Abstract Locomotion consumes a large proportion of individual energy budgets and may impose energetic constraints on other fitness‐related traits particularly under variable environmental conditions. Here, we tested whether increased energetic cost of transport (CoT) leads to reduced movement to minimise cost, and impacts reproductive investment and success at different temperatures and water flow conditions. CoT integrated across a range of speeds within individuals (∫CoT) was greater in fish acclimated to 30°C compared with 24°C, but acclimation to water flow did not alter ∫CoT. When tested in familiar groups, voluntary speed increased with increasing ∫CoT at 30°C, but nearest neighbour distance within groups did not change with individual differences in ∫CoT. Reproductive investment was lowest in fish acclimated to 30°C and flowing water. Increasing ∫CoT was negatively correlated with reproductive investment when fish were acclimated in still water. Egg quality and offspring survival decreased with acclimation to 30°C and water flow, compared with 24°C and still water, respectively. We show that reproductive investment was lowest under energetically demanding environmental conditions, and that energetic cost of locomotion can constrain reproduction. These responses can explain how environmental variation such as altered river flows, global warming and even habitat fragmentation can reduce fitness and potentially impact populations. Read the free Plain Language Summary for this article on the Journal blog.
Jahn et al. (Wed,) studied this question.