Ecological implications of intertidal mariculture: observed differences in bivalve community structure between farm and reference sites

Abstract

Summary Despite recent growth in shellfish aquaculture in British Columbia, Canada, the impacts of common practices on non‐target species are poorly understood. Two practices employed on clam farms to increase production of the exotic clam Venerupis philippinarum include the addition of juvenile ‘seed’ clams to the sediment and covering seeded clam beds with protective netting, ostensibly to exclude large mobile epibenthic predators. We expected the effects of predator exclusion to be most evident among other bivalves, which made up more than 80% of the infaunal macrobenthos at all sites surveyed. A field study across three regions collected infaunal bivalve density and biomass data. We compared species richness, composition and abundances of communities between clam farms and reference sites, paired on the basis of physical characteristics such as sediment type, slope and aspect. Venerupis philippinarum was the only species found in higher abundance on farm sites in low intertidal areas (227 ± 241·6 clams m−2, P= 0·02; 872·9 ± 792·9 g m−2, P= 0·037). Farmed sites showed no difference in mid‐intertidal areas, nor in density of the other 25 bivalve species, although an increase would be expected if netting excluded important predators. Although statistically non‐significant, there were indications that biomass of species other than V. philippinarum may have been lower on farm sites. Bivalve species composition was not significantly different between farm and reference sites. Nevertheless, farm sites were more similar to each other as a group than reference sites, leading to a loss of regional distinctness that was evident among reference sites. Synthesis and applications. Our findings support the hypothesis that predation and competition play minor roles in structuring communities in soft‐bottomed environments. Given the potential for cumulative effects of seeding and netting at large scales, a precautionary approach is recommended in future development of intertidal clam aquaculture.