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

No abstract

W a r y Harbour, a fjordic inlet on the west coast of Ireland, is the site of intensive suspended cultivation of mussels. Data on biornass and production of the macrobenthic infauna and zooplankton in the system are combined with literature data on primary production and wild and cultured mussel production in Kdlary Harbour to produce a balanced carbon budget. Total particulate carbon input to the system is 1907 tonnes C yr-'. Carbon production by phytoplankton is partitioned between zooplankton, cultured mussels, wild mussels, phytoplankton respiration and the detritus pool. The detritus pool also receives inputs from river carbon, macroalgae, zooplankton faeces, mussel faeces and phytoplankton. There is a carbon deficit which is probably accounted for by DOC input in freshwater, or import of marine POC, which were not measured. The data are used to estimate the maximum potential yield of cultured mussels from the system. This is in the order of 2000 to 3000 tonnes live weight per year. At this level of production, changes in the ecosystem are predicted. These are expected to arise from competition between mussels and zooplankton for food resources, increased sebmentation in localized areas, and disruption of the nitrogen cycle. However, nitrogen should not limit mussel production. Mussel culture, unlike fish culture in sea cages, re-routes the flow of energy and materials in coastal marine ecosystems but does not cause eutrophication by nutnent input. This is discussed in relation to red tides.

Killary Harbour is a fjord-like inlet on the west coast of Ireland and is an important site for suspended culture of mussels. As part of a study of the production ecology of the inlet, the zooplankton fauna was sampled quantitatively at two week intervals, between February 1981 and January 1982 using nets with mesh apertures of 90 and 335 μm. The dominant holozooplankton species of Killary Harbour are typical of inshore or neritic waters but there are also representatives of estuarine and oceanic faunas. Inflow of water into the bay below the surface layer of low salinity apparently maintains conditions suitable for neritic species and accounts for similarities between the zooplankton faunas inside and outside the bay. Large populations of estuarine species presumably do not develop because of the variability of freshwater discharge. During September 1981 there was evidence of an intrusion of oceanic water into the bay.

Killary Harbour is a fjord or sea loch on the west coast of Ireland. With the growth of aquaculture the harbour has become the site of both raft mussel cultivation and more recently, salmon farming. The successful management of these resources requires a detailed knowledge of the harbour's hydrography and ecology. In 1980/1 an extensive survey of the harbour was made. Papers dealing with primary productivity (McMahon & Patching, 1984) zooplankton (Ryan et al. 1986) and the biology of mussels (Rodhouse et al. 1984 a, b, 1985) have already been published. In this paper, we describe the phytoplankton of the harbour and factors which influence distribution and growth.