Food resource, gametogenesis and growth of Mytilus edulis on the shore and in suspended culture: Killary Harbour, Ireland

Abstract

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

Growth of mussels (Mytilus edulis galloprovincialis) on cultivation rafts: influence of seed source, cultivation site and phytoplankton availability

“…The growth rates measured in the present study, ranging from 6.3 to 11.1 mm month -1 , were slightly higher than those reported by Perez Camacho and Roman (1979) for mussels from the Ria de Arousa, comparable or occasionally higher than those reported by Page and Hubbard (1987) for Californian Mytilus edulis, which were considered by the authors as 'among the highest rates recorded world-wide', and markedly higher than those measured for this species by Rodhouse et al (1984) in Ireland. These growth rates are also greater than the growth rates of mussels cultured in intertidal areas, for either M. edulis (Theisen, 1968;Lube& 1969;Rodhouse et al, 1984;Hager, 1970) or the Mediterranean Mytilus galloprovincialis (Ceccherelli and Rossi, 1984).…”

“…These growth rates are also greater than the growth rates of mussels cultured in intertidal areas, for either M. edulis (Theisen, 1968;Lube& 1969;Rodhouse et al, 1984;Hager, 1970) or the Mediterranean Mytilus galloprovincialis (Ceccherelli and Rossi, 1984).…”

“…This result may be explained by a higher detritic organic matter content at the inner most site (Site 1) (Oceanology group IIM, unpublished data), not detected by the chlorophyll a analysis, but a potential food source for the mussel (Rodhouse et al, 1984;Perez Camacho et al, 1991). This can also explain the lower condition index for Site 2 mussels, although the occurrence of a spawning event at this site cannot be discarded.…”

Annual variation in condition, respiration and remineralisation ofMytilus edulis L. in the Sound, Denmark

“…Rodhouse et al (1984) found annual dr,/weights in 30 mm mussels in England between 85 and 200 mg, which corresponds to a condition index of 3.1-7.4. In contrary, condition indexes even lower than those found in the present study are possible.…”

Experimental trials on the feasibility of offshore seed production of the mussel Mytilus edulis in the German Bight: installation, technical requirements and environmental conditions

“…For suspended culture methods, such as the deployed longlines, mussels feed continuously and therefore grow rapidly (Seed 1968a, b). Water depth can be a limiting factor in tidal seas as usually a minimum water column height is essential during the low water spring tides due to variations in food availability within the water column (Rodhouse et al 1984;Page and Hubbard 1987). At the site Nordergründe, we suggest to submerge the longline at least 6-7 m to avoid the energy forces from waves.…”