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Atlantic salmon smolts, previously unexposed to infectious pancreatic necrosis virus (IPNV), were placed into tanks of sea water at 10 °C. After 4 weeks, 40 fish were injected intraperitoneally (i.p.) with homogenized and filter‐sterilized kidney material obtained from salmon with clinical IPN in a marine farm in Shetland. The injected fish were cohabited with 40 untreated fish. Mortalities began in the injected fish on day 7 and reached a peak of 48% on day 14. In the cohabitation group, mortalities began on day 14 and reached a peak of 70% on day 27. The IPNV in the Shetland kidney homogenate was cultured in Chinook salmon embryo (CHSE) cells and passed twice. This cultured virus was injected i.p. into fish at various doses ranging from 10 to 107 TCID50 fish−1 4 weeks after seawater transfer. Challenge tanks contained 30 injected fish and 30 cohabitees. Mortality rates and levels were dose‐dependent. The highest dose used resulted in a similar mortality pattern as obtained with a similar dose of the Shetland kidney homogenate, indicating that virulence was retained after two passes in tissue culture. Even with the lowest dose, mortality reached 12% in the injected group and 23% in the cohabitees. The IPNV titres were high (106−109 i.u. g−1 kidney) in fish which died during the experiment and low (<105 i.u. g−1 kidney) or undetectable in surviving fish. The cultured virus (pass 3) was used in a challenge model where the population density of fish in the tanks was high (50 injected and 50 cohabitees) or low (15 injected and 15 cohabitees). In the high stocking density tank, mortalities peaked at about 35% in the injected group and at 52% in the cohabitees. In the low stocking density tank, mortalities peaked at about 40% in the injected fish but no mortality occurred in the cohabitees. However, IPNV was detected (up to 104 i.u. g−1 kidney) in 82% of cohabitees sampled on day 30. These data suggest that lethal lateral transmission of the virus is dependent on the infectious pressure from the injected group. A further trial was conducted to investigate the effect of time post‐seawater transfer on the susceptibility of post‐smolts to IPN. Groups of fish were challenged every 2 weeks from week 0–10. Few mortalities occurred at week 0 and virus titres were high in these fish. Most survivors became carriers, some with titres >106 i.u. IPNV g−1 kidney. From 2 to 10 weeks after seawater transfer, mortalities in both injected and cohabitees were substantial with viral titres >107 i.u. g−1 kidney. Survivors had lower titres and in many virus was undetectable. Throughout the experiments, moribund fish were sampled for histology and all showed typical IPN histopathology.

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Given the rapid development of intensive culture systems for cold-water marine gadoids such as cod (Gadus morhua), haddock (Melanogrammus aeglefinus), hake (Merluccius spp.), and ling (Molva molva), an urgent need exists to examine likely threats posed by disease in intensive gadoid culture. While little is known about diseases that may affect or be introduced by cold-water marine gadoid fish once the animals are brought into intensive commercial culture systems, certain risks can be inferred both from an examination of disease problems experienced during the introduction of intensive culture systems for other cold-water marine species and from an examination of the existing literature on gadoid pathogens. This paper provides an overview of the potential disease risks associated with the development of intensive gadoid aquaculture and focuses on previously characterized diseases that the authors consider as posing the greatest threat to intensive gadoid culture. In addition to disease risk, this paper discusses risk reduction, including vaccination strategies and the risk of inter-species infection. Specific viral, bacterial, fungal, protoctistan, and metazoan pathogens are discussed in light of their previous implication in the causation of disease in cultured gadoids or other cold-water species, their occurrence and pathogenicity in wild gadoids, and with respect to life strategies and biology that may favour their propagation within intensive aquaculture systems.

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