Dispersion processes of the ranched striped jack Pseudocaranx dentex after release

抄録

Marine ranching of striped jack Pseudocaranx dentex was undertaken at the Goto Station of the Japan Sea-Farming Association between 1988 and 1999. This stocking method aimed at protecting the released fish from predators and preventing dispersion by feeding for a certain period. The present study examined the reduction process in the number of ranched striped jack and the conditions required to keep the fish in the ranched area. We estimated the sighting rate and the dispersion coefficient from the ranched area per day from 1993 to 1999, on the assumption that natural mortality can be ignored, using the observed number of fish from the sighting observation. The number of fish that remained was represented by a decreasing exponential function of day. This decreasing process was divided into six phases at most. We found two phases when the number of fish that remained markedly decreased in 1993 and 1995. In the first phase, the dispersion was caused by lack of food due to an increased food demand caused by the growth of fish. In the second phase, the ranched fish moved out from the ranched area due to the cessation of feeding. This dispersion was also observed in other years when feeding was stopped.