Field assessment of Pacific oyster (Crassostrea gigas) growth and ingestion of planktonic salmon louse (Lepeophtheirus salmonis) larvae at an Atlantic salmon (Salmo salar) farm in British​ Columbia, Canada

Integrated culture of bivalves at marine finfish farms may help lessen nutrient loading while producing a commercially-valuable crop and increasing the farms' social license to operate. Integrated culture of filter-feeding bivalves at salmon farms may also provide natural mitigation of the occurrence of planktonic sea lice larvae. This study assessed the growth of Pacific oysters (Crassostrea gigas) cultured at a commercial Atlantic salmon (Salmo salar) farm in British Columbia (BC) and the extent to which the cultured oysters contributed to mitigation of the occurrence of the salmon louse (Lepeophtheirus salmonis). Oysters were deployed in trays at 1-, 3-, and 6-m depths around one end of the farm and at a reference site approximately 150 m away. The height, length, and width of the shell and weight [whole wet, soft-tissue wet, dry, and ash-free dry (organic) weights] of oysters were measured at intervals following deployment. Salmon louse reduction was assessed monthly by comparing the water-borne density of larval sea lice among three bivalve cages and three controls (non-bivalve cages), and by examining oyster digestive tracts for L. salmonis DNA using PCR. All seven oyster-size variables increased significantly over time with significant effects of depth and position around the farm. In general, oysters at 1 and 3 m were significantly larger than those at 6 m. Side of the fish cage was used as a blocking factor in the experimental design and had a significant effect on final oyster size; at the end of the study, oysters at the farm were either significantly larger or not significantly different than oysters at the reference site, depending on the side of deployment. There was no significant variation in mean larval density due to time or treatment (bivalve versus non-bivalve). Larval lice densities were highest in January 2014. However, at that time there was no evidence of L. salmonis DNA in oyster digestive tissues.