Brown bears selectively kill salmon with higher energy content but only in habitats that facilitate choice

Abstract

Pacific salmon return to spawn in thousands of streams across the Pacific Rim, from large rivers to tiny headwater streams. Once on the spawning grounds, salmon undergo dramatic biochemical changes as they metabolize stored lipid and protein reserves; at stream entrance, they will contain up to 85% more lipid and 40% more protein than at their senescent death a week or two later. Foraging brown and black bears that congregate at spawning streams thus encounter salmon that vary dramatically in their energy content and thus energetic reward. We hypothesized that bears would selectively kill salmon that are highest in energy content (fewest number of days on the spawning grounds) when they pursue salmon at small shallow streams where little effort is necessary to capture salmon, i.e. habitats that facilitate choice. In contrast, bears in environments where foraging is difficult (deeper, more complex streams) should be less selective and should capture salmon that are most available. We tested these ideas by examining predation rates on fish of different in‐stream ages (i.e. energy content) at three different streams that varied in physical habitat attributes. At a very shallow, simple stream, bears preferentially killed salmon that had spent the fewest days in the stream. At two streams where deeper water and woody debris provided refuges for salmon, predation rates increased with in‐stream age. At the shallowest streams encounter rates and capture success are likely equal among the high‐ and low‐energy salmon and thus predation rates reflect active choice by bears. In contrast capture success probably increases on the older salmon at the larger streams (due to a loss of vigor), and thus ‘preference’ for these fish increases due to decreasing effort necessary to capture them.