Academic literature on the topic 'Mesopelagic ecosystems'

Within the twilight of the oceanic mesopelagic realm, 200–1000 m below sea level, are potentially vast resources of fish. Collectively, these mesopelagic fishes are the most abundant vertebrates on Earth, and this global fish community plays a vital role in the function of oceanic ecosystems. The biomass of these fishes has recently been estimated using acoustic survey methods, which rely on echosounder-generated signals being reflected from gas-filled swimbladders and detected by transducers on vessels. Here, we use X-ray computed tomography scans to demonstrate that several of the most abundant species of mesopelagic fish in the Southern Ocean lack gas-filled swimbladders. We also show using catch data from survey trawls that the fish community switches from fish possessing gas-filled swimbladders to those lacking swimbladders as latitude increases towards the Antarctic continent. Thus, the acoustic surveys that repeatedly show a decrease in mesopelagic fish biomass towards polar environments systematically overlook a large proportion of fish species that dominate polar seas. Importantly, this includes lanternfish species that are key prey items for top predators in the region, including king penguins and elephant seals. This latitudinal community switch, from gas to non-gas dominance, has considerable implications for acoustic biomass estimation, ecosystem modelling and long-term monitoring of species at risk from climate change and potential exploitation.

AbstractIn the Hawaiian Islands, there is a distinct resident community of micronekton, distributed along a narrow band where the upper underwater slopes of the islands meet the oceanic, mesopelagic environment. This mesopelagic boundary community serves as an important food resource to many animals. The goal of this work was to examine spatial heterogeneity of the Hawaiian mesopelagic boundary biomass at a range of scales, in the context of its diel vertical and horizontal migrations. A modified echosounder was used to sample the coasts of three Hawaiian Islands, permitting a range of scales from several meters to several kilometers to be assessed rapidly. The Hawaiian mesopelagic boundary community fits the hierarchical model of patch structure with patches within patches that are part of a larger-scale matrix of patches. Large differences in the overall distribution patterns of the mesopelagic boundary community exist along with a wide range of overall mesopelagic-animal densities. High animal-density locations have boundary-community layers with a large (kilometers) horizontal extent, and low animal-density locations have small (tens of meters), discrete patches. Higher animal-density locations are also more complex than low-density sites, with more levels of patchiness within the same range of spatial scales. Both time of day and distance from shore significantly affected the geometric and density characteristics as well as the distribution of aggregations within the boundary layer. Horizontal and vertical structures of the mesopelagic boundary community are also coupled. In high-density sites, there is strong vertical layering in acoustic-scattering strength while in low-density sites vertical acoustic structure is absent. The differences observed in the distribution of the mesopelagic boundary community at different levels of overall mesopelagic-animal density suggest biological forcing as the dominant mechanism. A description of heterogeneity in the mesopelagic boundary community in Hawaii is the first step in understanding its importance to both neritic and oceanic ecosystems and its potential for linking these two systems.

Abstract Acoustic survey methods are useful to estimate the distribution, abundance, and biomass of mesopelagic fish, a key component of open ocean ecosystems. However, mesopelagic fish pose several challenges for acoustic biomass estimation based on their small size, wide depth range, mixed aggregations, and length-dependent acoustic reflectance, which differentiate them from the larger epipelagic and neritic fish for which these methods were developed. Foremost, there is a strong effect of depth on swimbladder resonance, so acoustic surveys of mesopelagic fish must incorporate depth-stratification. Additionally, the 1–3 cm juveniles of many species are not only more abundant, but can also be stronger acoustic backscatterers than the larger adults that comprise most of the biomass. The dominant species in terms of biomass may thus be weak acoustic backscatters. Failure to properly incorporate depth, the full size distribution, and certain less-abundant species into mesopelagic acoustic analyses could lead to errors in estimated biomass of up to three orders of magnitude. Thus, thorough validation, or “ground-truthing ”, of the species composition, depth structure, population size distribution, capture efficiency of the sampling device, and acoustic properties of the fish present is critical for credible acoustic estimates of mesopelagic fish biomass. This is not insurmountable, but requires more ancillary data than is usually collected.

