Research Document - 2001/143

Further Investigations of the Fisheries Potential of the Exotic Varnish Clam (Nuttallia obscurata) in British Columbia

By Gillespie, G.E., Rusch, B., Gormican, S.J., Marshal, R., and D. Munroe

Abstract

arnish clams, Nuttallia obscurata, are a recently introduced exotic bivalve that have become well established in southern British Columbia. This species has attracted attention from commercial and recreational fishers and clam culturists, and has been identified as a potentially valuable fishery resource. This paper presents results of several projects to collect biological and ecological information on varnish clams, in support of fishery development.

The distribution of varnish clams in British Columbia continued to expand beyond the Strait of Georgia into Johnstone Strait and north along the west coast of Vancouver Island. They were found associated with other commercially important bivalves including Manila and littleneck clams, Venerupis philippinarum and Protothaca staminea, respectively, although generally higher in the intertidal zone. Varnish clam distribution extended lower in the intertidal zone on beaches that did not support large populations of Manila clams.

Experiments to examine competitive relationships between varnish and Manila clams showed evidence of competition when the two were placed together, with varnish clams having some competitive advantage in the upper intertidal zone and Manilas in the mid-intertidal zone.

Characteristics of harvest and processing of varnish clams were examined. Varnish clams >30 mm total length were harvested from mixed populations. Harvest efficiency was relatively high, 60-80%, for this size class. Breakage during harvest was low, approximately 2%, and shrinkage during processing was approximately 4%, evenly divided between weight loss due to water loss and losses due to mortality. Grit was purged readily from clams during wet storage within 48 hours. Commensal pea crabs, Pinnixia faba, were not purged from varnish clams even after 34 days.

Successful development of a varnish clam fishery depends on a consistent supply for the market. This could be achieved by allowing harvests of varnish clams from tenured foreshore under aquaculture permits. As the market becomes established, demand and price may allow for economically viable commercial harvest opportunities. Further work is required to develop biological information to support management of a sustainable commercial fishery

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