Perspectives on Pink Salmon and Sea Lice: Scientific Evidence Fails to Support the Extinction Hypothesis

Using a Ricker (1975) model and escapement data for a subset of Broughton Archipelago, British Columbia watersheds, Krkosek et al. (2007a) predicted that sea lice infections originating on Atlantic salmon (Salmo salar) farms will cause the extinction of pink salmon (Oncorhynchus gorbuscha) in the archipelago by 2010. The purpose of this article is to examine this issue in the context of all of the escapement data available for the archipelago and to review additional scientific reports and information not discussed by Krko. sek et al. (2007a). Additional research during the last five years is not consistent with the Krkosek et al. (2007a) conclusion that sea lice routinely cause in excess of 80% mortality of fry. Rather, the literature reviewed herein indicates that pink salmon fry mount an effective immune response at sizes as small as 0.7 g, resulting in the rapid shedding of lice within two weeks. Pink salmon returns are shown to be highly variable throughout the Northeast Pacific in areas without salmon farms. Following periods of high abundance, pink salmon populations typically fall to low levels, and they may remain depressed for several generations. However, in most cases, the populations then gradually increase to begin the cycle anew. An examination of returns to all of the documented Broughton Archipelago watersheds indicates that following exceptionally high returns in 2000 and 2001, the populations declined to very low numbers in 2002 and 2003. Contrary to the conclusions reached by Krkosek et al. (2007a), Broughton pink salmon returns have steadily increased since then, with no indication that they are threatened with extinction. Other unsubstantiated assumptions used in Krko. sek et al. (2007a) are also discussed in light of additional scientific reports and theoretical considerations.