2014 Volume 62 Issue 1 Pages 89-97
Ayu Plecoglossus altivelis altivelis is one of the most important fish in inland fisheries in Japan. However, there has been a growing concern over its recent stunted growth in the Nagara River, prompting scientists to determine the cause of this problem. This study evaluated the actual conditions for ayu growth in the river and compared these in the native population and the following stocks: hatchery-reared, originating from Lake Biwa, and originating from amphidromous seed. Seventy-two specimens were caught by night-time gill net fishery with lighting in late October 2009 and discriminated by otolith Sr/Ca and morphological characteristics. Our analysis showed that 62 specimens were native, whereas 7 were hatchery-reared, and 3 were originating from amphidromous seed. The standard length of the native fish was significantly smaller than that of the stocks. Furthermore, the standard length of the native fish was not significantly correlated with hatching date or the initial date of their freshwater period, although a significant positive correlation was observed with the specific growth rate in the stream-dwelling period. This study thus shows that the observed small size of ayu in the autumn was due to the stunted growth of native fish in the stream.