Abstract Mesopelagic fish are an important component of marine ecosystems, and their contribution to marine biogeochemical cycles is becoming increasingly recognized. However, major uncertainties remain in the rates at which they remineralize organic matter. We present respiration rate estimates of mesopelagic fish from two oceanographically contrasting regions: the Scotia Sea and the Benguela Current. Respiration rates were estimated by measuring the enzyme activities of the electron transport system. Regression analysis of respiration with wet mass highlights regional and inter-specific differences. The mean respiration rates of all mesopelagic fish sampled were 593.6 and 354.9 µl O2 individual−1 h−1 in the Scotia Sea and Benguela Current, respectively. Global allometric models performed poorly in colder regions compared with our observations, underestimating respiratory flux in the Scotia Sea by 67–88%. This may reflect that most data used to fit such models are derived from temperate and subtropical regions. We recommend caution when applying globally derived allometric models to regional data, particularly in cold (<5°C) temperature environments where empirical data are limited. More mesopelagic fish respiration rate measurements are required, particularly in polar regions, to increase the accuracy with which we can assess their importance in marine biogeochemical cycles.

The importance of spatial pattern in ecosystems has long been recognized. However, incorporating patchiness into our understanding of forces regulating ecosystems has proved challenging. We used a combination of continuously sampling moored sensors, complemented by shipboard sampling, to measure the temporal variation, abundance and vertical distribution of four trophic levels in Hawaii's near shore pelagic ecosystem. Using an analysis approach from trophic dynamics, we found that the frequency and intensity of spatial aggregations—rather than total biomass—in each step of a food chain involving phytoplankton, copepods, mesopelagic micronekton and spinner dolphins ( Stenella longirostris ) were the most significant predictors of variation in adjacent trophic levels. Patches of organisms had impacts disproportionate to the biomass of organisms within them. Our results are in accordance with resource limitation—mediated by patch dynamics—regulating structure at each trophic step in this ecosystem, as well as the foraging behaviour of the top predator. Because of their high degree of heterogeneity, ecosystem-level effects of patchiness such as this may be common in many pelagic marine systems.

The water and calorific content of fifteen species of mesopelagic sub-Antarctic fish from Macquarie Island were determined. Mean percent water content was 69–82%. Calorific content was highly variable between species, especially in the Myctophidae, where it ranged between 22.6–59.3 kJ·g−1 dry weight. The water and calorific content varied with size class within a species, with the smallest size classes generally having the lowest water content but highest calorific content. These values will be useful for future assessment of energetic transfer between trophic levels and energetic modelling of Southern Ocean ecosystems.

Gulf of Mexico (GOM) ecosystems are interconnected by numerous physical and biological processes. After the Deepwater Horizon (DWH) disaster, these ecological processes facilitated dispersal of oil-spill toxicants or were damaged and broken. A considerable portion of post-DWH research focused on higher levels of biological organization (i.e., populations, communities, and ecosystems) spanning at least four environments (onshore, coastal, open ocean, and deep benthos). Damage wrought by the oil spill and mitigation efforts varied considerably across ecosystems. Whereas all systems show prolonged impacts because of cascading effects that impacted functional connections within and between communities, deep-sea and mesopelagic environments were particularly hard hit and have shown less resilience than shallow environments. In some environments, such as marshes or the deep-sea benthos, products from the spill are still biologically accessible. Some shallow ecosystems show signs of recovery, and populations of some species show resilience; however, a return to a “pre-spill” state is questionable. Importantly, habitats in which large amounts of energy flow through the ecosystem (marshes, coastal regions) recovered more quickly than low energy habitats (deep-sea benthos). Functional interactions between Gulf of Mexico systems are more complex and widespread than generally recognized. Moreover, ecosystems in the Gulf are subject to multiple stressors that can combine to impart greater, and less predictable, impacts. To help mitigate the effects of future insults, we identified four salient areas of research that should be addressed for each of the major environments within the GOM: establishing monitoring systems; quantifying coupling between GOM ecosystems; developing criteria for assessing the “vulnerability” and “resilience” of species, communities, and ecosystems; and developing holistic predictive modeling.

