Brown bear genetic detections as the basis for an analysis of stream use in relation to spawning salmon abundance and stream morphology

Brown bears (Ursus arctos (Linnaeus, 1758)) are famous for exploiting
annual pulses of Pacific salmon (Oncorhynchus sp.), but studies of how
bears do so have largely been confined to anomalous locations where they
congregate to feed at natural salmon migration bottlenecks. We addressed
this knowledge gap using nine years of non-invasive genetic detections of
brown bears along six small streams – grouped into trios along the
northern and southern shores of Lake Aleknagik that constitute foraging
neighborhoods where distinct bear populations prey on sockeye salmon (O.
nerka (Walbaum, 1792)) – to test hypotheses about sex-specific patterns of
relative stream use in relation to salmon abundance and stream morphology.
Numbers detections per bear on a stream during the summer salmon spawning
season were inversely proportional to water depth in the northern stream
neighborhood; by contrast, greater in-stream salmon abundance boosted
detection rates, and females were more likely to be detected repeatedly
than males, in the southern stream neighborhood. Thus, bear behavior along
small streams was consistent with optimal foraging theory, as more
intensive use of streams in a season corresponded with greater abundance
and accessibility (reduced stream depth) of salmon, and with the idea that
female bears are less sensitive to humans, as these streams received a low
but regular level of human visitation. Our findings also reveal, however,
that the factors affecting bear movement patterns along small streams can
vary, even among stream networks are separated by just a few km,
underscoring the complexity of the bear-salmon relationship. Finally, we
documented marked variability in the use of streams by individual bears,
with some consistently visiting particular streams over the summer but
most being detected just once, implying that impacts and reliance on
salmon on any stream may differ considerably among individuals in brown
bear populations.