Influence of environmental factors on spat collection and mussel (Mytilus edulis) culture in raft systems in two Scottish sea lochs

Abstract

Growth, mortality, production, spat collection, seasonal cycles of condition index, biochemical composition, carrying capacity of commercial raft culture systems and population genetic characteristics of blue mussel (Mytilus edulis) were studied at different sites in Loch Etive and Loch Kishorn on the west coast of Scotland between May 1993 and May 1995. The main objective of the study was to evaluate current suspended mussel culture production in raft systems and to obtain basic information on the biology and the genetic structure of the two mussel populations in the lochs. There were some water quality differences between the sites in relation to seston, salinity and transparency but not to temperature, particulate organic matter and chlorophylla. When food is available (as particulate organic matter and chlorophyll-a), there was a clear seasonal cycle in mussel somatic growth and shell growth. Mussel growth was relatively high from mid-spring until late autumn, but very slow during the rest of the year. The spring-summer period of rapid shell length and somatic growth coincided with relatively optimum environmental conditions and positive relationships were indicated between growth rates, temperature and salinity, indicating the limiting effect of these two primary factors on growth from late-autumn to mid-spring when there is also a lack of available food. Mussel growth was higher at 2m depth on the raft-rope systems, but in lantern nets experimental growth did not show differences between depths. Growth was found to be similar in the lantern nets and on culture ropes in the two lochs in the first year of experiments (from May 1993 to May 1994). Overall, mean length increments were 31.01mm in Loch Etive and 28.75mm in Loch Kishorn over a 15 month period. The mussels reached marketable size (>50mm) in two years from the known time of spat settlement. A cross-transplantation experiment showed that site rather than stock is the main factor explaining differences in mussel growth in Loch Etive and Loch Kishorn. The position of the mussels within a raft has a significant effect on their growth; mussels at the inflow of a raft have a better growth than those near the outflow (p<0.05) due to greater availability of food. Mean mussel biomass was higher in Loch Kishorn while production was higher in Loch Etive, but there were seasonal and monthly fluctuations in both biomass and production at both sites. Biochemical composition and energy content were similar in both sites, while mussel meat yield and condition indices were significantly higher in Loch Kishom than Loch Etive. Meat content, condition index and carbohydrate values were high during the summer and low from autumn to spring, reaching minimum values in March and April at the time of spawning. Spat settlement occurred in June-July in Loch Etive and June-December in Loch Kishorn. Sea squirt, starfish and eider duck are problems effecting spat collection at the Loch Kishorn site, whereas spat collection in Loch Etive is unaffected by these pests/predators. The carrying capacities for cultured mussels were found to be about 24 metric tons per raft for Loch Etive and 38 metric tons per raft for Loch Kishorn using a particulate organic matter based model; these are reasonable estimates in comparison to the known mussel production levels reported by producers. However, a seston-based model gave an overestimate of carrying capacity for both sites. Cross-transplantation of mussels, electrophoresis and shell morphological measurements showed significant differences between the Loch Etive and Loch Kishorn mussel populations. Mortality rates were higher in transplanted mussels than in the native mussels (p<0.001).