Farmed pink salmon (Oncorhynchus gorbuscha) have been identified as an extinction threat to wild salmon in a study published in the most recent issue of the journal Science.
Salmon lice (Lepeophtheirus salmonis) carried and transmitted to wild salmon by farmed fish are shown to be capable of wiping out the wild fish populations living in waters adjacent to fish farms in the study by Martin Krkoek, Jennifer Ford, Alexandra Morton, Subhash Lele, Ransom Myers and Mark Lewis.
The salmon louse is an ectoparasitic copepod that feeds on surface tissues of salmonid fishes, causing stress, osmotic failure, bacterial or viral infection, and ultimately death.
The louse is directly transmitted via planktonic nauplii and copepodids that can persist for several days.
The authors examined fisheries data for wild pink salmon population in rivers in the Broughton Archipelago in western Canada (a group of islands and channels about 420 km northwest of Vancouver) from 1970 to the present.
The study site covered an area in which 64 rivers were not exposed to salmon farms and seven rivers where salmon must migrate past at least one farm.
Several outbreaks of louse infestations of wild pink salmon have been recorded in this area from 2001 and 2005, with evidence showing salmon farms situated near rivers through which wild salmon migrate being the primary source of these infestations (it was observed that when farms on a primary migration corridor were fallowed, lice numbers dropped and salmon populations increased).
The researchers used mathematical models to estimate the effects of louse infestations on population parameters and to estimate the mortality of pink salmon caused by lice.
The results provide strong evidence that farm-induced louse infestations have depressed wild pink salmon populations and may cause local extinction.
The models showed that the estimated mortality from louse infestations ranged from 16% to over 97% and was commonly above 80%, exceeding previous fishing mortality.
The authors also found louse infestation to be capable of causing a 99% collapse of the population of wild pink salmon in four salmon generations.
For more information, see the paper: Krko_ek, M, JS Ford, A Morton, S Lele, RA Myers and MA Lewis (2007) Declining wild salmon populations in relation to parasites from farm salmon. Science 318, pp. 1772"1775.