Temporal Patterns of Sea Louse Infestation on Wild Pacific Salmon in Relation to the Fallowing of Atlantic Salmon Farms

—We report on a 3-year study of the infestation rates of the sea louse, Lepeophtheirus salmonis, on wild juvenile pink salmon Oncorhynchus gorbuscha and chum salmon O. keta in the Broughton Archipelago, British Columbia. In 2002, the British Columbia Ministry of Agriculture, Fisheries, and Food ordered farm fallowing (i.e., the removal of farmed Atlantic salmon Salmo salar from net-cages) along the presumed migration route of wild juvenile Pacific salmon in this area. The goal was to protect wild juvenile fish from sea louse infestation. We assessed the effectiveness of this decision by comparing sea louse infestation rates on wild juvenile salmon near three Atlantic salmon farm sites prior to, during, and after fallowing. Overall, L. salmonis levels were significantly reduced (P , 0.0001) at the study sites during fallowing but returned to the original level after fallowing. The decline was age specific. While the abundance of the earliest attached sea louse phase (the copepodid stage) declined by a factor of 42, the mean abundance of adult L. salmonis did not decline significantly. Changes in salinity and temperature could not account for the decline. This study provides evidence that the fallowing of Atlantic salmon farms during spring juvenile salmon migrations can be an effective conservation and management tool for protecting wild salmon. While this correlation adds to the increasing weight of evidence linking Atlantic salmon farms to increased parasite loads on wild salmon, greater cooperation between researchers and farmers will be necessary to isolate the causal mechanisms and provide safe seaward passage to wild juvenile salmon. The farming of Atlantic salmon Salmo salar is correlated with infestation of the sea louse Lepeophtheirus salmonis on juvenile salmon in Europe (Tully et al. 1993; Birkeland 1996; Heuch et al. 2005) and Canada (Morton and Williams 2003; Morton et al. 2004). While other nonsalmonspecific sea louse species have been recorded on salmon, they are not known to have pathogenic effects on salmonid hosts. Lepeophtheirus salmonis is a salmon-specific marine sea louse endemic to the Northern Hemisphere (Pike and Wadsworth 1999). Its naupliar larvae are free-swimming for approximately 4 d at 108C before attaining infective capability (copepodid stage) (Johnson and Albright 1991a). Copepodid-stage sea lice can survive without a * Corresponding author: [email protected] Received September 2, 2004; accepted December 8, 2004 Published online June 30, 2005 host for approximately 5 d at 108C (Boxaspen and Næss 2000). Lepeophtheirus salmonis molts through eight stages from attachment to adulthood at approximately 4 weeks of age, permitting uncomplicated approximation of age within this period (Johnson and Albright 1991a, 1991b). Optimal salinity for L. salmonis survivorship and reproduction is 30‰ (Johnson and Albright 1991b), and copepodids can tolerate salinity levels as low as 15‰ (Pike and Wadsworth 1999). Sustained temporal presence of the earliest attached juvenile stages (copepodid and chalimus I) of sea lice on a population of fish indicates chronic re-infestation and a local source of gravid female sea lice. This characteristic is specific to infestations found on wild salmon near salmon farms (Pike and Wads-