Evaluation of the influence of an alien salmonid, Salmo trutta, on the native freshwater fish community

本研究的主要目的是阐明外来鲑鱼的生活史模式、迁徙、摄食生态，以及与本地或外来鲑鱼在同域的种间竞争，以评估对本地鱼类群落的影响。在北海道南部的黑尻河和北海道中部的四足湖进行了野外调查，得到以下结果：在Hekiriji河，在河口上游约7 km处的大坝水库中发现了大量的Salmo trutta， 2岁雌鱼（bbb275 mm FL）和雄鱼（>172 mm FL）均已成熟，表明它们参与了产卵。秋季长到海拔210-270毫米时，有少数个体蜕变成小鳟鱼，表明它们应该成为海鳟。发现200 ~ 300 mm赤潮鱼主要以毛毛虫幼虫、陆生动物和底栖鱼类（如Noemacheilus toni）为食。在四足湖，两种外来鲑科鱼类，S.trutta和Oncorhynchus mykiss共存，前者以底栖动物为食，后者以比富河的入口溪流中的游泳类动物为食，这表明它们的食物资源分配。在湖中，S. trutta比三刺棘鱼等鱼类更早出现，而斑点鲑和kokanee salom4。结果表明，沙螽捕食动物的生态位宽度小于沙螽。该研究强烈表明，S. trutta对本地鱼类群落的影响很大，主要是由于它的食鱼性摄食。

The main objectives of the present study were to elucidate the life-history pattern, migration, feeding ecology of an alien salmonid, Salmo trutta, and the interspecific oompetition with native or alien salmonld species in the sympatry, to evaluate tbe influence on the native fish community. The field survey was carried out in both the Hekiriji River of southern Hokkaido and the Lake Shikotsu of central Hokkaido, and following results were obtained:1. In the Hekiriji River, Salmo trutta was found to be abundantly stocked in the dam reservoir which was located about 7 km upstream from the mouth, and that both age 2-year females (>275 mm FL) and males (>172 mm FL) reached to mature, suggesting their participation into the spawnings.2. A few individuals of S.trutta metamorphosed into smolts when they grew up to 210-270 mm FL in the autumn, suggesting that they should become sea trout.3. Salmo turtta individuals of 200-300 mm FL were found to feed maily on trichopteran larvae, terrestrial animals and the benthic fishes such as Noemacheilus toni.4. In the Lake Shikotsu, where two alien salmonid species, S.trutta and Oncorhynchus mykiss, coexist, the former fed on benthic animals whereas the latter fed on swimming-type aminals in an inlet stream, the Bifue River, suggesting their food resource partitioning. In the lake, S. trutta predated the fishes such as threespine stickleback, while-spotted charr and kokanee salom.5. The niche width of prey animals in S. turtta was found to be lower than that in O. mykiss.6. This stydy strongly suggests that S. trutta provides a large impact on the native fish community mainly due to its piscivorous feeding.