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 churn 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.