In situ observations of the association between juvenile fishes and scyphomedusae in the Bering Sea

In September 1995, dense aggregations of large scyphomedusae (mainly Chrysaora melanaster) were observed in 27 midwater deployments of a remotely operated vehicle (ROV) near the Pribilof Islands in the Bering Sea. Age-0 walleye pollock Theragra chalcogramma appeared to be frequently associated with these gelatinous zooplankton. During daytime, up to 30 pollock were observed swimming within the tentacles of these medusae, but when approached by the ROV, the pollock generally left the vicinity of the jellyfish. At night, few such associations were observed and juvenile pollock moved closer to the surface, apparently feeding in loose aggregations, while the medusae remained close to the thermocline (35-40 m). Prowfish Zaprora silenus were also observed near large medusae, but tended to be found closer to the bell rather than within the tentacles. The lack of any gelatinous material in the stomachs of the pollock suggests that juvenile pollock associate to gain shelter from predation or possibly as a thigmotactic response to biotic structure. The implications of this commensal behavior with gelatinous zooplankton are discussed relative to pollock recruitment in the Eastern Bering Sea.