Abstract
This study examined the effects of a concrete groundsill on the survival rate of fish falling off the groundsill. The survival of river sculpin (Cottus nozawae), Siberian stone loach (Noemacheilus barbatulus toni), and four age groups of 0+, 1+, 2+ and 3+ of rainbow trout (Oncorhynchus mykiss) in Shirigishimanai Stream, Hokkaido, Japan, was surveyed before improvement in November 2000. Immediately after the experiment, we observed the survival of the fish in a tank for 10 days. The survival analysis using the Kaplan-Meier method and Log-rank test showed that rainbow trout 0+ juveniles suffered seriously from falling off the groundsill (survival rate: 73%). After constructing a water cushion on the apron, the water on the apron became deeper (before: 11.3cm, after: 77.9cm) and the current velocity slower (before: 0.75 m/sec, after: 0.09 m/sec). We conducted a second survey after the construction using 0+ juvenile rainbow trout in November 2002. The improvement increased the survival of juvenile fish (survival rate: 95%). Our study suggests that the reduced height and the water cushion on the concrete apron should benefit juvenile salmonids during up- and downstream migration.
