Constraints of digestion on swimming performance and stress tolerance vary with habitat in freshwater fish species.

Limited aerobic scope (AS) during digestion might be the main constraint on the performance of bodily functions in water-breathing animals. Thus, investigating the postprandial changes in various physiological functions and determining the existence of a shared common pattern because of possible dependence on residual AS during digestion in freshwater fish species are very important in conservation physiology. All species from slow-flow habitats showed impaired swimming speed while digesting, whereas all species from fast-flow habitats showed strong swimming performance, which was unchanged while digesting. Only two species from slow-flow habitats showed impaired heat tolerance during digestion, suggesting that whether oxygen limitation is involved in the heat tolerance process is species-specific. Three species from slow- or intermediate-flow habitats showed impaired hypoxia tolerance during digestion because feeding metabolism cannot cease completely under hypoxia. Overall, there was no common pattern in postprandial changes in different physiological functions because: (1) the digestion process was suppressed under oxygen-limiting conditions, (2) the residual AS decreased during digestion, and (3) performance was related to residual AS, while digestion was context-dependent and species-specific. However, digestion generally showed a stronger effect on bodily functions in species from slow-flow habitats, whereas it showed no impairment in fishes from fast-flow habitats. Nevertheless, the postprandial change in physiological functions varies with habitat, possibly due to divergent selective pressure on such functions. More importantly, the present study suggests that a precise prediction of how freshwater fish populations will respond to global climate change needs to incorporate data from postprandial fishes.