Estimated Sea Louse Egg Production from Marine Harvest Canada Farmed Atlantic Salmon in the Broughton Archipelago, British Columbia, 2003–2004

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Abstract
Recent infestations of sea lice Lepeophtheirus salmonis on wild juvenile pink salmon Oncorhynchus gorbuscha and subsequent declines in the number of returning adult pink salmon have raised concern for the health of wild fish relative to salmon farming activities in the Broughton Archipelago, British Columbia. I used available (but limited) industry data to estimate sea louse egg production from Atlantic salmon Salmo salar farmed by Stolt Sea Farm (now Marine Harvest Canada, Inc., Campbell River, British Columbia) in 2003 and 2004. The 12 active farms contained between 1 and 5 million Atlantic salmon during the 2 years and about 800,000 fewer mature salmon at the start of 2003 than in 2004. Sea louse egg production peaked during winter–spring in both years prior to the seaward migration period of the area's small and vulnerable juvenile pink salmon and chum salmon O. keta. Marine Harvest Canada salmon hosted over 6 million gravid sea lice that produced 1.6 × 109 eggs during 2 weeks in the winter of 2003–2004. Only half as many eggs were produced from the fewer hosts present during this period in 2003. Sea lice on farmed fish were further reduced to near zero each year through multiple uses of emamectin benzoate (Slice). Fewer farmed Atlantic salmon and sea lice in 2003 coincided with lower abundance and prevalence of L. salmonis on juvenile pink salmon and chum salmon near farms. A recent agreement between industry and conservationists may help improve data quality, our understanding of the dynamics sea louse–salmon interactions, and our chances of conserving wild salmon.

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