, and b) to estimate the total amount of catches considering several hydrological and fishing pressure scenarios. Sampling was performed in the Mondego estuary between November 2011 and March 2013 during the autumn and winter. Results showed differences between the two fishing seasons: a higher number of bycatch was observed in 2012-2013 (rainy year), particularly in autumn. For the glass eel, no differences between years were observed. Results also indicated higher species diversity in the rainy year, during the autumn months, as well as the highest number of individuals. Higher catches of the most abundant species were also observed in the rainy year, when compared to the regular one, suggesting that the magnitude of impact of these illegal practices is related with the hydrological cycles. The present work shows that Illegal and non-selective fishing practices such as this one have high impacts on the captures species, and ultimately on the functioning of the estuarine ecosystem.

碩士
國立臺灣大學
漁業科學研究所
102
Natural stocks of A. japonica’s glass eels have decreased dramatically due to overfishing, environmental destruction and change of ocean conditions in recent years. Therefore, in order to reduce the consumption of wild glass eels, developing techniques of artificial production for improving Japanese eel farming industry has become a major goal. As a result of no appropriate feed conversion method currently applied on the breeding process of eel larvae, the growth rate and survival rate are extremely low. In this study, three leptocephali, five preleptocephali, and ten embryos were collected near the Mariana ridge by cooperating with the Japanese team, and one glass eel was collected at the estuary of Yilan River. All the samples were submitted for RNA-seq by next-generation sequencing for reconstructing complete transcriptome of Japanese eel. Furthermore, the expressions of digestive enzymes and nutrient transporters existing in small intestine were investigated for realizing the digestive and absorptive capacities of Japanese eel larvae. A total of ~30 million raw reads (100 PE) were generated in our experiment and assembled into 224,043 transcripts after eliminating adaptors and low quality reads. Moreover, 116,146 transcripts were predicted to have open reading frames, and these putative protein-coding transcripts were submitted to blast against nr database. Subsequently, 70,096 transcripts were found positive hits, 90.2% of which were homologous genes of Actinopterygii. Besides, our analytic results showed that the digestive capacity of protein is terrific in preleptocephalus, but the digestive capacities of carbohydrate and lipid are very poor. In addition, a similar pattern is also found in leptocephalus, but the digestive capacities of carbohydrate and lipid are relatively better than in preleptocephalus. Furthermore, glass eel has average capacities for digesting protein, carbohydrate and lipid. In nutrient absorption, the absorptive capacity of amino acids is actually the best in preleptocephalus and leptocephalus, followed by the absorptive capacity of glucose which is better than cholesterol. Additionally, in glass eel, the absorptive capacity of amino acids is also superb, but the absorptive capacity of glucose is significant decreased than previous stages. Interestingly, the cholesterol transporters almost have no expression in glass eel. The digestive capacity of carbohydrate is poor in preleptocephalus and leptocephalus, but absorptive capacity is good. This may be related to gut microbiota in them, or they prefer to directly absorb monosaccharide or disaccharide in the food. Moreover, the digestive capacity of lipid is terrific in glass eel, but the absorptive capacity of cholesterol is poor, which may be associated with the well-functioned lipase and structural properties of plasma membrane because the small molecules of lipid end-products usually can pass through the plasma membrane. Based on above results, much useful information can be provided for developing artificial feed and feed conversion method. An online database for accessing the de novo assembled transcripts of four transcriptome data and annotated result of each transcript has been established. This database will be public for researching community of Japanese eel, and it can assist researchers in studying fundamental eel biology and artificial production.

碩士
國立臺灣大學
漁業科學研究所
106
Anguilla japonica is an important feeding aquaculture fish in Northeast Asian countries. Although the artificial reproduction of cultured A. japonica eel was succeeded in Japan and Korea, the production cost is too high for huge demand of eel. Therefore, all the fry need to capture from the wild. According to previous research, the spawning ground should be on the west side of the western Mariana Ridge due to glass eels never appear on the east side of western Mariana Ridge. The larvae of A. japonica were passively transported by the North Equatorial Current, the Kuroshio, the Taiwan warm Current, the Taiwan Strait Current, the Tsushima Warm Current, the Jeju Warm Current, and the Yellow Sea Warm Current to Korea, China, Taiwan and Japan. Previous studies have found that the main fishing seasons in each region are different, Taiwan is from November to February, Japan is from December to April, South Korea is from January to May and China is from December to May. The main reason for the different fishing seasons is the differences of estuary temperature and the distance of the transportation route. Due to complex environmental factors, the detailed drifting pathway of the larvae is not well understood. Previous studies show that the growth increments in otoliths of larvae were found to be deposited daily, thus the daily age of glass eels can be examined through the number of increment of otolith. This study combing information from (1) otolith increments, (2) East Asian coastal currents and (3) local main fishing seasons to analyze the drifting path of A. japonica glass eels on the East Asian coast. Samples were collected from Taiwan, China, Japan, and South Korea. The hypothesis of this study was based on the above data: (1) the time that larvae had drifted is proportional to the number of the increment in the otolith, the farther from the spawning ground, the longer drifting time and larval duration it would have. Thus, it is speculated that there were multiple diffuse paths for A. japonica in East Asia. Our results find that the glass eels captured from Taiwan have the smallest mean larval duration and glass eels captured from Yalu River have the biggest mean larval duration. The mean larval duration of glass eel from sample sites is on an upward trend along the transport distance. The later the recruitment of glass eel at the sampling site, the age of the glass eel will be underestimated easier. Five possible routes were found : (1) The main Kuroshio transport glass eel from Taiwan I-Lan to Japan Tanega-Shima Island, Mikawa Bay, Sagami River and Ichinomiya River; (2) Taiwan Strait Current transport glass eel from Taiwan Ping-Tung then to Taiwan Dan-Shui, and to Xiang-Shan by ESCC; (3) Taiwan Warm Current transport glass eel from I-Lan to China Fu-Ding, Min River, Wen-Zhou, Ning-Bo and Shang-Hai; (4) Yellow Sea Warm Current transport glass eel from I-Lan to Korea Je-Ju Island, Geum River and China Yalu River and (5) glass eels in northern Fu-Jian may transport by the ESCC from Fu-Ding to southern Fu-Jian and Xiang-Shan (Guang-Dong). The results of this study clarify the transport mechanism of A. japonica in East Asia which is meaningful for the distribution and management of A. japonica in East Asia.