| Summary: | The close proximity of net-pen salmon farms and wild Pacific salmon stocks in British Columbia (BC) is an incentive for precautionary management of the environmentally and economically damaging parasites known as sea lice. Bivalves cultured as part of an integrated multi-trophic aquaculture (IMTA) system may contribute natural, preventative louse control through the ingestion of planktonic sea lice larvae. A field trial was conducted to test sea lice mitigation by bivalves at a commercial Atlantic salmon (Salmo salar) farm in BC using Pacific oysters (Crassostrea gigas). Oysters were cultured in trays around one end of the farm and at a reference site approximately 150 m away from August 2013 until August 2014. Parasitic and planktonic sea lice (Lepeophtheirus salmonis and Caligus clemensi) were monitored before and during oyster deployment, beginning in December 2012. Parasite abundance peaked in January 2013 (6.5 lice/fish, >85% C. clemensi), and the following year in February 2014 (3.3 lice/fish, >80% L. salmonis). Larval density within cages peaked in January, both in 2013 (1.28 larvae/m3) and 2014 (0.96 larvae/m3). Parasite abundance was significantly correlated with both surface salinity (r2= 0.28, p=0.04) and sea lice larval density (r2= 0.65, p=0.01). Observed densities were significantly lower (t=3.41, p=0.009) than those calculated for the site based on water temperature and salinity, the number of adult female lice present, and the approximate number of fish. Sea lice mitigation by oysters was assessed by comparing monthly sea lice larval densities inside bivalve and non-bivalve fish cages, and by analyzing preserved oyster digestive tracts from January 2014 (when larval densities were highest) for presence of L. salmonis DNA using PCR. Using these methods, no significant evidence of sea lice mitigation was detected. Oyster growth was monitored by measuring whole wet weight, soft tissue wet, dry, and ash-free dry weight, and shell length, width, and height approximately every four months. ...
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