Cephalopods are important in Arctic marine ecosystems as predators and prey, but knowledge of their life cycles is poor. Consequently, they are under-represented in the Arctic ecosystems assessment models. One important parameter is the change in ecological role (habitat and diet) associated with individual ontogenies. Here, the life history of Gonatus fabricii, the most abundant Arctic cephalopod, is reconstructed by the analysis of individual ontogenetic trajectories of stable isotopes (δ13C and δ15N) in archival hard body structures. This approach allows the prediction of the exact mantle length (ML) and mass when the species changes its ecological role. Our results show that the life history of G. fabricii is divided into four stages, each having a distinct ecology: (1) epipelagic squid (ML < 20 mm), preying mostly on copepods; (2) epi- and occasionally mesopelagic squid (ML 20–50 mm), preying on larger crustaceans, fish, and cephalopods; (3) meso- and bathypelagic squid (ML > 50 mm), preying mainly on fish and cephalopods; and (4) non-feeding bathypelagic gelatinous females (ML > 200 mm). Existing Arctic ecosystem models do not reflect the different ecological roles of G. fabricii correctly, and the novel data provided here are a necessary baseline for Arctic ecosystem modelling and forecasting.

Eastern boundary upwelling provides the conditions for high marine productivity in the Canary Current System off NW-Africa. Despite its considerable importance to fisheries, knowledge on this marine ecosystem is only limited. Here, parasites were used as indicators to gain insight into the host ecology and food web of two pelagic fish species, the commercially important species Trichiurus lepturus Linnaeus, 1758, and Nealotus tripes Johnson, 1865. Fish specimens of T. lepturus (n = 104) and N. tripes (n = 91), sampled from the Canary Current System off the Senegalese coast and Cape Verde Islands, were examined, collecting data on their biometrics, diet and parasitisation. In this study, the first parasitological data on N. tripes are presented. T. lepturus mainly preyed on small pelagic Crustacea and the diet of N. tripes was dominated by small mesopelagic Teleostei. Both host species were infested by mostly generalist parasites. The parasite fauna of T. lepturus consisted of at least nine different species belonging to six taxonomic groups, with a less diverse fauna of ectoparasites and cestodes in comparison to studies in other coastal ecosystems (Brazil Current and Kuriosho Current). The zoonotic nematode Anisakis pegreffii occurred in 23% of the samples and could pose a risk regarding food safety. The parasite fauna of N. tripes was composed of at least thirteen species from seven different taxonomic groups. Its most common parasites were digenean ovigerous metacercariae, larval cestodes and a monogenean species (Diclidophoridae). The observed patterns of parasitisation in both host species indicate their trophic relationships and are typical for mesopredators from the subtropical epi- and mesopelagic. The parasite fauna, containing few dominant species with a high abundance, represents the typical species composition of an eastern boundary upwelling ecosystem.

The conservation of marine ecosystem structure and functioning is a priority target within the context of the environmental management. This is especially important in oligotrophic systems such as the Balearic Sea, where food availability is a limiting factor. For such a target is fundamental to know the array of predator-prey interactions as a basis to understand the food web trophodynamics. In this Thesis, the trophic structure of the pelagic and benthic habitats off the Balearic Islands and the feeding ecology of three taxonomic groups (cephalopods, elasmobranchs and mesopelagic fishes) playing a key role in marine ecosystems, were investigated A relatively large food web (89 species) encompassing both the pelagic and the benthic habitats were analysed. The food webs extended up to 4 trophic levels in both habitats, and most species occupied intermediate trophic levels. A high partitioning rate of trophic resources was found, which might be related to the reduction of competition for food. The energy link between the pelagic and benthic communities (benthopelagic coupling; BPC) was stronger on the shelf break, where higher hydrodinamism mix the water column and associated nutrients, than on the slope. The geographical variability of oceanographic drivers (i.e. eddies, currents) influenced the strength of the BPC. Regarding species trophic interactions, stomach contents provided evidence of mesopelagic prey coupling pelagic and benthic habitats, especially down the shelf break. Rays from the shelf consumed mainly decapod crustaceans and teleosts, whereas sharks from the slope fed primarily upon mesopelagic prey (i.e myctophids, euphausiids). Sharks and rays variation in diet was driven by species specific ontogenetic stages and habitat use. The diet of the two squid Loligo vulgaris and L. forbesii, examined for the first time in the Mediterranean, showed their highly piscivorous habits. Their prey composition revealed a lack of diet overlap as a result of their bathymetric segregation. Both squids showed shifts in diet related to size and their reproduction period. The squid L. vulgaris feed on small benthic prey until they reach a size threshold and feed on benthopelagic fish thereafter. Seasonal changes in diet in both squid species, likely related to reproduction, might help improving their individual body condition. During this period, L. vulgaris increased the consumption of highly nutritive polychaetes, whereas the adults of L. forbesii carried out movements to deeper waters to feed on lipid-rich mesopelagic prey. Mesopelagic prey are important food resources for demersal species in the Balearic Sea and, as a result of their nycthemeral movements, they have important implications for the transport of mass and energy through the water column. The trophodynamics of this group revealed that food sources (δ13C) varied little over the spatial scales sampled (location and depth) but showed high seasonality, reflecting intra-annual changes in the species composition of the phytoplankton community. By contrast, spatio-temporal variations of mesopelagic trophic interactions (δ15N) were minimal. Important niche segregation was observed between the non-migratory stomiiforms and some of the extensive migratory myctophids. Finally, the comparative analysis of the trophic ecology of deep-sea cephalopods and elasmobranchs showed that they displayed different feeding strategies with a clear resource partitioning between and within taxa. Segregation of the isotopic space indicated a contrasting food source gradient (δ13C) stretching from pelagic (squids and cuttlefishes) to nektobenthic (octopuses and elasmobranchs) sources. However, deep-sea sharks off the Balearic Islands frequently preyed on mesopelagic species, as did squids and cuttlefishes. Cephalopods, in turn, constituted an important food resource for deep-sea sharks. Squid and shark species identified as benthopelagic feeders, play a key role in the transport of energy from midwater regions to the benthic community.

