Near-bottom copepod aggregations around the nursery ground of the juvenile red sea bream in Shijiki Bay.

Abstract

Near-bottom copepod aggregations were investigated in Shijiki Bay by means of stomach content analysis of juvenile red sea bream, near-bottom towing of plankton nets and direct under-water observation with the aid of scuba-equipment. The stomach contents of 220 red sea bream juveniles, 12.7 to 43.7mm in FL, were examined. Nearly 80% of the total food items consisted of copepods, 97% of which were pelagic species. The overwhelming majority of them were Acartia omorii and A. steueri. The near-bottom towing of plankton nets revealed a landward increase in the abundance of pelagic copepods. Two types of near-bottom aggregations were recognized by scuba-diving; a flat carpet-like type with thickness of about 30cm, and a discrete ball-like swarming type with a long axis of about 50cm. The former appeared to be formed by A. omorii and the latter by A. steueri. The maximum density found was 326 per liter far A. omorii and 511 for A. steueri, these being 100 times greater than the background density. The three approaches used in the present study clearly demonstrated the existence of near-bottom aggregations of Acartia omorii and A. steueri; the former rather widely distributed, whereas the latter is restricted to the innermost area shallower than 15m. These two species each shows a distinct macro- and microdistribution pattern which results in a high degree of habitat segrega-tion.