| Summary: | 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 ...
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