Mesopelagic (open ocean, 200-1000 m depth) fishes are important consumers of zooplankton and are prey of oceanic predators. Some mesopelagic fishes (e.g., myctophids and stomiids) undertake a diel vertical migration where they ascend to the near-surface waters during the night to feed and descend into the depths during the day to avoid predators. Other mesopelagic fishes (e.g., Sternoptyx spp.) do not vertically migrate and remain at deep depths throughout the day. While in the epipelagic zone (surface – 200 m depth), vertically migrating fishes become prey to upper-trophic level predators, such as: tunas and billfishes. Benthic fishes (e.g., macrourids) often vertically migrate as well, ascending into the pelagic zone to feed on pelagic organisms. Fishes of different depths and vertical migration habits likely have a different ecological role in food webs. The relationship between parasites and gut contents provides insights into ecological processes occurring within assemblages, as prey items are often vectors for parasites. This study examined the differences between the prey items present in the gastrointestinal cavity and parasites of 26 mesopelagic fish species in the Gulf of Mexico. Results showed that based on the proportionally dominant prey items per species, six different feeding guilds existed within this assemblage, five based on planktivory: copepodivores, predators of copepods and other zooplankton, predators of copepods and euphausiids, gelatinivores, generalists, crustacean decapodivores, and upper-trophic level predators. Larger fishes preyed on larger prey items and harbored more parasites. Sigmops elongatus exhibited an ontogenetic diet shift at 75 mm standard length, progressing from eating primarily copepods at small sizes to eating primarily euphausiids at large sizes. Compared to similar studies, this study revealed a higher parasitic infestation by trematodes, an endoparasite (parasite within the host) class often restricted to nearshore hosts, in Gulf of Mexico fishes. Helicometrina nimia, the dominant parasite of the gempylid Nealotus tripes, has not previously been recorded in hosts below 200 m depth, suggesting a foodweb pathway that transitions from nearshore to offshore. These data can be used to develop and refine models aimed at understanding ecosystem structure and connectivity.

