Sex and stream depth affect detection rates of brown bears during annual salmon pulses in southwestern Alaska

Understanding how consumers exploit transient resource pulses requires studying the localized responses they trigger. To explore how brown bears (Ursus arctos (Linnaeus, 1758)) exploit Pacific salmon pulses outside of migration bottlenecks, we used nine years of genetic detections along six small streams – grouped into trios along the northern and southern shores of Lake Aleknagik, Alaska where distinct bear populations prey on sockeye salmon (Oncorhynchus nerka (Walbaum 1792)) – to test hypotheses about stream use in relation to salmon abundance and stream morphology. Bear detections in the northern trio were inversely proportional to water depth, whereas in the southern trio greater salmon abundance boosted detection rates and repeat detections were more common for females. Thus, bear behavior comported with optimal foraging theory, as stream use increased with salmon abundance and accessibility, and with the idea that females are less sensitive to anthropogenic distirbance, as these streams are visited by humans. Our findings reveal that factors affecting bear presence along small streams can vary, even among streams separated by a few km, underscoring the complexity of the bear-salmon relationship. Finally, some bears consistently visited streams but most were detected once, implying that impacts and reliance on salmon differ among individuals in bear populations.