Why do fish fail at high temperatures?

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There is increasing evidence that fish will be badly affected by increases in global temperatures due to their body temperatures matching that of the surrounding water. This is a particular problem for freshwater fishes since they have little scope for moving to colder water. The widely-quoted mechanistic explanation for the upper temperature limit for fish is that, as the temperature rises, there comes a point where the animal cannot supply sufficient oxygen to meet the increasing metabolic demands of its cells. However, recent tests of this hypothesis have found that it often does not correctly predict upper temperature limits, with fish failing at a temperature when their supply of oxygen is still adequate. This suggests that the currently-adopted hypothesis is inadequate.In this project we will use newly-developed methods to provide the first comprehensive test of an alternative hypothesis, and in so doing potentially provide the physiological mechanism that will underpin predictions of the consequences of warming temperatures for the distribution and functioning of fish species - and potentially other cold-blooded aquatic species. We will test whether the upper temperature limit for fish is determined by the point at which their mitochondria - the powerhouses of animal cells - fail to deliver enough of the energy molecule ATP to allow their cells to function properly. Using Brown trout as a test species we will investigate whether the changing functioning of their mitochondria allows us to predict the upper temperature at which they can still feed and grow. We will also test whether fish living at high temperatures age faster, as is predicted by this alternative hypothesis. Information from this project will allow us to predict what types of species of fish will be most vulnerable to high temperatures and which features of the temperature regime are most critical to survival (e.g. whether it is the daily average or the peak temperature). This information is needed if we are to prioritise species or sites for protection measures.