Research Document - 1999/116

Stock assessment and management frameworks for the proposed fishery for sea mussels (Mytilus californianus) in British Columbia.

By G. Gillespie

Abstract

Sea mussels (Mytilus californianus) are the largest species of mytilid mussel. They are found only on the west coast of North America, and range from Baja California to Alaska. Sea mussels form dense beds, generally in the intertidal zone of exposed rocky shores. Sea mussels spawn throughout the year, with peaks in activity in the spring and fall. Fertilization is external, and pelagic larvae may spend 3-5 weeks drifting planktonically before settling to adult habitat. Growth rates are variable, depending on availability of food, intertidal elevation, temperature and mussel density. Sea mussels can grow to approximately 270 mm in length, and may live 50-100 years. Size at maturity is approximately 70 mm in length. Sea mussels are filter feeders, and are in turn preyed upon by sea stars, whelks, crabs, fish, birds and marine mammals.

Mussel beds are highly structured, and provide habitat for nearly 300 other species. If greatly disturbed or destroyed, mussel beds require many years (5-100+) to recover to the climax community. Rate of recovery is dependent on size of the disturbance, season of disturbance, intertidal elevation, substrate angle and intensity of larval recruitment.

A commercial fishery for sea mussels existed in Baja California, Mexico from 1967-1981. Mussel beds were quickly depleted, and the fishery was extended through discovery of previously unexploited beds until its collapse. A small fishery for sea mussels existed in Oregon from 1979-1997. Landings peaked in 1989 at nearly 30 t, but have since declined to <1 t. Reasons for the decline are unknown, but may include overharvest. Attempts to develop commercial fisheries for sea mussels in California and British Columbia did not succeed.

Programs to assess, manage and monitor proposed sea mussel fisheries are presented and evaluated. Because of the longevity of sea mussels, and the sensitivity of the mussel bed community to disturbance, very low harvest rates and specialized means of responsible harvesting are discussed. A preliminary survey protocol and suggestions for collecting fishery-dependent and fishery-independent information are provided.

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