FOOD RESOURCE, GAMETOGENESIS AND GROWTH OF MYTILUS-EDULIS ON THE SHORE AND IN SUSPENDED CULTURE - KILLARY-HARBOUR, IRELAND

Mussels, M. edulis L., grow on the shore and are cultured on ropes in Killary Harbor, a fjordic inlet on the Irish west coast. The food resource available to cultured mussels differs from that available to wild mussels on the shore. Although phytoplankton densities as estimated from chlorophyll a concentrations are similar, the shore environment in the inner part of the inlet is characterized by high mean particulate organic C concentrations, due to presence of variable amounts of allochthonous detrital C. The annual cycles of flesh weight and ash content of wild and cultivated mussels were followed over 2 yr. These cycles were related to the reproductive cycle observed by taking histological samples of mussel gonad, by plankton sampling for larvae and by monitoring larval settlement. Shell growth was measured in wild mussels by reading seasonal growth patterns on sectioned shells and in cultured mussels by following progress of the modal shell length of cohorts on ropes. Wild mussels have a partial spawning in early spring and spawn completely in the summer. Cultured mussels spawn twice during the summer, in the year following settlement. Growth rate of wild mussels decreases with increasing aerial exposure. The fastest growing mussels, at 0% exposure, take .apprx. 6 yr to attain the length attained by the mode of the cultured mussels after 18 mo., when they are harvested. Wild mussels utilize a mix of phytoplankton and detritius as food during the summer and large wild mussels can use detritus during the autumn and early winter for an increase in flesh weight and gametogenesis. This results in a partial spawning restricted to large individuals in the spring. Cultured mussels are mainly dependent on phytoplankton for food. This supports fast growth and 2 spawning bouts during the summer, but flesh weight declines once phytoplankton densities fall in the autumn.