Effects of Salmon-Derived Nutrients and Habitat Characteristics on Population Densities of Stream-Resident Sculpins

Movement of nutrients across ecosystem boundaries can have important effects on food webs and population dynamics. An example from the North Pacific Rim is the connection between productive marine ecosystems and freshwaters driven by annual spawning migrations of Pacific salmon (Oncorhynchus spp). While a growing body of research has highlighted the importance of both pulsed nutrient subsidies and disturbance by spawning salmon, their effects on population densities of vertebrate consumers have rarely been tested, especially across streams spanning a wide range of natural variation in salmon densities and habitat characteristics. We studied resident freshwater prickly (Cottus asper), and coastrange sculpins (C. aleuticus) in coastal salmon spawning streams to test whether their population densities are affected by spawning densities of pink and chum salmon (O. gorbuscha and O. keta), as well as habitat characteristics. Coastrange sculpins occurred in the highest densities in streams with high densities of spawning pink and chum salmon. They also were more dense in streams with high pH, large watersheds, less area covered by pools, and lower gradients. In contrast, prickly sculpin densities were higher in streams with more large wood and pools, and less canopy cover, but their densities were not correlated with salmon. These results for coastrange sculpins provide evidence of a numerical population response by freshwater fish to increased availability of salmon subsidies in streams. These results demonstrate complex and context-dependent relationships between spawning Pacific salmon and coastal ecosystems and can inform an ecosystem-based approach to their management and conservation.

营养物质跨生态系统边界的输移，会对食物网与种群动态产生重要影响。北太平洋沿岸（North Pacific Rim）的一个典型案例，便是由太平洋鲑属（Oncorhynchus spp.）鱼类的年度产卵洄游所驱动的高生产力海洋生态系统与淡水生态系统之间的养分联系。尽管越来越多的研究均强调了脉冲式养分补贴（pulsed nutrient subsidies）与产卵鲑鱼带来的生境扰动的双重重要性，但二者对脊椎动物消费者种群密度的影响却极少得到实证检验，尤其是在横跨鲑鱼密度与栖息地特征均存在广泛自然变异的各类溪流的研究中。我们在沿岸鲑鱼产卵溪流中对定居性淡水鱼类棘侧杜父鱼（Cottus asper）与海岸带杜父鱼（C. aleuticus）开展了研究，以检验它们的种群密度是否会受到粉鲑（O. gorbuscha）与狗鲑（O. keta）的产卵密度，以及栖息地特征的影响。海岸带杜父鱼的种群密度在粉鲑与狗鲑产卵密度较高的溪流中达到最高。此外，其种群密度在pH值更高、流域面积更大、深潭覆盖占比更低、坡度更缓的溪流中同样更高。与之相反，棘侧杜父鱼的种群密度在拥有更多大型枯木与深潭、冠层覆盖度更低的溪流中更高，但其种群密度与鲑鱼密度并无显著关联。上述针对海岸带杜父鱼的研究结果，为淡水鱼类对溪流中鲑鱼补贴资源供给增加产生的种群数量响应提供了实证依据。本研究结果表明，产卵的太平洋鲑鱼与沿岸淡水生态系统之间存在复杂且依赖于环境背景的关联，可为基于生态系统的鲑鱼管理与保护工作提供科学参考。