Disease interaction between farmed and wild fish populations

This paper reviews the literature on disease interaction between wild and farmed fish and recommends strategies to reduce the disease risks to both populations. Most, if not all, diseases of farmed fish originate in wild populations. The close contact between farmed and wild fish readily leads to pathogens exchange. Aquaculture creates conditions ( e.g. high stocking levels) conducive to pathogen transmission and disease; hence pathogens can overspill back, resulting in high levels of challenge to wild populations. This is exemplified by sea lice infections in farmed Atlantic salmon. Stocking with hatchery reared fish or aquaculture escapees can affect disease dynamics in wild populations. Whirling disease has been spread to many wild rainbow trout populations in the US with the release of hatchery reared stock. The greatest impact of aquaculture on disease in wild populations has resulted from the movement of fish for cultivation. Examples of exotic disease introduction following movement of live fish for aquaculture with serious consequences for wild populations are reviewed. The salmon parasite, Gyrodactylus salaris, has destroyed wild salmon populations in 44 Norwegian rivers. Crayfish plague has wiped out European crayfish over much of Europe. Eels numbers have declined in Europe and infection with the swimbladder nematode Anguillicola crassus has in part been blamed. The impact of disease in farmed fish on wild populations can mitigated. Risk analysis methods need to be refined and applied to live fish movement and new aquacultural developments. Appropriate biosecurity strategies, based on risk assessments, should be developed to reduce pathogen exchange and mitigate the consequences.