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

Cover is 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 the 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 (the ''poor'' patch), the other providing more food but no cover (the ''good'' patch). Because fish distributions in the absence of cover conformed to an ideal free distribution (IFD) for unequal competitors (i.e., the distribution of competitive abilities matched the distribution of food), we used IFD theory to quantify the energetic equivalence of cover to the fish. In the presence of cover and a model avian predator, use of the poor patch increased relative to the predictions of the IFD model. Using this observed deviation from an IFD, we calculated how much extra food must be added to the good patch to return the distribution of fish to the previously observed IFD of unequal competitors. As predicted, adding this amount of food caused the fish to return to their previous distribution, demonstrating that IFD theory can be used to relate energy intake and risk of predation in a common currency.