Relationship between salmon egg subsidy and the distribution of an avian predator

<strong>ABSTRACT</strong> As a spatial subsidy, which is the phenomenon of transferring resources from a donor system to a recipient system, anadromous salmonids contribute to the supply of marine-derived nutrients to freshwater and terrestrial systems. Live salmon and salmon carcasses and eggs are utilized by various organisms and affect their abundance and distribution. However, the evaluation of the effect of salmon subsidies on the abundance and distribution of terrestrial animals is biased towards predators or scavengers that utilize spawning adults and carcasses, and few studies have focused on the effect of salmon eggs as a subsidy. To avoid underestimating the function of salmon subsidies, the response to the availability of salmon eggs in various systems should be investigated. Here, we investigated the abundance and feeding behaviour of the brown dipper <em>Cinclus pallasii</em>, as salmon egg a consumer, based on the hypothesis that the availability of salmon eggs affects the diet composition and stream distribution of this small predator. In addition, to test whether changes in the abundance of brown dippers are determined by salmon spawning, their abundance was compared upstream and downstream of the dam. Brown dippers used salmon eggs during the spawning season (53.7% of diet composition), and their abundance increased as the number of spawning redds increased. In contrast, this pattern was not observed upstream of the dam. These results suggested that the abundance and stream distribution of brown dippers vary according to the variation in the spatiotemporal availability of salmon eggs.<strong>摘要</strong> 作为一种空间补贴（spatial subsidy）——即资源从供体系统向受体系统转移的现象——溯河洄游鲑科鱼类（anadromous salmonids）可为淡水与陆地生态系统提供海洋源性营养盐。各类生物会利用活鲑鱼、鲑鱼尸体及鱼卵，并因此改变自身的丰度与分布格局。然而，当前针对鲑鱼补贴对陆地动物丰度和分布影响的评估，多偏重于利用产卵成体与尸体的捕食者或食腐动物，鲜有研究关注作为补贴来源的鲑鱼鱼卵的生态效应。为避免低估鲑鱼补贴的生态功能，亟需探究各类生态系统中生物对鲑鱼鱼卵可获得性的响应。本研究以褐河乌（*Cinclus pallasii*）为对象，将其视作鲑鱼鱼卵的消费者，提出假说：鲑鱼鱼卵的可获得性会影响这一小型捕食者的食物组成与溪流分布格局，并据此调查了褐河乌的丰度与取食行为。此外，为验证褐河乌的种群丰度变化是否由鲑鱼产卵活动决定，本研究对比了大坝上下游区域的褐河乌种群丰度。结果显示，褐河乌在鲑鱼产卵季会取食鲑鱼鱼卵（占其食物组成的53.7%），且其种群丰度随产卵坑数量增加而升高；而该模式在大坝上游区域并未出现。上述结果表明，褐河乌的丰度与溪流分布格局，会随鲑鱼鱼卵时空可获得性的变化而改变。