Changes in the composition of stock origin and standard length of ayu Plecoglossus altivelisaltivelis during the Tomozuri angling season in the Nagara River, central Japan

Delay of upward migration from the coast is considered to be a cause of stunted growth in ayu Plecoglossus altivelis altivelis in the Nagara River. To evaluate the effect of stocked seeds (hatchery reared, originating from Lake Biwa and amphidromous stock) on the growth of native ayu in the river, monthly changes in origin composition and standard lengths of territorial ayu caught by Tomozuri angling were examined. Stock origin of 94 individuals caught in the middle reaches of the river from June to October 2010 were discriminated using the otolith Sr/Ca ratio and morphological characteristics. Percentage frequency of native ayu increased with time from 4.2 % in June to 73.3 % in September and 71.4 % in October. Hatchery-reared stocks were consistently caught throughout the sampling period (26.7–41.7 %), while the catch of both Lake Biwa and amphidromous stocks were limited to before August. Native ayu were significantly smaller than the others in July; however, no significant differences were found from August. Apparent growth of stocked seed stopped after August, while native ayu stopped in September. Thus, we suggest that large stocked ayu disturb the native ayu from making territories and stunt their growth in the early part of the season.