Seasonal changes in the metabolism of cultured mussels (Mytilus edulis L.) from a Nova Scotian inlet: the effects of winter ice cover and nutritive stress

This study examined the seasonal changes in standard respiration and ammonium excretion rates of mussels (Mytilus edulis L.) from suspended culture in Nova Scotia. The measurements included the harsh winter period when the mussel culture lines are held under a thick ice cover. In order to assess the role of short-term energy reserves, mussels were removed from the field and changes in metabolism in the absence of food were measured for up to 11 days. The time required to exhibit maintenance metabolism is a useful index of the magnitude of stored reserves serving as catabolic substrates and a measure of the degree of coupling of mussels to ambient food supplies. The experimental mussels were collected from commercial lines on an aquaculture farm in Upper South Cove, Nova Scotia. The winter mussels, which were collected from under the ice-covered cove, showed signs of nutritional stress, and respiration rates were less than or close to those required for maintenance metabolism. The standard summer and autumn metabolic rates were within the range expected due to the difference in water temperature from winter, with a Q(10) of around 2.0. However, the rates measured during spring were much higher than expected based on water temperature, and decreasing rates during particle-starvation suggested that these mussels had accumulated metabolic reserves during the early spring phytoplankton bloom. High mantle index values during spring and their correlation with both respiration and ammonium excretion indicated that the energy demands of reproduction influenced metabolism in this season. These results demonstrate that food availability is a significant control on the seasonally-changing metabolism of mussels regardless of water temperature. It is proposed that low values for the O:N ratios in experimental mussels may reflect the metabolic acclimation of mussels in suspended culture to a high quality food source with a consistently low C:N ratio. (C) 1997 Elsevier Science B.V.