Population structure, growth and reproduction of the common mussel (Mytilus edulis L.) off the Island of Seili (SW Finland)

We studied the reproduction, population structure and growth of the common mussel (Mytilus edulis L.) off the Island of Seili in summer 1996. The discharge of gametes occurred from 26 July to 2 August. The breeding period was short and repeated spawning was not found. Density of mussels ranged from 1 250 to 16 730 ind. m–2 and biomass from 250 to 2 100 g m–2. Size of individuals (shell length) ranged from 0.6 mm (spat) to 44.0 mm (nine years old). The distribution of age groups in the mussel population demonstrates the stability of recruitment over several previous years. The most rapidly growing individuals (reaching up to 41 mm at the end of the seventh year) were registered at the depth of 8 m. The average growth rate of molluscs from different habitats did not vary. A Bertalanffy model of ontogenetic growth, based on seasonal changes in the water temperature, was constructed for the “average individual” of this population.