Significance of Near-bottom Copepod Aggregations as Food Resources for the Juvenile Red Sea Bream in Shijiki Bay

Abstract

The significance of copepod swarming near the bottom as food resources for juvenile red sea breams was investigated around the nursery ground of Shijiki Bay. Juveniles of 12.7 to 43.7 mm FL were found to predate upon a large number of pelagic copepods composed exclusively of Acartia omorii and A. steueri. The maximum number of copepods detected in a stomach increased markedly with juvenile length, reaching about 100 at 13 mm FL, 200 at 15 mm FL and exceeding 300 at 20 mm FL. However, few copepods were found in the stomachs of juveniles larger than 35 mm FL. The large quantities of ingested copepods observed were inferred to be caused by effective predation on highly aggregated copepods near the bottom. The estimation for the daily ration of the red sea bream demonstrated that early juveniles can obtain enough energy for maintenance and growth soley through feeding on copepods. Within the bay, densities of copepods swarming near the bottom showed a landward increase. The density gradient induces the early juveniles to migrate into the innermost part of the bay, i.e. the nursery ground. Together, these quantitative estimates and with qualitative evidence confirm that copepods swarming near the bottom are of great importance for the survival and growth of the early juveniles of the red sea bream, particularly as emergent food resources under deficient conditions of benthic prey organisms. © 1987, The Japanese Society of Fisheries Science. All rights reserved.