Understanding the energy pathways through Southern Ocean mesopelagic communities

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