In order to estimate the loss of the juvenile masu salmon at the intake of agricultural diversion weirs during downstream migration in the Kaji River, Niigata Prefecture, a new method was presented. A total of 70, 205 fish with fork length of about 11.5cm were divided into three groups marked differently by one another by otolith tagging and released at the upstream of the upper weir-I (upper group), the downstream of the lower weir-II (lower group), and the middle area between wear-I and weir-II (middle group) on 13-20 October 1994. We assumed that the difference of the recapture rates of these three groups would be derived from the difference of entrainment of fish at each weir, and estimated the rates of entrainment based on the recapture rates in the sea after their downstream migration in spring.
The recapture rates were 0.0229, 0.0388 and 0.0432% for upper, middle and lower groups, respectively, thus the upper group showed the lower recapture rate because of the larger loss of fish by entrainment at the weirs. The rate of entrainment with 95% confidence intervals were estimated as 41.0% (30.2-51.9) at the weir-I, 10.2% (-1.32-21.5) at the weir-II, and 47.0% (30.5-63.5) for a sum of loss at both weirs.