L'océan Indien Sud abrite une riche diversité de milieux océaniques, de communautés pélagiques et de populations de prédateurs qui ont récemment abouti à des zones de conservation et un classement au patrimoine mondial de l'UNESCO. Cependant, des populations de prédateurs de la région ont montré des signes de déclin, probablement liés aux effets du changement climatique et à la dynamique de leurs proies. Ces dernières sont principalement composées de zooplancton et de micronecton, qui occupent une place centrale dans les écosystèmes marins en influençant les cycles biogéochimiques et le transfert d'énergie et de biomasse le long du réseau trophique. Cependant, l'étude de leur structuration est complexe en raison de leur distribution en profondeur dans la zone mésopélagique (200-1000 m) et de leurs migrations verticales quotidiennes, en faisant l’une des zones les moins connues à l'échelle globale. Dans cette thèse, nous utilisons l'acoustique active, qui permet de suivre ces organismes avec une haute résolution spatiale et verticale, pour étudier la structuration des écosystèmes mésopélagiques dans deux systèmes océaniques contrastés. Nous avons d'abord examiné leur réponse dans une zone de transition entre ces océans, dans la région des îles Saint-Paul et Amsterdam. Nos résultats indiquent que les migrations nycthémérales sont le principal facteur structurant verticalement les organismes pélagiques, même dans une zone de transition marquée. Trois régions ont été identifiées, cohérentes spatialement et partageant des caractéristiques environnementales et acoustiques communes. L'observation à 38 kHz (couramment utilisée en hydroacoustique) a confirmé une activité biologique accrue dans la zone subtropicale et un minimum dans la zone subantarctique. Cependant, nos analyses révèlent également une réponse acoustique distincte en fonction de la fréquence acoustique considérée, en termes de structuration verticale et de biomasse intégrée. De plus, les faibles niveaux mesurés dans l'océan Austral semblent ne pas correspondre aux fortes biomasses consommées par les importantes populations de prédateurs marins de cette région. Ces observations mettent en évidence un problème de résonance acoustique pour l'étude de ces organismes. Pour étudier ce phénomène, notre seconde étude s'est focalisée sur la structuration des communautés mésopélagiques à l'échelle régionale, proposant une classification des paysages acoustiques à deux fréquences permettant de sonder jusqu'à 1000 m de profondeur. Quatre régions spatialement cohérentes ont été identifiées en se basant sur la combinaison de la structuration verticale aux deux fréquences considérées. Les résultats de cette étude révèlent une inversion de la dominance de la réponse acoustique entre le 38 et le 18 kHz au niveau du front subantarctique, confortant l'hypothèse d'un changement de communauté à ces latitudes, et questionnent l'utilisation du 38 kHz comme descripteur des communautés mésopélagiques. Les données acoustiques étant complexes (potentiellement multivariées, en quatre dimensions, et aux portées verticales distinctes), les travaux présentés dans cette thèse ont également contribué au développement de méthodes d'analyse fonctionnelle permettant de concilier la variabilité horizontale, verticale et temporelle de ces écosystèmes. La gestion des écosystèmes marins nécessitant une compréhension des dynamiques de ces systèmes et de la structuration des communautés qui les composent, nos travaux ont également eu des échos en conservation marine; au niveau national, en contribuant au dossier d'extension de la réserve naturelle des îles Saint-Paul et Amsterdam, puis international en contribuant à l'écorégionalisation de la zone pélagique de la région subtropicale et subantarctique de l'océan Indien Sud
The Southern Indian Ocean hosts diverse oceanic environments, pelagic communities, and predator populations that have recently led to the establishment of conservation areas and UNESCO World Heritage recognition. However, predator populations in this region have displayed signs of decline, likely attributed to the impacts of climate change and prey dynamics. These prey populations consist predominantly of zooplankton and micronekton, playing a crucial role in marine ecosystems by influencing biogeochemical cycles and the transfer of energy and biomass along the trophic web. Nevertheless, the study of their structuring is intricate due to their deep distribution in the mesopelagic zone (200-1000 m) and their daily vertical migrations, making it one of the least explored areas on a global scale. In this thesis, we employ active acoustics, which provides high spatial and vertical resolution for tracking these organisms, to investigate the structuring of mesopelagic ecosystems in two contrasting oceanic systems. We initially examined their responses in a transition zone between these oceans, within the Saint-Paul and Amsterdam Islands region. Our findings indicate that nycthemeral migrations are the primary factor vertically structuring pelagic organisms, even within a pronounced transition zone. Three spatially coherent regions were identified, sharing common environmental and acoustic features. Observations at 38 kHz (commonly used in hydroacoustics) confirmed increased biological activity in the subtropical zone and a minimum in the subantarctic zone. However, our analyses also reveal a distinct acoustic response based on the acoustic frequency considered, in terms of vertical structuring and integrated biomass. Additionally, the low levels measured in the Southern Ocean appear inconsistent with the high biomasses consumed by the large marine predator populations in this region. These observations highlight an acoustic resonance issue in the study of these organisms. To investigate this phenomenon, our second study focused on the structuring of communities at a regional scale, proposing a two-frequency acoustic landscape classification, both probing depths of up to 1000 m. Four spatially coherent regions were identified based on the combination of vertical structuring at the two considered frequencies. The results of this study reveal a shift in the dominance of acoustic responses between 38 and 18 kHz at the subantarctic front, supporting the hypothesis of a community change at these latitudes and raising questions regarding the use of 38 kHz as a descriptor for mesopelagic communities. Given the complexity of acoustic data (potentially multivariate, with distinct vertical ranges, and four-dimensional), the work presented in this thesis also contributed to the development of functional analysis methods to reconcile the horizontal, vertical, and temporal variability of these ecosystems. As marine ecosystem management requires an understanding of the dynamics of these systems and the structuring of the communities within them, our work has also had implications in marine conservation. At the national level, it contributed to the extension of the Saint-Paul and Amsterdam Islands Nature Reserve, and at the international level, it contributed to the ecoregionalization of the pelagic zone in the subtropical and subantarctic region of the South Indian Ocean

