Stocking effectiveness of hatchery-released kuruma prawn Penaeus japonicus in the Xiangshan Bay, China

Stock enhancement is a fisheries management approach involving the release of hatchery organisms to increase abundance and commercial landing of natural fish or invertebrate population. Kuruma prawn Penaeus japonicus is one of the most important fishing and cultured crustaceans in China coast, and thus is listed as one of the important target species for stock enhancement. In 2010, a total of about 167 million hatchery kuruma prawn juveniles (body length greater than or equal to 11.0 mm) were released through two phases into the Xiangshan Bay, Zhejiang Province. The first batch of 30.68 million prawn juveniles were released on July 6, while the second batch of about 135.9 million prawn juveniles were released on July 15-16. In order to evaluate the potential effectiveness of the stock enhancement program, the present study monitored survival, growth rate, and migratory pattern of the hatchery-released kuruma prawn in the bay through recapture method monthly. It was necessary for assessing the stocking effectiveness to distinguish the released individuals from natural partners. It was found that there were obvious differences in hatching time and body size between the natural and hatchery kuruma prawn juveniles, and thus we could use cohort analysis with the least-squares method to identify released hatchery individuals in the study. The results indicated that the released prawns mainly inhabited in inner or middle part of the Bay where the sediments were sandy or sandy-mud substratum. The released juvenile prawns started recruiting to the fishery in mid/late August, and then it might migrate from the Bay to coastal open water for overwintering in October. In mid-September, the released prawns reached the mean body length of 95.4 mm and 71.4 mm, with the survival rate of 0.79% and 1.06%, respectively, for the first and second batch. The relative abundance of the released prawns in the total catch was 14.0% and 84.2%, respectively, for the first and second batch. The exponents for the body length-body weight relationships differed significantly (P < 0.05) between released and natural stock, with greater value for the released stock, which showed that released individuals were more robust than natural ones. During the fishing season, it was found that lots of released prawns were caught by local fishermen through small beam trawls and fish traps within the bay. During the study period from late August to early October, a total of about 420, 600 released prawn