Heritable differences in metabolic stability underpin thermal tolerance of threespine stickleback (Gasterostues aculeatus) ecotypes

Assessing intraspecific variation in thermal tolerance can provide insights on how rising global temperatures will differentially impact individual organisms and populations. The oxygen and capacity limited thermal tolerance hypothesis (OCLTTH) posits that aerobic scope (AS), the difference between standard and maximal metabolic rate, underpins other ecologically relevant metrics of performance. Here we assess short- and long-term thermal tolerance of benthic and limnetic ecotypes of threespine stickleback (Gasterosteus aculeatus) fish through common garden measurements of heritable variation in AS and parasite immunity. We predicted that benthic fish would display higher thermal tolerance resulting from adaptation to the relatively large thermal fluctuations in the shallow lakes that they inhabit. Based on the OCLTTH, we also expected that AS would decline with temperature and be a precursor for reduced condition. Although data connecting parasite immunity and temperature are limited, we predicted oxygen limitation would impair parasite resistance at higher temperature, particularly in the high-immunity limnetic fish. Despite exposing fish to temperatures below their thermal maxima, survival decreased with temperature and was most pronounced in limnetic fish at the highest temperature. The evolved thermal resilience of benthic stickleback was associated with their AS stability, suggesting that at least some populations of this keystone species are capable of withstanding hotter climates. However, contrary to the OCLTTH, temperature associated changes in AS were not associated with declines in fish condition or immunity.