Foraging behavior of bears at salmon streams: intake, choice, and the role of salmon life history

Gende, Scott

Foraging behavior of brown bears was examined at several small streams and ponds where pink and chum, or sockeye salmon spawn. Capture and consumption choice by bears was consistent with a lipid-maximizing strategy, but varied with salmon density (a function of the number of fish present and creek size), and with social dominance among bears. Bears preferentially killed salmon that were highest in lipid content, but only when streams were sufficiently shallow. As the availability of salmon decreased, bears consumed a larger proportion of each captured fish among both years and habitats. When availability was high, bears consumed more biomass from ripe salmon, and selectively consumed the eggs(roe) and skin in females and the brains in males. Examination of the biochemical differences in salmon revealed that these captuer and consumption choices elevate lipid intake by as much as 80%. However, they also suggest that bears may be sacrificing intake efficiency to maximize gros lipid intake. Body parks perferentially consumed were highest in lipid density, but were often small, constituting only a small fraction of the total available lipid in that fish. Observations of bears were consistent with this hypothesis. Little time was needed for bears (regardless of social status) to capture a salmon. However, dominant bears were able to forage longer and thus selectively kill and consume salmon that were highest in lipid density resulting in greater energy intake than other (subdominant) bears. The total biomass of salmon captured by dominant bears during a sling foraging bout (often 60 minutes or less) often exceeded that which could be consumed, allowing for selective predation and consumption of lipid-rich body parts. Each small stream supported one or two dominat bears (often large females with dependent young), which visited steams more often than other bears and thus, over the couse of the salmon run, killed a disproportional number of the salmon. However, intake rates of subordinate bears foraging at our sites would still be much greater than in feeding on high-density blueberry patches, so foraging on salmon was still beneficial.