Migration No Longer Separates Salmon From Sea Lice
Analyses of several sets of data finds mortality rates of 9 to 95% among various juvenile wild salmon populations as they migrate past salmon farms east of Vancouver Island. The farms have short-circuited the separation of adult salmon from juveniles that is maintained by migration patterns. When wild salmon swim past Atlantic salmon farms, they come in close proximity with adult salmon and pick up the parasites at an unnaturally young age.
Sea lice parasitize adult wild salmon without killing them. But for juvenile salmon, exposure to sea lice is deadly. Young fish under natural conditions do not encounter lice until they are at sea for several months. The migrations of Pacific salmon separate wild adults from juveniles, minimizing transfer of lice to the vulnerable young fish.
Along the 80 km migration route surveyed in BC's Broughton Archipelago, a distance that takes about 2.5 months for these fish to travel, sea lice on juvenile chum and pink salmon originated primarily from salmon farms. Wild salmon carrying three or more motile sea lice generally succumbed, while nearly all uninfected salmon survived. Lice further along in their life cycle were more lethal to the fish.
Martin Krkosek, Mark A. Lewis, Alexandra Morton, L. Neil Frazer and John P. Volpe. 2006. Epizootics of wild fish induced by farm fish. PNAS Proceedings of the National Academy of Sciences. 103(42): 15506–15510.