Mussels, Mytilus edulis L. grow on the shore and are cultured on ropes in Killary Harbour, 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 POC concentrations. This is because of the presence of variable amounts of allochthonous detrital carbon.The annual cycles of flesh weight and ash content of wild and cultivated mussels were followed over two years. 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 o % exposure, take about 6 years to attain the length attained by the mode of the cultured mussels after 18 months, when they are harvested.We conclude that wild mussels utilize a mix of phytoplankton and detritus as food during the summer and that 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 two spawning bouts during the summer, but flesh weight declines once phytoplankton densities fall in the autumn. I N T R O D U C T I O NCulture of mussels, Mytilus edulis L., on ropes suspended from rafts or long lines is a well-established industry in several countries (Mason, 1976;Lutz, 1980). In Ireland, the industry centres on Killary Harbour, a fjordic inlet on the west coast (Fig. 1). A dense population of mussels occurs on the shores of the inner part of the inlet, and the experience of commercial growers has shown that larval settlement is heaviest here. Ropes are seeded here during peak settlement periods

An experiment was performed to assess the productivity, food chain efficiency and nitrogen conversion efficiency of a semi-closed nursery system for bivalve spat held in upwelling columns. Algal blooms were encouraged by nutrient enrichment of onshore tanks and maintained for short periods under grazing pressure from bivalves.

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