Do small tributaries function as refuges from floods? A test in a salmonid-dominated mountainous river.

摘要

The potential role of tributaries as refuges from flooding was examined. Population census was conducted in four small tributaries of a river system 1-2 days before the typhoon in 8-10 August 2003, and an additional census in these four tributaries on 13-14 August. The censuses were conducted at the Shiisorapuchi River, one of the major branches to the Sorachi River that drains into the Kanayama Reservoir in a mountainous area of central Hokkaido, Japan. Before the typhoon, all the tributaries were dominated by small (< 100 mm) Dolly Varden with 5-10 individuals of relatively large (100-149 mm) Dolly Varden. Small numbers of white-spotted charr and freshwater sculpin Cottus nozawae were also present in some of the tributaries. No individual exceeded 150 mm in the tributaries before the flood. Sakhalin taimen Hucho perryi, Japanese/rosyface dace Tribolodon spp. and Siberian stone loach Barbatula barbatula toni were present in the main stem. After the typhoon, no change in fish species and size structures in the tributaries was observed. No timen, dace or loarch was detected in the tributaries, nor white-spotted charr larger than 100 mm that were abundant in the main stem near the tributaries. Results did not support the hypothesis that many large fishes immigrate to small tributaries during floods. Despite the lack of evidence of mass movement, result suggested a few immigrants from the main stem.