Stable isotopes reveal variation in consumption of Pacific salmon by brown bears, despite ready access in small streams

Brown bears Ursus arctos consume a wide range of organisms, including ungulates and plants, but Pacific salmon Oncorhynchus spp. are especially important to their diet where their ranges overlap. Although some brown bears minimize antagonistic encounters with other brown bears or infanticide by avoiding streams where salmon spawn, studies generally assume that brown bears with ready access to salmon feed heavily on them. To test this assumption, and the hypothesis that male brown bears would feed more heavily on salmon than females (owing to their sexual size dimorphism), we collected hair samples from brown bears by using barbed wire placed on six small tributaries of Lake Aleknagik, Alaska, USA, where adult Sockeye Salmon Oncorhynchus nerka are readily accessible and frequently consumed by brown bears. Analysis of DNA distinguished among the different brown bears leaving the hair samples, some of which were sampled multiple times within and among years. We assessed the contribution of salmon to the diet of individual brown bears by using carbon and nitrogen stable isotope signatures. The 77 samples analyzed from 31 different bears over 4 y showed isotopic ratios consistent with reliance on salmon, but the wide range of isotopic signatures included values suggesting variable, and in one case considerable, use of terrestrial resources. Stable isotope signatures did not differ between male and female brown bears, nor did they differ between two sides of the lake, despite marked differences in Sockeye Salmon density. We collected the hair samples when salmon were present, so there was some uncertainty regarding whether they reflected feeding during the current or previous season. Notwithstanding this caveat, the results are consistent with the hypothesis that salmon were sufficiently available to provide food for the brown bears and that the considerable isotopic variation among brown bears with access to salmon reflected their age, status, and behavior.

棕熊（Ursus arctos）的摄食对象涵盖多种类群，包括有蹄类与植物，但当分布范围重叠时，太平洋鲑属（Oncorhynchus spp.）鱼类对其食谱尤为重要。尽管部分棕熊会通过避开鲑鱼产卵的溪流，以减少与同类的对抗性遭遇或杀婴行为（infanticide），但现有研究普遍默认：能够便捷接触鲑鱼的棕熊会大量捕食鲑鱼。为验证这一默认假设，以及“雄性棕熊因体型性二态性（sexual size dimorphism），会比雌性更多依赖鲑鱼为食”的假说，我们在美国阿拉斯加州阿列克纳吉克湖的6条小型支流布设带刺铁丝网，采集棕熊的毛发样本——该区域内成年红大马哈鱼（Oncorhynchus nerka）极易被棕熊获取，且常成为其捕食对象。DNA分析可区分不同个体留下的毛发样本，部分个体在年内及跨年度间被多次采样。我们通过碳、氮稳定同位素特征（stable isotope signatures），评估鲑鱼在单只棕熊食谱中的占比。4年间共对31只不同棕熊的77份样本进行分析，同位素比值结果与棕熊依赖鲑鱼的结论一致，但宽泛的同位素值范围也包含了部分指示个体更多利用陆地资源的数值，其中1例个体的陆地资源利用占比颇高。尽管红大马哈鱼种群密度在湖两岸存在显著差异，但雌雄棕熊的稳定同位素特征并无显著差异，湖两岸的棕熊同位素特征也无明显区别。我们在鲑鱼洄游期采集毛发样本，因此无法确定样本反映的是当前还是上一季节的摄食情况。尽管存在这一局限，研究结果仍支持以下结论：鲑鱼的充足可及性可为棕熊提供食物来源，而接触鲑鱼的棕熊间存在的显著同位素差异，反映了它们的年龄、生存状态与行为模式的不同。