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.
Highlights
Each year, hundreds of millions of adult Pacific salmon (Oncorhynchus spp.) transport vast quantities of energy and marine-derived nutrients to freshwater ecosystems in Asia and North America when they return from the ocean to spawn in streams, rivers, and lakes (Gresh, Lichatowich, & Schoonmaker, 2000; Hocking & Reynolds, 2011; Schindler, Scheuerell, et al, 2003)
TA B L E 1 Numbers of detections and unique individual brown bears (Ursus arctos) identified (IDs) using hair sampling barbed wires deployed along six sockeye salmon (Oncorhynchus nerka) spawning streams flowing into Lake Aleknagik (Wood River System, Alaska) over the course of four summers (2012–2015)
TA B L E 2 Huggins multi-session, closed-capture models estimating brown bear abundance over the course of three summers (2013, 2014, 2015) along six salmon spawning streams flowing into Lake Aleknagik, AK
Summary
Hundreds of millions of adult Pacific salmon (Oncorhynchus spp.) transport vast quantities of energy and marine-derived nutrients to freshwater ecosystems in Asia and North America when they return from the ocean to spawn in streams, rivers, and lakes (Gresh, Lichatowich, & Schoonmaker, 2000; Hocking & Reynolds, 2011; Schindler, Scheuerell, et al, 2003). Whereas previous studies have quantified the influence of stream traits and salmon density on carcass deposition by bears (e.g., Quinn et al, 2009), the difficulty associated with estimating bear abundance at relevant spatial and temporal scales has hindered assessment of the relationship between bear density and the delivery of salmon-derived nutrients to terrestrial food webs. This knowledge gap is especially pronounced in areas where small salmon spawning streams predominate, which typify the habitats along the north Pacific Rim where bears and salmon interface but.
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