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The effect of predator netting on clam recruitment in Baynes Sound, B.C. with a special focus on the response of the Manila clam (Venerupis philippinarum) Munroe, Daphne Marie


Passive and active forces determine the patterns of settlement of invertebrate larvae. Research efforts into larval settlement have been dominated by attached and conspicuous species in hard substrate environments. Here, data on early recruitment patterns of a mobile bivalve species from a soft-sediment habitat is provided. In particular, how intertidal clam aquaculture netting influences the distribution of settling pediveliger larvae was investigated. Early recruitment patterns of Manila clam larvae (Venerupis philippinarum) were examined in relation to predator netting used in farming clams in British Columbia. A method for sampling recent settlers from intertidal sediments was developed, proven effective and employed to sample settled clams (<600 μm shell length) from four sites in Baynes Sound, on the eastern side of Vancouver Island, B.C. in 2003 and 2004. Paired netted and non-netted plots were compared for number of early recruits. Plots with the netting and high density of adult clams experienced lower levels of settlement. Settlement varied annually with 2003 experienceing [sic] an order of magnitude less recruitment than 2004. In addition, laboratory tests were run using flumes to examine the retention of competent clam larvae within flumes with netting on the bottom. No difference in the retention of clam larvae was observed due to netting or sediment treatments. Sediment properties (sediment grain size, organic carbon and inorganic carbon) were also compared between netted and non-netted plots. No difference was seen in the sediment properties measured except for slightly higher levels of organic carbon beneath nets; this was likely due to the higher number of adult clams beneath the nets. The netting buffers temperature at the sediment surface during tidal exposure by up to 3°C, the biological relevance of this remains untested. No increase in sedimentation was measured beneath netting; however, decreased bivalve settlement beneath netting was observed but only in the year when overall settlement was high. This decrease in recruitment was not supported by the flume trials; however these were run at one velocity. Trials at different velocities may produce different results. These field observations are an important contribution to understanding larval settlement of mobile species in a soft-sediment habitat.

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