Impact of suspension culture using mesh sleeves on genetic characteristics of Mytilus edulis L. in Canada

Previous studies have suggested that a large proportion of the more heterozygous mussels are lost while in suspension culture. Such a reduction in multi-locus heterozygosity (MLH) could impact mussel production because more heterozygous individuals have better growth and survival than more homozygous ones. The present study followed the changes in MLH over a year in two different cohorts and found a reduction in MLH during the culture cycle for both cohorts. This reduction did not occur during the early phases of the culture operations, such as sleeving (= socking), or during the first weeks in suspension culture, but took place during the following growing season starting in early summer for both cohorts. The MLH decrease was not associated with different environmental conditions at the growing site. It is hypothesized that mussels characterized by high MLH are more mobile than those with lower MLH and migrate out of the mesh tubes faster after sleeving to get access to better growing conditions (space and food). However, being at the periphery of the sleeves, these more heterozygous individuals are more vulnerable to fall-offs due to turbulence and/or self-thinning. We suggest that the MLH decrease on the sleeves is not caused by the selective mortality of more heterozygous individuals but rather by higher losses through fall-offs.