A sediment bioassay to assess the effects of aquaculture waste on growth, reproduction, and survival of Sphaerium simile (Say) (Bivalvia: Sphaeriidae)
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Increasing concerns regarding the environmental sustainability of freshwater cage aquaculture have encouraged new research on the potential impacts of this industry. A trout aquaculture facility was operated in Lake 375 at the Experimental Lakes Area (ELA) in northwestern Ontario, Canada, in 2003 and 2004. As part of this experiment, the spatial extent and magnitude of the potential impacts of aquaculture waste on the benthic community were assessed in 2004 through a novel sediment bioassay using the fingernail clam Sphaerium simile (Say) (Bivalvia: Sphaeriidae). Survival, growth, reproduction, and metal body burdens were evaluated for individual clams exposed for 6 weeks to sediment collected from directly below the fish cage, and from 1, 3, 5, 8, and 50 m away. The clams exposed to sediment from directly under the cage experienced 100% mortality, and this was attributed to the lower bulk density of the sediment (0.09 g cm-3 dw) that was heavily influenced by the sedimentation of farm waste, as compared with sites further from the cage (0.11-0.12 g cm-3 dw). In addition, copper and zinc concentrations were 5 and 7 times greater in the sediment from directly under the cage at mean concentrations of 109 and 1054 μg g-1 dw, respectively, compared to all other sites at 14-22 and 116-152 μg g-1 dw, respectively, and may have also contributed to the mortality of these clams. Clams in all other sediment treatments experienced no mortality and had low tissue concentrations of copper (3.9-5.4 μg g-1 dw) and zinc (23.9-32.3 μg g-1 dw), consistent with the low concentrations of copper and zinc observed in all of these sediments. Clams exposed to sediment from 1 m from the cage had significantly greater growth than clams exposed to sediments collected 3, 5, and 8 m from the cage. There were no significant differences in average embryo size, but the number of embryos per adult was significantly greater in the clams exposed to the 1 m sediment than those exposed to the 50 m sediment. No significant differences were detected in sediment nutritional quality as measured by C:N ratios and organic content, which were both similar between the 1, 3, 5, 8, and 50 m sites. Results from this study suggest that the effects of rainbow trout (Oncorhynchus mykiss Walbaum) aquaculture on fingernail clams would mainly occur directly under or within a close radius of the cage. This sediment bioassay was an effective method for testing the impacts of cage culture on the growth, reproduction and survival of benthic invertebrates such as the fingernail clam.
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