The energetic equivalence of cover to juvenile coho salmon (Oncorhynchus kisutch): ideal free distribution theory applied

Cover U often thought to be an important habitat characteristic for juvenile stream salmonids. In addition to providing protection from predators, cover may be associated with reduced food availability. Thus, an individual's use of cover is likely to reflect a trade-off between the conflicting demands of growth and survival. We measured die influence of cover on foraging-site selection in groups of eight juvenile coho salmon (Oncorhynchus kisutch) by examining their distribution across two stream channel patches, one providing access to cover but little food (die "poor" patch), die other providing more food but no cover (die "good" patch). Because fish distributions in the absence of cover conformed to an ideal free distribution (IFD) for unequal competitors (Le., die distribution of competitive abilities matched die distribution of food), we used IFD dieory to quantify die energetic equivalence of cover to die fish. In die presence of cover and a model avian predator, use of die poor patch increased relative to die predictions of die IFD model. Using tiiis observed deviation from an IFD, we calculated how much extra food must be added to die good patch to return die distribution of fish to die previously observed IFD of unequal competitors. As predicted, adding this amount of food caused die fish to return to dieir previous distribution, demonstrating that IFD dieory can be used to relate energy intake and risk of predation in a common currency.