L'écosystème océanique Néo-Calédonien dans le Pacifique sud-ouest contient une forte diversité de prédateurs, comme les oiseaux marins ou les cétacés. La récente création du Parc naturel de la mer de corail a ouvert un besoin d'informations solides sur le fonctionnement de cet écosystème remarquable, notamment sur la dynamique du micronecton (organismes entre 1 et 20cm constituant la nourriture des prédateurs supérieurs) et de son rôle central dans les réseaux alimentaires, d'autant plus que ce maillon est le plus méconnu. Dans ce contexte, cette thèse a montré une forte influence de l’environnement sur sa distribution spatiale et saisonnière avec en moyenne plus de micronecton dans le sud que dans le nord, et plus en hiver qu’en été. Les espèces composant le micronecton sont plus riches en crustacés au nord en céphalopodes et poissons au sud. L’abondance de micronecton aurait tendance à diminuer sous l’influence du changement climatique. Enfin, nous avons montré que la répartition spatiale de ce groupe influençait la présence de six groupes de prédateurs : thon jaune, thon germon, dorade coryphène, dauphins, puffin et fou à pieds rouges
The New Caledonian pelagic ecosystem in the southwest Pacific contains a high diversity of predators, such as seabirds and cetaceans. The recent creation of the Natural Park of the Coral Sea created a need for solid information on the functioning of this remarkable ecosystem, particularly on the dynamics of the micronecton (organisms between 1 and 20 cm constituting food of top predators) and its central role in food webs, especially since is the most poorly understood. In this context, this PhD showed a strong influence of environment on its spatial and seasonal distribution with on average more micronekton in the south than in the north, and more in winter than in summer. The species composing the micronecton are riche in crustaceans in the north and in cephalopods and fish in the south. The micronecton abundance would tend to decrease under the influence of climate change. Finally, we showed that the spatial distribution of this group influenced the presence of six groups of predators: yellowfin tuna, albacore tuna, dolphin, dolphin, shearwater and red-footed body

