Brown Bear Consumption of Pacific Salmon Varies Greatly Among Individuals but Increases With the Bear's Age and Salmon Availability

TL;DR

Brown bears show varied salmon consumption, which increases with age and salmon availability, but differs greatly among individuals.

Contribution

The study reveals significant individual variation in salmon consumption among brown bears, despite high salmon availability and no consistent sex-based differences.

Findings

Salmon consumption increases with bear age and is higher in areas with more salmon.

There is no consistent difference in salmon consumption between male and female bears.

Some bears consume little salmon even in areas with abundant salmon.

Abstract

As animals age and grow, their diet often changes as food demand increases, and because their size and social dominance may allow them to capture and consume larger and different prey. Diet also can vary between sexes and individuals. In this study we used stable isotope analysis of hair samples collected from free‐living brown bears,
Ursus arctos
, identified by DNA analysis, in an area with easy access to Pacific salmon (Oncorhynchus sp.). The goals were to test the hypotheses that salmon consumption would increase as bears aged, be higher in males than females, and higher in bears with access to more than fewer salmon, in the context of the level of variation among individuals. Samples (n = 165 from 85 different bears) were collected from six streams, three on each side of Lake Aleknagik, Alaska from 2012 to 2022 (missing only 2020) and processed for isotopes of N and C. These data confirmed our predictions of greater salmon consumption as individuals aged and in those sampled along streams with higher salmon densities. However, inferred salmon consumption varied greatly among bears and did not differ consistently between sexes, though far more females were detected than males. Thus, at the population level, salmon availability affected inferred consumption by bears, and average consumption increased as the bear aged and presumably grew larger with higher social status. Nevertheless, some individuals consumed little salmon, though the hair samples were collected from bears traveling along small streams with dense salmon populations where predation is common. They may have been using the streams primarily as traveling corridors rather than foraging areas, despite the abundance of salmon. In any case, the varied reliance on salmon, without sex bias, observed here differs from systems where males consume more salmon and some females avoid salmon‐bearing streams.

Hair was sampled in a noninvasive manner from free‐living brown bears in Alaska and processed for genetic identification and stable isotopes. These data revealed great individual variation in bear reliance on salmon, despite being sampled along streams full of salmon. Bear reliance on salmon increased as they aged, but did not differ consistently between males and females.

Figures