Alternative energetic strategies for a cold-water fish: Gains in the mainstem versus efficiency in floodplain refuges

Alternative responses to environmental change can buffer populations from climate variation. Variation in behavioural responses to thermal stress may mediate population-level responses to warming, but relatively little is known about the trade-offs associated with alternative strategies. In freshwater ecosystems cold-water fish exhibit intra-population variation in refuge seeking behaviour during periods of thermal stress. Moving to refuge may present a conflict between temperature and food, since consumption rates should decline with increasing fish densities (and potentially lower food production) in refuges. In comparison, fish that do not seek refuge incur higher metabolic demands but may also have higher foraging rates. While this trade-off is fundamentally important for cold-water fish in a warming world, it is difficult to quantify, and the relative fitness of alternative strategies remains largely unknown. We integrated field measurements of adult coastal cutthroat trout diet, fish growth and water temperature within a bioenergetics framework to quantify the energetic trade-offs associated with alternative responses to summer maximum temperatures. While the energetic costs and benefits of each strategy were markedly different, both yielded comparable growth (i.e. net energy balance), suggesting that both strategies represent evolutionarily stable strategies at present. However, warming temperatures are expected to penalize fish remaining in non-refuge habitat, making this strategy less energetically profitable in the future. Our findings underscore the importance of considering individual variation in response to climate change and floodplain heterogeneity in climate adaptation planning.