Species-specific patterns of refuge use in fish: The role of metabolic expenditure and body length

Being in a refuge has benefits in terms of predator avoidance and costs in terms of lost feeding opportunities. We investigated how the relative importance of these costs and benefits changes with increasing body length in two sympatric fish species, the minnow, Phoxinus phoxinus, and the stickleback, Gasterosteus aculeatus, which differ in their morphological anti-predator adaptations, Minnows were slower to emerge after taking refuge than sticklebacks, and spent less time outside it, which is consistent with the idea that minnows, which lack the morphological defences (such as spines) of sticklebacks, I-e more cautious of predators than the latter. Food-deprivation experiments indicated that the costs of missed feeding opportunities in terms of relative weight loss were lower for minnows than sticklebacks. Therefore hiding in a refuge comes at a lower metabolic cost for minnows than stickleback. Both species reduced their hiding time when food deprived and increased it following a predation threat. Furthermore. they both showed a strong trend for longer hiding periods and shorter exploration times outside the refuge with increasing body length. Our results suggest that in sticklebacks the body length-dependence of hiding times was a result of perceived predation risks being constant with increasing body length whereas relative weight losses decreased. Thus larger fish could metabolically afford to be more cautious. In minnows, both the predation list and metabolic expenditure decreased with increasing body length suggesting that the longer hiding times in larger fish represented a trade-off between the two factors.