Commercial Manila clam (Tapes philippinarum) culture in British Columbia, Canada: The effects of predator netting on intertidal sediment characteristics

Quantifying risks posed by aquaculture to adjacent coastal ecosystems is necessary to ensure long term stability of coastal systems and the sustainability of industries that exist therein. Research has demonstrated that the use of predator netting in shellfish aquaculture increases sedimentation rates and productivity; here we examine the influence of netting on the west coast of Canada. Changes in percent silt (sediment particles <63 μm), percent gravel (sediment particles >2 mm), organic and inorganic carbon levels and temperature, and differences in clam populations were monitored on paired netted and non-netted Manila clam (Tapes philippinarum) plots on four farmed beaches at Baynes Sound, British Columbia in 2003 and 2004. There were no significant differences in the levels of silt (p = 0.129, n = 8), gravel (p = 0.723, n = 8), or inorganic carbon (p = 0.070, n = 8) between netted and non-netted plots. However, the level of organic carbon was significantly higher on netted plots (p = 0.014, n = 8) and a slight temperature buffering effect of the netting during low-tide events over the period of study. There were significantly more T. philippinarum on netted plots compared to non-netted plots (p = 0.001, n = 8) and the length frequency distribution of the populations also differed (p < 0.00001) with non-netted plots containing slightly smaller clams. The observed increase in organic carbon levels beneath netting is possibly due to biodeposition by T. philippinarum beneath nets and removal of organics by the deposit feeding Nuttallia obsurata on non-netted plots; however that was not tested here. For the locations and parameters monitored in this study, it appears that netting and clam farming in Baynes Sound British Columbia, has limited effect on the sediment.