Association behavior of young striped jack Pseudocaranx dentex to a shade of flotsam

Behaviors of the striped jack Pseudocaranx dentex to the shade of flotsam were observed in experimental tanks to determine whether or not the shade would act as a factor to trigger association behavior of fishes to a flotsam.During the daytime, 20 fish of 23cm TL preferred to distribute under the cover which was set on the right half side of a 90m3 outdoor tank, while there was no difference in fish distribution between the right and left sides in a control tank without cover. During the nighttime, however, there was no difference in horizontal distribution between the test and the control tanks. Fish in the test tank tended to distribute at shallower depths than those in the control tank during the daytime, while both fish distributed at the surface during almost all of the nighttime. When the light intensity was increased by more than 10, 000 lx in an outdoor 60m3 tank with a 90×180cm plywood board at the surface, 21 striped jack of 24cm TL distributed in the shade of the plywood board, while they went out of the shade and swam around the tank when the decrease in light intensity exceeded 10, 000 lx. These data suggested that the shade might attract the striped jack and trigger the association behavior. It was also suggested that in marine ranching of the striped jack in coastal waters the feeding station should provide fish with shade to enhance their association behavior and to keep fish at a fixed area in the sea.