Sensitivity of Antarctic fish to ocean warming - an energy budget approach

Like the Arctic, the Antarctic region hosts some of the hot spots of climatic change. At the western Antarctic Peninsula, alterations of air and water temperature, pH, salinity and sea-ice regime were reported and associated shifts in species abundance and changes in food web structure have already become evident. In contrast, for most high-Antarctic regions, no climate related changes have yet been found. However, future temperature increases are also projected for these areas. Ocean warming affects marine ectotherms by directly impacting their body temperature and thus physiology. Antarctic marine ectotherms, such as fish, are highly adjusted to the very cold and stable conditions of the Southern Ocean and are suggested to be highly temperature sensitive. Fish constitute an important link in Antarctic food webs by being prey and predator alike. While various studies focused on the impact of elevated temperature on lower organisational levels in Antarctic fish, trade-offs of increased temperature for the whole organism remain unclear, but are highly relevant from an ecological perspective. Thus, this thesis aimed to assess the impact of increasing temperature on Antarctic fish at the whole-organism level from an energy budget perspective. The energy taken up by an organism can be allocated to different vital functions, such as routine metabolism, growth, reproduction and excretion. When routine metabolic costs are covered, energy can be allocated to growth and reproduction, the factors influencing a species abundance and population structure. In the first study of this thesis, energy allocation to routine metabolism as well as response patterns to an acute increase of temperature in the fish species Lepidonotothen squamifrons, Trematomus hansoni and Lepidonotothen nudifrons were analysed using oxygen consumption measurements. While metabolic responses to changing temperature were comparable in all species, metabolic costs of high-Antarctic fish were higher at habitat temperatures. Starting from higher metabolic rates at habitat temperature, it was hypothesised that high-Antarctic species might achieve critical thermal thresholds much earlier than low-latitude species when temperature increases. In the second study, temperature-dependent trade-offs at the whole-organism-level in Antarctic fish were analysed measuring different energy budget parameters. The results indicated a lower thermal tolerance of the high-Antarctic Trematomus bernacchii compared to the low-Antarctic Lepidonotothen nudifrons. After nine weeks of acclimation to elevated temperatures (4 degree Celsius), routine metabolic rates of T. bernacchii returned to baseline levels (0 degree Celsius). However, mass growth was reduced by 84% at 2 degree Celsius, likely due to less efficient food assimilation. In nature, such severe reductions in fish growth could delay sexual maturity and reduce production. In the third study, temperature-dependent growth rates of fish species from different latitudes were assessed. Polar and especially Antarctic species showed low growth and a narrow thermal tolerance window for growth performance compared to temperate species. A further climate induced reduction of already low growth rates could significantly affect population structures and abundances of polar fish. In conclusion, this thesis indicates differences in energy allocation, such as potentially higher routine metabolism, among low- and high-Antarctic fish. These could contribute to a high thermal sensitivity of high-Antarctic species. On the whole-organism level, this thermal sensitivity was displayed by significant reductions of already low growth rates at elevated temperatures. Finally, these results suggest that ocean warming may have far-reaching consequences for Antarctic fish production and population structures with potential extensive implications for entire Antarctic ecosystems and food webs.