Mesopelagic fishes and squids play critical roles in mediating the transfer of energy through Southern Ocean pelagic food webs. However, the energy pathways comprised of mesopelagic fishes and squids remain understudied compared to those mediated by Antarctic krill Euphausia superba. In part, this has been due to the difficulty of sampling and observing taxa in the remote and harsh environment of Southern Ocean pelagic ecosystems. This has limited our understanding of mesopelagic fish and squid functional roles and baselines of community structure and function. Increasingly, evidence suggests that environmental change is likely to have effects on mesopelagic communities that may result in shifting food web dynamics. It is therefore essential to establish baseline understanding of the structure and function of mesopelagic groups to predict and assess the magnitude of future change. This thesis examined the poorly studied trophic pathways mediated by mesopelagic fishes and squids across regional and circumpolar spatial scales. Specifically, biochemical tracer techniques were used to obtain new empirical knowledge of fish and squid functional roles and key food web linkages in the Southern Ocean. Additionally, I integrated survey data of mesopelagic fishes from seven national Antarctic research programs into a comprehensive circumpolar dataset. An ensemble modelling approach was then used to investigate key determinants of mesopelagic fish abundance and distribution, highlighting important geographic areas of mesopelagic fish occurrence. Chapter 2 investigated the trophic role of mesopelagic fishes in the region of the southern Kerguelen Plateau in the Indian sector of the Southern Ocean and the role of body size on the trophic structure of the community. I used bulk carbon and nitrogen stable isotopes to quantify species’ isotopic niches and to examine the relationship between body size and trophic position. I found high overlap in the trophic niches among species although evidence indicated latitudinal variation in the trophic position estimates of taxa, broadly partitioned by the southern Antarctic Circumpolar Current Front; and a size-based trophic structure between species. Chapter 3 provided the first cross-basin comparison of the trophodynamics of pelagic squids across two major oceanic sectors in the Southern Ocean (West Pacific and Indian sectors) using archived squid beaks collected from predator stomachs. I combined bulk nitrogen stable isotopes and compound specific stable isotopes to investigate the trophic role of species within each community and to assess body size relationships at the species, community, and ocean basin levels. This revealed similar trophic structuring between both locations characterised by almost three trophic levels, from mid-trophic levels to higher predators. Further, body size was not a strong determinant of trophic structure in either community suggesting that feeding mode and/or prey availability may be stronger determinants of trophic position. Chapter 4 presents Myctobase, a circumpolar database of mesopelagic fishes from survey data. Here, I carried out the important step of integrating and standardizing published and unpublished survey data on the abundance, biomass, biodiversity, and methodological metadata for mesopelagic fishes of the Southern Ocean. Myctobase will enhance research capacity by providing the broadscale spatiotemporal perspective and baseline data necessary for international effort toward observing and modelling mesopelagic fish. Chapter 5 showcases the utility of Myctobase by presenting important information on the key determinants of mesopelagic fish abundance. I used data from Myctobase to develop speciesspecific models of the abundance of eight key myctophid species and the genus Bathylagus. I used a set of environmental variables, previously correlated to myctophid occurrence records, to develop boosted regression tree models for each species. I then used these models to obtain a circumpolar prediction of abundance for each species. I found that abundance is predicted by meso- and sub-mesoscale oceanographic features, in particular the Polar Front was a major delimiting feature for the distribution of species. Furthermore, the interactions between depth and solar position were key predictors of abundance indicating diel vertical migratory behaviours in some species. Overall, this thesis presents novel insights into the functional roles of mesopelagic fish and squid taxa and key determinants of mesopelagic fish abundance and distribution. Importantly, this thesis takes fundamental steps towards the development of baselines of community structure and function and the broadscale spatiotemporal perspective necessary for the holistic management and conservation of open-ocean pelagic ecosystems in the Southern Ocean.

In recent decades, the prevalence of plastics in the marine environment has increased and is amongst the most pervasive problems affecting the marine environment globally. Numerous studies have documented microplastic ingestion by marine species with more recent investigations focusing on the secondary impacts of microplastic ingestion on ecosystem processes. However, few studies so far have examined microplastic ingestion by mesopelagic fish which are one of the most abundant pelagic groups in the oceans and their vertical migrations are known to contribute significantly to the rapid transport of carbon and nutrients to the deep sea. Therefore, any ingestion of microplastics by mesopelagic fish may adversely affect this cycling and may aid in the transport of microplastics from surface waters to the deep-sea benthos. Microplastics are ubiquitous in the marine environment and are increasingly contaminating species in the marine ecosystem and the food chain, including food stuffs intended for human consumption. The effects of microplastics on aquatic organisms are currently the subject of intense research. Here, we provide a critical perspective on published studies of microplastic ingestion by aquatic biota. We summarize the available research on Microplastic presence, behavior, and effects on aquatic organisms monitored in the field and laboratory studies of the ecotoxicological consequences of microplastic ingestion. Finally, researchers plan further studies to learn more about how these fish are ingesting and spreading microplastics. It will be particularly interesting to see whether the fish ingest these microplastics directly as mistaken prey items, or whether they ingest them through eating prey species, which have previously ingested the microplastics. Also, there is a need to understand the mechanism of action and ecotoxicological effects of environmentally relevant concentrations of microplastics on aquatic organism health.