Selective consumption of sockeye salmon by brown bears: patterns of partial consumption, scavenging, and implications for fisheries management

Abstract

Animal foraging requires a series of complex decisions that ultimately end with consumption of resources. The extent of consumption varies among consumers, including predator-prey systems; some predators always completely consume their prey but others may partially consume prey that are too large to be completely consumed, or consume only parts of smaller prey and discard the remains. Partial consumption of prey may allow predators to maximize energy intake through selectively feeding on energy-rich tissue, as is observed in bears (Ursus spp.) selectively feeding on Pacific salmon (Oncorhynchus spp.). Here, we examined selective and partial consumption of sockeye salmon (O. nerka) by brown bears (U. arctos) in western Alaska. First, we tested a series of hypotheses to determine what factors best explain why some salmon are killed and abandoned without tissue consumption, and what tissues are consumed from the salmon that are fed upon. We found that a foraging strategy consistent with energy maximization best explained patterns of selective prey discard and partial consumption, as traits of the fish itself (size, sex, and condition) and the broader foraging opportunities (availability of salmon as prey) were important. Second, we documented empirical relationships between salmon availability and consumption by bears to evaluate whether current salmon management in Bristol Bay, Alaska limits salmon intake by bears foraging in small streams. Bears reached maximum salmon consumption every 2-4 years in the systems studied, and management scenarios revealed comparatively small increases in salmon consumption by bears in response to large increases in salmon availability, suggesting that current management does not severely limit salmon intake in these populations. Finally, we conducted a field prey choice experiment to explore scavenging as another mechanism of prey consumption. Limited data revealed lower rates of scavenging compared to previous years, and we observed similar patterns of selectivity and partial tissue consumption in scavenged carcasses as were seen in freshly killed salmon. Through exploring the intricacies of selective and partial consumption of salmon by bears, this work aims to increase understanding of the bear-salmon predator-prey relationship, and to add to knowledge about the flow of salmon-derived nutrients through aquatic and terrestrial ecosystems.