Quinn Thomas P, Kinnison Michael T
Fisheries Research Institute, School of Fisheries, University of Washington, Box 357980, Seattle, WA 98195, USA e-mail:
Oecologia. 1999 Nov;121(2):273-282. doi: 10.1007/s004420050929.
Breeding activity increases the vulnerability of many animals to predation, and such predation can affect the subset of animals successfully reproducing. To study the ways in which predation might affect the evolution of Pacific salmon, we measured the intensity and selectivity of predation by bears (primarily brown bears, Ursus arctos) on mature sockeye salmon (Oncorhynchus nerka) breeding in a series of small, spring-fed ponds and creeks near Pedro Bay, Alaska, from 1994 to 1998. Bears killed male salmon more often than females; males constituted 60% of the kills but only 35% of the salmon that died of senescence. The bears also killed fish that were larger, on average, than those dying of senescence (males: 462 vs 452 mm; females: 453 vs 443 mm). The level of predation varied greatly, from 4% (females) and 10% (males) in 1994 to 100% of both sexes in 1996 and 1997. The rate of predation also varied among habitats, being lower in larger ponds than in smaller, shallower ponds and the very small interconnecting creeks. Despite the intense and size-selective predation, the salmon in safer habitats (large ponds) were not larger than those in riskier habitats, and salmon densities were only slightly higher in the safer areas. Compared to a nearby population that experiences no bear predation (Woody Island), the male sockeye salmon from the Pedro Pond system had shallower bodies (i.e., less exposure in shallow water) for a given length, consistent with the hypothesis that selective predation can affect the extent of sexual dimorphism among populations. However, the average length at age for both males and females was greater in the Pedro Pond fish, indicating that selective factors besides predation affect length. Overall, the results indicate that bears can be an agent of natural selection within (and perhaps between) sockeye salmon populations, and predation can greatly affect reproductive success among individuals and years for the population as a whole.
繁殖活动增加了许多动物被捕食的脆弱性，而这种捕食行为会影响成功繁殖的动物群体。为了研究捕食行为可能影响太平洋鲑鱼进化的方式，我们在1994年至1998年期间，对阿拉斯加佩德罗湾附近一系列由泉水补给的小池塘和溪流中繁殖的成熟红大麻哈鱼（Oncorhynchus nerka），测量了熊（主要是棕熊，Ursus arctos）的捕食强度和选择性。熊捕杀雄性鲑鱼的频率高于雌性；被熊捕杀的雄性占60%，但自然衰老死亡的雄性仅占鲑鱼总数的35%。熊捕杀的鱼平均比自然衰老死亡的鱼更大（雄性：462毫米对452毫米；雌性：453毫米对443毫米）。捕食水平差异很大，从1994年的4%（雌性）和10%（雄性）到1996年和1997年两性均为100%。捕食率在不同栖息地也有所不同，在较大的池塘中比在较小、较浅的池塘和非常小的连通溪流中更低。尽管存在强烈的大小选择性捕食，但较安全栖息地（大池塘）中的鲑鱼并不比风险较高栖息地中的鲑鱼更大，而且较安全区域的鲑鱼密度仅略高。与附近没有熊捕食的种群（伍迪岛）相比，佩德罗池塘系统中的雄性红大麻哈鱼在给定长度下身体更浅（即在浅水中暴露更少），这与选择性捕食会影响种群间两性异形程度的假设一致。然而，佩德罗池塘的雄鱼和雌鱼的平均年龄体长都更大，这表明除了捕食之外，还有其他选择因素影响体长。总体而言，结果表明熊可能是红大麻哈鱼种群内部（也许还有种群之间）自然选择的一个因素，捕食会极大地影响个体间以及整个种群数年的繁殖成功率。
