Foraging theory, habitat selection and the ecology of a guild of benthic estuarine fishes

Estuaries offer high availability of resources for certain species while presenting significant risks. I examined the extent to which resources drive the patterns of distribution of the coastrange sculpin, Cottus aleuticus , between estuarine and upstream habitats in a Pacific Northwest estuary. High food availability in the estuary results in a substantial potential fitness increase compared with the stream. Resource-matching models such as the ideal free distribution (IFD) were not sufficient to explain the relative abundance of C. aleuticus in estuarine and upstream habitats. Both fish and their prey were around 2 times more abundant in the estuary, which was consistent with the IFD model, but growth experiments indicated that the prey resources in the estuary were sufficient to support even more individuals than I observed there.; Predation risk from the marine cottid Leptocottus armatus and competition from the congener Cottus asper limit the extent to which C. aleuticus benefits from the estuary. Using models that consider costs of habitat selection, I explored how each of these factors affects the ability of C. aleuticus benefit from the estuary. The mortality suffered by C. aleuticus was around 4 times greater in the presence of L. armatus compared with the predator free situation which was consistent with the expectations of habitat selection theory.; I then used methodological tools derived from foraging theory to examine the effects of predation risk on both Cottus aleuticus and C. asper. Foraging theory predicts that as the costs of foraging increase, the harvest of food at a given patch is abandoned at a higher food density. When differences between species in foraging effort exist, such differences indicate the response to predation costs as a potential mechanism for their coexistence. I used giving-up densities (GUDs) to determine the relative foraging effort of C. aleuticus and C. asper in the non-lethal presence or absence of L. armatus in the estuary. Cottus aleuticus reduced its resource consumption in response to predation risk as a foraging cost, whereas only juvenile C. asper showed similar behavior. For C. aleuticus and juvenile C. asper, behaviors such as vigilance, or simply short term reductions in foraging effort contribute to fitness in complex ways.