Growth and production of mussels ( Mytilus edulis L.) suspended at salmon cages and shellfish farms in two Scottish sea lochs

Abstract

Shell length and tissue growth, biomass and production of 1–2-year-old mussels suspended from salmon sea cages, mussel rafts and long-lines were monitored during May 1990–May 1992 in Lochs Etive and Leven on the west coast of Scotland. Water temperature, salinity and food availability were also determined. These showed a clear seasonal cycle and, in consequence, growth of mussels was relatively rapid from May to October and very slow or absent during the rest of the year. Particulate organic matter and chlorophyll-a were higher at salmon farms than at adjacent mussel farms (significant at P < 0.05 with POM in Loch Etive). Maximum tissue growth occurred during April–May and shell length during June–August. Tissue growth and hence production were negative during winter (October–April). Annual length increments of 25.1–25.9 mm at sites in Loch Etive were significantly greater than those of 20.1–22.8 mm in Loch Leven ( P < 0.001). This difference is due to greater temperature and food availability in Loch Etive and possible heavy metal stress in Loch Leven. Shell length growth of mussels at salmon farms was slightly higher than at neighbouring shellfish farms, but differences were significant only during 1991–1992 ( P < 0.001). A multiple regression model of the effects of environmental factors on shell length increment gave significant coefficients for water temperature and chlorophyll-a, together accounting for 67–88% of total variance, but the effect of non-algal POM (estimated by difference between total POM and that associated with chlorophyll) was significant only in Loch Etive in 1990–1991, where it explained 6% of the variance ( P < 0.05). In addition there were significant differences in live (whole) and wet and dry meat weights ( P < 0.05), and in situ production losses at salmon farms during winter were less than at shellfish farms, indicating that mussels at salmon farms did not deplete their reserves as much as those at mussel farms. The differences in performance between salmon and shellfish farms probably resulted from the utilization of higher organic matter concentrations at salmon farms. The advantages and disadvantages of integrating filter-feeding bivalves with cage culture of finfish are discussed.