Evidence for Declines in the Native Leukoma staminea as a Result of the Intentional Introduction of the Non-native Venerupis philippinarum in Coastal British Columbia, Canada

Abstract

On the west coast of British Columbia (BC), Canada, the intertidal regions are under ever increasing pressure to be used primarily for near-bottom mariculture practices. These include seeding the foreshore with the nonindigenous Venerupis philippinarum (Manila clam) followed by the application of antipredator netting. The Manila is confamilial with the indigenous Leukoma staminea (littleneck clam), and seeding could possibly provide a competitive advantage for the Manila leading to the extirpation of the littleneck within coastal BC. Over two survey years, nine and seven farm-reference paired beaches (18 and 14 beaches) from three geographically distinct regions of BC were sampled for abundances and size class structure of the Manila and littleneck clam. Several lines of evidence suggest that the Manila is replacing the native littleneck; (1) comparison of average abundances of the littleneck versus Manila showed a significantly greater number of Manila on both farmed and reference sites across all three regions (p < 0.05; years I and II); (2) as distance between farmed and reference sites increased, numbers of littlenecks on reference sites also increased suggesting that close proximity to farmed sites increases the effect of seeding on numbers of the native species; (3) comparison of percent similarity of the population structure as determined by size class distribution for the Manila versus the littleneck clam indicated that intertidal reference sites in close proximity to farmed sites tended to be more similar to each other with respect to similar Manila population structure versus that of the littleneck (r = 0.4; p = 0.08); (4) within the region where active farming is the most aggressive (Baynes Sound), Manila clams are the dominant bivalve on all sampled intertidal regions; and (5) where the dispersal of “wild” (not-seeded) Manila clam is prevented because of thermal tolerance limits, the littleneck is the dominant bivalve. Ecological consequences of such species replacement on intertidal function are as yet known.