Low to moderate wave exposure did not impact blue mussel (Mytilus edulis) growth in a mesocosm study

Little is known about the causes of the decline in blue mussel populations in the North Atlantic. If mussel beds are to be protected, and maybe even restored, we need knowledge about environmental conditions under which blue mussels can survive and grow. Wave exposure impacts the growth and abundance of blue mussels by impacting food availability, predation, competition and sedimentation. In the field it is difficult to separate the effects of the different variables, and we therefore wanted to perform a simple, but controlled, mesocosm study on the impact of wave exposure on blue mussel (Mytilus edulis) growth. We placed three replicate blue mussels in each of 12 mesocosm basins, of which six had high and six had low wave level. Each of the 36 blue mussels were measured weekly for 13 summer weeks and the measured parameters (length, width, thickness, weight and displacement volume) were analysed against wave exposure and time using a non-parametric Generalised Additive Model (GAM). Surprisingly, we found no effect of wave exposure on any of the parameters. This could be because wave exposure is not as important as we have believed, but that it usually captures other factors, such as sedimentation, predation and competition. It could also be explained by the level and span in wave exposure being too low, failing to generate measurable effects. Our advice for future studies is to increase the difference in wave exposure levels, but still perform controlled studies to separate the effect of wave exposure from other variables.