Abstract
In March 2006, six spur dikes consisting of several sets of the wired boulders, were experimentally placed to restore gravelbed in a 50-m bedrock outcrop reach of the Makomanai River, central Hokkaido, northern Japan. The study purposes were to examine the influence of bedrock outcrop on summer habitats of wrinklehead sculpin (Cottus nozawae) and masu salmon (Oncorhynchus masou), and spawning habitat of masu salmon returning from ocean. The effects of spur dikes on habitat improvement were also examined. We set three sections representing gravelbed (reference), bedrock (control) and experiment (treatment), and investigated physical habitat variables, fish density and the number of masu salmon redd from July to October 2007. In the bedrock section, areal percentage of gravelbed was distinctly lower than that in the gravelbed section. Sculpin abundance and the number of redd were also lower in the bedrock section than in the gravelbed section. In the experiment section, abundance of small-sized sculpin (<90 mm) and the number of redd were similar to those in the gravelbed section, owing to an increase of gravel-pebble (2-64 mm) area in the experiment section facilitated by the spur dikes. Abundance of large-sized sculpin (≥90 mm) in the experiment section was lower than that in the gravelbed section because area of cobble-boulder (>64 mm) suitable for large-sized sculpin habitat did not significantly increase in the experiment section. These results suggested that the rehabilitation measures using the spur dikes foster the accumulation of medium-sized bed materials and accordingly improve fish habitat in bedrock rivers. Gravelbed is essential habitat component not only for anadromous fish but also for benthic fish at various stages of their life histories. We believe that restoration of sediment balance in a catchment is a fundamental measure to preserve gravel bed rivers.
