Alaskan brown bears (Ursus arctos) aggregate and display fidelity to foraging neighborhoods while preying on Pacific salmon along small streams

Abstract

The interaction between brown bears (Ursus arctos) and Pacific salmon (Oncorhynchus spp.) is important to the population dynamics of both species and a celebrated example of consumer‐mediated nutrient transport. Yet, much of the site‐specific information we have about the bears in this relationship comes from observations at a few highly visible but unrepresentative locations and a small number of radio‐telemetry studies. Consequently, our understanding of brown bear abundance and behavior at more cryptic locations where they commonly feed on salmon, including small spawning streams, remains limited. We employed a noninvasive genetic approach (barbed wire hair snares) over four summers (2012–2015) to document patterns of brown bear abundance and movement among six spawning streams for sockeye salmon, O. nerka, in southwestern Alaska. The streams were grouped into two trios on opposite sides of Lake Aleknagik. Thus, we predicted that most bears would forage within only one trio during the spawning season because of the energetic costs associated with swimming between them or traveling around the lake and show fidelity to particular trios across years because of the benefits of familiarity with local salmon dynamics and stream characteristics. Huggins closed‐capture models based on encounter histories from genotyped hair samples revealed that as many as 41 individuals visited single streams during the annual 6‐week sampling season. Bears also moved freely among trios of streams but rarely moved between these putative foraging neighborhoods, either during or between years. By implication, even small salmon spawning streams can serve as important resources for brown bears, and consistent use of stream neighborhoods by certain bears may play an important role in spatially structuring coastal bear populations. Our findings also underscore the efficacy of noninvasive hair snagging and genetic analysis for examining bear abundance and movements at relatively fine spatial and temporal scales.

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

“…Although the proportion of years with salmon limitation was apparently high, bears detected at Hansen Creek routinely moved between salmon‐bearing streams within a season (Wirsing et al. ). Given the lack of complete synchrony in salmon abundance among streams (Rogers and Schindler ), salmon limitation at one stream does not imply salmon limitation in all neighboring streams.…”

“…We estimated that 17–34 bears foraged at Hansen Creek each year from 2013 to 2016 (Wirsing et al. ), so a bear population consuming a maximum of 7,732 kg of salmon at Hansen Creek would consume 227–455 kg per capita annually, corresponding to an estimated maximum daily intake rate of 8–15 kg·bear −1 ·d −1 over the 30‐d season. Actual daily intake depends on bear size, sex, age, and social status (Van Daele et al.…”

“…, Wirsing et al. ), small increases in consumption resulting from increased escapements, if realized across multiple foraging streams, could provide an ecologically meaningful increase in salmon consumption. This scenario is plausible, but we were unable to investigate it in the current study.…”

“…We checked hair snares every other day from mid‐July through late August each year, and distinguished the bears using DNA microsatellites (Wirsing et al. ). For each year, we generated a genetic capture–mark–recapture population estimate (Wirsing et al.…”

“…For each year, we generated a genetic capture–mark–recapture population estimate (Wirsing et al. ), and then estimated salmon consumption per bear by dividing the biomass consumed by the estimated number of bears.…”