Scottish Mussel Culture in the Natural Environment: Observations and Implications for Industry

Shellfish aquaculture is growing rapidly with the expanding human population, offering high-quality animal protein as well as economic benefits to producing areas. Cultivation relies on coastal and estuarine habitats, dynamic ecosystems where marine organisms are exposed to natural environmental variability, and which will be affected by climate change.

This thesis investigated the variability in blue mussel (Mytilus edulis) performances under suspended rope-culture in a stratified Scottish sea loch, Loch Leven. Environmental conditions were monitored over one year and at different culture depths and related to the mussel’s product quality (meat yield, condition index), genetic composition (genotype, extent of hybridisation) and shell phenotype (shell strength and shape).
Environmental conditions varied over time (seasonal and short-term fluctuations) but also across depth, generating different microhabitats for mussels depending on their position on the rope. Conditions varied most at shallow culture depths, for salinity in particular, but presented warmer temperature and higher food availability compared to greater depths and promoted mussel growth and abundance. Meat yield and condition index followed a seasonal cycle, with maximum values in early summer and minimum values in winter, associated with environmental (nutritional) and reproductive cycles. The genetic composition and shell morphology differed across depth. On average, every sixth mussel carried alleles of Mytilus trossulus, but the level of introgression was overall low. However, highly introgressed mussels (≥ 75 %) presented distinct shell morphology (significantly lower shell strength, elongated shell shape) and originated mainly from shallow culture depths. This phenotype distinguished them from their congeners and allowed for their identification based on shell characters.

This thesis presents the variability in suspended mussel culture in a heterogeneous environment, highlighting the complex relationships between habitat conditions and the genetic and phenotypic make-up in naturally occurring mixed-species stocks. The knowledge gained offers guidance for the farm operators to optimise production (site selection, spat collection etc.) and provides better predictions for the industry on the possible effects associated with climate change on future mussel production.