The Effects of Water Temperature, Salinity, and Currents on the Survival and Distribution of the Infective Copepodid Stage of Sea Lice (Lepeophtheirus Salmonis) Originating on Atlantic Salmon Farms in the Broughton Archipelago of British Columbia, Canada

Abstract

Recent reports claim that pink salmon fry are heavily infected by Lepeoptheirus salmonis as they pass salmon farms in the Broughton Archipelago. Hydrodynamic studies reveal that the top 25 to 40 m of water generally flows seaward through the archipelago under the influence of freshwater, reducing surface salinity from 15–25% from June through November of most years. Sea lice larvae do not consistently develop to an infectious stage at salinities < 30‰—providing a natural control in the archipelago. The larvae require 4.2–5.8 days to develop to the infective stage at the temperatures typical during the time that pink salmon fry are migrating. Net current speeds measured at 15 salmon farms are predicted to carry most sea lice nauplii 7.3–10.0 km downcurrent and out of the archipelago before they become infective. A particle-tracking model predicts that nauplii released in the vicinity of two farms located deep in the archipelago are carried 10–40 km from the farms before they become infective. These predictions are consistent with the European experience and suggest little potential for salmon farms to be self-infecting or to infect migrating wild fish in their immediate vicinity, except where net current vectors are zero. A clearer picture of sea lice epizootics requires detailed knowledge of site-specific oceanographic characteristics and the affects these have on sea lice survival and dispersal during development to the infectious copepodid stage.