Underwater observations on copepod swarms in temperate and subtropical waters

Abstract

Swarms of 8 copepod species were visually observed in temperate and subtropical waters along the coast of Japan from 1975 to 1981. Irregular ball-shaped swarms, with diameters less than 1 m. were common for Acartia plumosa, A. erythraea, A. steueri, A. japonica, A. hamata and Oithona oculata. They generally occurred on or a little above the bottom near the shore during the day and maintained their position against slight water currents. A. clausi in Shijiki Bay and 0. oculata in Tanabe Bay formed continuous flat swarms and carpeted the bottom with copepods. A. steueri in Shijiki Bay. 1978, swarmed within sea grass beds. Labidocera pavo formed small swarms, in which the copepods swirled with each other. Copepod swarms were almost rnonospecific aggregations except for A. erythraea, which was observed to swarm jointly with A. plumosa. Within swarms, adult copepods generally dominated and females outnumbered males. For some of Acartia spp., swarms consisted almost entirely of adults, especially females, indicating that the swarming of these species is age-and sex-specific. Adaptive functions of copepod swarms are discussed. Swarming of copepods appears to be a common phenomenon in temperate shallow waters and of significant importance as food resource for some demersal fishes.