Managing salmon for wildlife: Do fisheries limit salmon consumption by bears in small Alaskan streams?

Abstract

Ecosystem‐based management requires consideration of overlapping resource use between humans and other consumers. Pacific salmon are an important resource for both fisheries and populations of wildlife around the Pacific rim, including coastal brown bears (Ursus arctos); salmon consumption has been positively linked to bear density, body size, and reproductive rate. As a case study within the broader context of human–wildlife competition for food, we used 16–22 yr of empirical data in four different salmon‐bearing systems in southwestern Alaska to explore the relationship between sockeye salmon (Oncorhynchus nerka) availability and consumption by bears. We found a negative relationship between the annual biomass of salmon available to bears and the fraction of biomass consumed per fish, and a saturating relationship between salmon availability and the total annual biomass of salmon consumed by bears. Under modeled scenarios, bear consumption of salmon was predicted to increase only with dramatic (on the order of 50–100%) increases in prey availability. Even such large increases in salmon abundance were estimated to produce relatively modest increases in per capita salmon consumption by bears (2.4–4.8 kg·bear−1·d−1, 15–59% of the estimated daily maximum per capita intake), in part because bears did not consume salmon entirely, especially when salmon were most available. Thus, while bears catching salmon in small streams may be limited by salmon harvest in some years, current management of the systems we studied is sufficient for bear populations to reach maximum salmon consumption every 2–4 yr. Consequently, allocating more salmon for brown bear conservation would unlikely result in an ecologically significant response for bears in these systems, though other ecosystem components might benefit. Our results highlight the need for documenting empirical relationships between prey abundance and consumption, particularly in systems with partial consumption, when evaluating the ecological response of managing prey resources for wildlife populations.

Stable Isotopes Reveal Variation in Consumption of Pacific Salmon by Brown Bears, Despite Ready Access in Small Streams

“…We also hypothesized that males would have isotopic values indicating more dietary reliance on salmon than females due to social dominance (e.g., Van Daele et al 2013;Adams et al 2017), but no difference was detected. This pattern is consistent with results of research elsewhere in Alaska (Fortin et al 2007), and with the idea that salmon are sufficiently available and accessible in these streams to both sexes of bears, as evidenced by partial consumption of salmon carcasses (Lincoln and Quinn 2018;Lincoln et al 2020a). Whatever enhanced access males might have through social interactions may be mitigated by some greater wariness compared to females.…”

“…20-50% of the sockeye salmon present each year (Quinn et al 2017). This interpretation, that salmon are sufficiently abundant to meet the needs of the bears, is also consistent with the prevalence of partial consumption of salmon by bears in this system (Lincoln and Quinn 2019;Lincoln et al 2020a).…”

“…This contrasts with well-studied sites like the McNeil River falls, where bears congregate in close proximity to each other and display alternative fishing tactics (Gill and Helfield 2012). Deacy et al (2018) drew the same conclusion as did Lincoln et al (2020a); beyond some threshold, increased salmon density may not substantially affect their consumption by bears, and variation in diet reflects individuality in bear behavior more than salmon availability.…”

“…In this study, we used stable isotope analyses to test the null hypothesis, that the isotopic signatures of brown bears would indicate reliance on salmon by all individuals because heavy predation on salmon (ca. 20-50% annually) occurs in streams within the home ranges of sampled bears (Quinn et al 2017;Wirsing et al 2018), and because partial consumption of salmon carcasses indicated that they are available in surplus (Lincoln and Quinn 2019;Lincoln et al 2020a), against two alternative hypotheses. The samples might indicate (1) diverse diets, because bears tend to congregate on salmon streams from much wider ranges occupied earlier in the season (Glenn and Miller 1980;Sellers and Aumiller 1994), adding many other items to their integrated diets, and (2) less reliance on salmon and feeding on lower trophic levels by females than males, due to behavioral differences (Rode et al 2006;Van Daele et al 2013;Adams et al 2017).…”

The influence of human disturbance on Pacific salmon (Oncorhynchus spp.) in the diet of American black bears (Ursus americanus) in two areas of coastal British Columbia, Canada

“…The prominence of ecosystem management has led some authors (Hilderbrand et al 2004;Piccolo et al 2009;Lincoln et al 2020) to call for the management of Pacific salmon (genus Oncorhynchus Suckley, 1861) to consider not only escapement goals for fisheries harvest, but also for the needs of wildlife. Salmon are an important source of nutrients and energy for riparian ecosystems and terrestrial wildlife (Quinn et al 2018;Reimchen 2018).…”