Mortality of juvenile Japanese flounder Paralichthys olivaceus caused by scavenging amphipods and ostracods in trap experiments

Trap experiments were conducted using wild and artificially reared juvenile Japanese flounder in various conditions as baits in a flounder nursery habitat of Mano Bay. All of the live wild flounder placed in the traps with sand survived in good health. However, most of the live wild flounder in the traps without sand and live-reared flounder in the traps with and without sand, as well as injured and dead-reared flounder, were consumed and stripped to skeletons overnight. A large number of the lysianassoid Orchomene sp. and Orchomene naikaiensis, and the cypridinid Vargula hilgendorfii, which are typical scavengers, accumulated in the traps with consumed flounder; however, few amphipods and ostracods occurred in traps without consumed flounder. Thus, once flounder are detected and reached by a swarm of these crustaceans, flounder may be preyed on, irrespective of the condition of the fish. This finding is supported by aquarium observations that amphipods and ostracods were able to kill and consume live flounder. Burial in sand will potentially prevent the crustaceans from detecting wild flounder by interfering with the odor from the fish. The results of our study raise the possibility that scavenging crustaceans are able to predate hatchery-reared flounder released in the sea.