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Epizootics of fish - Wild and Farmed

Salmonids Health

By Martin Krkosek, Mark A. Lewis, Alexandra Morton, L. Neil Frazer, and John P. Volpe - The continuing decline of ocean fisheries and rise of global fish consumption has driven aquaculture growth by 10% annually over the last decade.


The association of fish farms with disease emergence in sympatric wild fish stocks remains one of the most controversial and unresolved threats aquaculture poses to coastal ecosystems and fisheries. We report a comprehensive analysis of the spread and impact of farm-origin parasites on the survival of wild fish populations.

We mathematically coupled extensive data sets of native parasitic sea lice (Lepeophtheirus salmonis) transmission and pathogenicity on migratory wild juvenile pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon. Farm-origin lice induced 9–95% mortality in several sympatric wild juvenile pink and chum salmon populations.

The epizootics arise through a mechanism that is new to our understanding of emerging infectious diseases: fish farms undermine a functional role of host migration in protecting juvenile hosts from parasites associated with adult hosts. Although the migratory life cycles of Pacific salmon naturally separate adults from juveniles, fish farms provide L. salmonis novel access to juvenile hosts, in this case raising infection rates for at least the first 2.5 months of the salmon's marine life (80 km of the migration route).

Spatial segregation between juveniles and adults is common among temperate marine fishes, and as aquaculture continues its rapid growth, this disease mechanism may challenge the sustainability of coastal ecosystems and economies.

Further Information

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To read the response from the BC Salmon Farmers Association, click on the title below.

An analysis presented to the BC Government Special Committee on Sustainable Aquaculture (PDF)

The recent paper by Krkosek et al. (2006) entitled “Epizootics of Wild Fish Induced by Farm Fish” has been widely advertised as “proof” that wild pacific salmon juveniles in British Columbia (specifically in the Broughton archipelago) are being “devastated” by farmed salmon. The paper reports the development of a mathematical model which aspires to describe the acquisition of sea lice by out-migrating juvenile pink and chum salmon, with the assumption that the principal source of infective larval sea lice are farmed salmon.

November 2006