Influence of Fish Stocks on the Recapture Rate of Ayu Released in the River Tsubusa

Abstract

To estimate the influence of fish stocks on stocking effectiveness of the ayu, Plecoglossus altivelis, the dispersal patterns and recapture rates of wild and hatchery reared fish were determined through a stocking experiment in the River Tsubusa.
During short-term dispersion just after release in May, two groups of hatchery fish (HR-A, HR-B) were mainly dispersed downstream, while wild fish (LAKE) swam upstream. However, another wild fish (ACCL) reared for 43 days before release had a lesser tendency to disperse than the others. Fish swimming upstream after release were larger in size for every group than those going downstream.
During settlement and downstream migration periods (June-October), most recapture occurred in the area within ±2 km from the point of release. LAKE distributed more upsteram and HR-A, more downstream than the others. Recapture rate during fishing season was highest in LAKE (29.2%), and HR-B (5.9%), ACCL (1.3%) and HR-A (0.8%) following in order, indicating the superiority of the wild stock (LAKE) and a disagreeable effect of temporal rearing on stocking effectiveness of wild fish (ACCL).
In conclusion, the stocking effectiveness represented by recapture rate depends on the dis-persal pattern after release: fish with stronger tendency to swim upstream resulted in higher re-capture rate, and are thus regarded as having a good fish quality.