Physiological mechanisms underlying fish distributions along elevational gradients: estimating fundamental thermal niches

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the Department of Ecology, Environment and Evolution, School of Life Sciences, College of Science, Health and Engineering, La Trobe University, Bundoora.

There is much concern about how climate change will affect the thermal regimes of freshwater ecosystems and, by extension, the performance of ectothermic animals within those systems. The aim of this PhD thesis was to improve our understanding of the thermal niches of two congeneric species of fish distributed along elevational gradients: the river blackfish (Gadopsis marmoratus), and two-spined blackfish (G. bispinosus). By studying the thermal niches of these two species, this thesis aimed to 1) determine whether the distributions of the blackfish are constrained by their thermal physiology, and 2) determine how changes in thermal regimes predicted by climate change will impact these two species. The distribution of the two species, and associated temperature regimes suggest they have different realized thermal niches, with G. bispinosus occupying cooler habitats than G. marmoratus. Despite this, there were only small differences in their thermal niche when defined by aerobic swimming performance, aerobic scope, growth rate and thermal tolerance. Predictions of how increasing temperatures due to climate change may impact these two species were made based on temperature-dependent growth models and upper thermal limits (UTL). The impact of increasing temperatures on growth rates were highly seasonally specific, with strong decreases predicted for summer. These reductions in growth will occur during the recruitment season, which may result in blackfish distribution contracting to higher elevations. Additionally, climate change is likely to cause maximum temperatures to frequently exceed blackfish UTLs across large parts of their distribution. This thesis demonstrates that modelling the thermal niches of ectotherms distributed along the river continuum is not straightforward. If we are to effectively and efficiently forecast impacts of different thermal futures, riverine ecologists must do more to experimentally decipher the relative influence of temperature and other abiotic drivers on the fitness of riverine ectotherms.

This thesis was a recipient of the Nancy Millis Award for theses of exceptional merit.