Evaluation of suspended mussel (Mytilus edulis L.) culture and integrated experimental mariculture with salmon in Scottish sea lochs
Growth. mortality. production. physiology and seasonal cycles of condition index and proximate biochemical composition of experimental populations of blue mussels (Mytilus edulis L.) were studied at different sites in Loch Etive and Loch Leven on the West coast of Scotland between May 1990 and September 1992. The main objective of the study was to evaluate current suspended mussel culture practices and to establish the basis for their possible integrated cultivation with salmon cage fanning. In addition. a preliminary investigation on employment of the . Charm IT' system as a rapid method for detecting residues of drugs used for treating cultured salmon in the tissues of mussels was carried out. There were some differences between sites in salinity, seston and particulate organic matter, but not in chlorophyll-a. Food availability (as particulate organic matter and chlorophyll-a) showed a clear seasonal cycle and in consequence growth of mussels were relatively rapid from late-spring until mid-autumn (g 6 months) and very slow or absent during the rest of the year. This period of rapid length and tissue growth coincided with relatively optimum environmental conditions and there were apparent positive relationships between monthly growth rates and temperature and chlorophyll-a values, indicating the limiting effect of these two primary factors on growth during autumn-winter and even in early spring. Almost all growth parameters examined were showed significant differences between the lochs. Growth performance of both native and transplanted mussels in Loch Leven was quite poor. Overall annual length increments were 25.1-25.9 mm at sites in Loch Etive and Dunstaffnage Bay, but 20.1-22.8 mm in Loch Leven. A crosstransplantation experiment showed that site rather than stock is the main reason for differences in growth parameters between Lochs Etive and Leven. These observed growth differences between sites and stocks were also confirmed by physiological measurements and estimated growth potential or scope for growth. Growth of mussels at salmon farms was faster than at neighbouring mussel farms during two annual experiments, but only meat weight at one salmon farm during experiment I, and length and live weight at the salmon farm in Loch Etive as well as all growth parameters at the salmon fann in Loch Leven during experiment II were significantly greater (}>sO.05). These differences were most likely a result of high particulate organic matter levels at salmon fanns. Similar to growth, biomass and production, the condition index and biochemical composition of mussels showed a clear seasonal cycle. Meat content, condition index and glycogen values were high during summer, started to decline in late autumn and reached minimum values in April before showing maximum increases in May. This reflects the typical storage and reproductive cycle of mussels in Northern Europe: accumulation of reserves during summer and their utilization during winter and early spring as energy resources for metabolism and reproduction. This cycle clearly showed that the main spawning of mussels on the West coast of Scotland occurred during March-May, and primary spat settlement from June to August. Heavy losses occurred from French socks, causing substantial amounts of eliminated biomass during experiment I, but when these fall outs were eliminated during experiment II by using lantern nets, it was clear that natural mortality rates were quite low and similar at all sites. Apart from growth characteristics and physiological responses, there were persistent morphological differences between the Loch Etive and Loch Leven populations. Cross-transplantation and physiological measurements after various acclimatization periods showed that, while morphological differences might be related to genetic origin, all other differences between the two popUlations are governed by environmental factors. The practical implications of these findings for developing suspended mussel culture on the West coast of Scotland and the possibility of a simple integrated salmon-mussel fanning system, which could be effective in controlling potential eutrophication from intensive salmon cage fanning and the removing large amounts of organic matter by mussels leading alterations in ecosystem, are discussed. A preliminary study with the Chann II Test has showed that the system is not so appropriate method as expected for screening mussel tissue sampled straight from the field, since mussel tissues require purification due to interference from bacteria or microbial detritus, before screening.