EFFECT OF PREDATOR NETTING ON RECRUITMENT AND GROWTH OF MANILA CLAMS (VENERUPIS PHILIPPINARUM) ON SOFT SUBSTRATE INTERTIDAL PLOTS IN BRITISH COLUMBIA, CANADA

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This paper examines patterns of larval clam settlement on soft-bottom intertidal plots in British Columbia, Canada relative to predator netting (used in beach culture of clams) and adult populations. We studied four sites over two years. Each site had a netted and nonnetted plot where level of clam recruitment was compared. Density (spat/m2) of early recruits was measured for each plot at each site. We found that year (P = 0.0001) was a significant factor, and netting was weakly significant (P = 0.064) in influencing early recruitment of clams. Netted plots also contained high densities of adult clams. Adult filter feeders have been presumed to decrease larval settlement by filtration of recruits. We found that at lower adult density, the relationship between density of spat and density of adults was negative (as predicted). However, at higher densities of adults (450/m2), this relationship no longer held and spat density increased slightly with increase in adult density. Neither presence of netting nor extremely high adult density prevented recruitment of clam larvae, and in plots with nets and high numbers of adults there remained a strong level of recruitment. Our results showed recruitment of bivalve larvae is variable annually and also varies among sites, highlighting the importance of adequate replication of sites and samples to ensure accurate reflection of recruitment patterns. We also found that other factors, netting and adult density, can influence the level of recruitment. Further research is needed to determine whether the magnitude of influence will translate into changes in ultimate population patterns.