Heat Tolerance as an Indicator of Climate Change Impacts: General Considerations and a Case Study in Poeciliid Fish

Heat tolerance of organisms is an important determinant of the potential impact of global warming and could thus be a very relevant bioindicator. If heat tolerance increases due to acclimation or adaptation, climate change impacts will be less than anticipated. In contrast, a decreased heat tolerance may indicate that a population is already stressed. As a case study, we compared heat tolerances among fish collected from pairs of sites with different thermal regimes, for the least killifish (Heterandria formosa) and the eastern mosquitofish (Gambusia holbrooki). We quantified heat tolerance as temperature-at-death and time-to-death when field-collected fish were exposed in the laboratory to increasing water temperatures. For the four pairs of populations that were compared, two differed in heat tolerance. Fish from a site with above-normal temperatures had a reduced heat tolerance indicative of being stressed. Fish from another site with above-normal temperatures had an elevated heat tolerance and this in...