Experiments on the survival, growth and yield of relaid seed mussels (Mytilus edulis L.) in the Menai Straits, North Wales

The survival, growth and yield of young mussels relaid on small plots, at densities of 8·75 and 13·25 kg/m² live weight, were studied at low-water mark of spring tides in the Menai Straits, North Wales. Populations grew from mean lengths of 27 mm to minimum market size (50 mm) in 15–20 months, and to 55–58 mm in two years, but good growth was offset by high mortality so that no significant gains in live crop weight resulted. Heavy predation by shore-crabs (Carcinus maenas (L.)) upon mussels smaller than 45 mm shell length was the major factor restricting crop growth. During the first year, mortality of unprotected mussels was very high (70–85%), whereas losses within crab-proof cages were significantly less (17–41%). Second-year mortality on open plots fell significantly to 22–57%, most losses then being caused by storms and bird predators, but that in cages rose to 27–54% due to overcrowding and smothering in biodeposits (‘mussel-mud’).

Dry-flesh biomass (ash-inclusive) of unprotected mussels amounted to 4–6% of live crop weight (shell inclusive) and fluctuated seasonally, attaining peak values in late summer and early autumn. Standing crops were usually between 0·4 kg/m² and 0·8 kg/m², with a maximum of 1·2 kg/m². Total production of dry-flesh over the two year growing period was 0·75–1·43 kg/m², most production (about 69%) occurring in spring and summer of the first year. For optimum utilisation of mussel stocks and cultivation grounds the relaid mussels should be harvested at peak biomass, in late summer and autumn, and at a smaller size (45–50 mm) than at present.