Comparative experiments for estimating the optimal size and timing of release of herring Clupea pallasii juveniles

¹Tohoku National Fisheries Research Institute, Fisheries Research Agency, Miyako, Iwate 027-0097, ²Seikai National Fisheries Research Institute, Fisheries Research Agency, Goto, Nagasaki 853-0508, ³Seto-Inland-Sea National Fisheries Research Institute, Fisheries Research Agency, Imabari, Ehime 794-2305, Japan

 We compared the return rates of adult herring from eight release groups of hatchery reared herring Clupea pallasii juveniles with four different conditions of fertilization month and fish size at release in Miyako Bay from 1998 to 2003: January-fertilized 50 and 60 mm TL, and February-fertilized 50 and 60 mm TL. Return rates of the released groups ranged widely from 0.01 to 0.87%, and tended to be higher in the January-fertilized groups than the February-fertilized groups. We did not detect a difference in the return rates between the two body size groups. The return rates were significantly correlated with the date of release but not with body size at release. Because the return rates of released herring juveniles are considered to be determined by survival of juveniles immediately after release, early fertilization and early release seemed to have a beneficial effect on the juvenile survival in the bay, and eventual return rate of adult herring.