Research Document 1998/167

Options for controlling disease and improving the health in farmed salmon, as a means of reducing risks posed by escapes

By A. McVicar

Abstract

Wherever possible, the prevention of infection is preferable to treating established disease in fish farms. As regional variations exist in the natural occurrence of fish disease, the prevention of the spread of new diseases to new areas should form a cornerstone of sensible disease control. At the international, national and local levels, there has been considerable success in controlling the spread of important diseases both by regulation and by voluntary codes of practice. Failures have occurred, leading to the appearance of previously exotic diseases and the evaluation of such cases has led to the continued evolution of preventative measures.

Most farmed fish are continually exposed to the range of naturally occurring infections present in the local environment and these are the main cause of disease in fish farms. Of potential importance to wild salmon is the risk of an abnormally high focus of infections being established in an area. However, not all of these infections can cause significant pathogenicity. The development of a disease condition from an infection and its severity is a consequence of the complex interaction between environment, host and pathogen. Options for control of disease can lie in any one of these areas. Management of stress in farmed fish populations is a recurring theme in disease control in fish farms, although it is inevitable that the level which can be achieved has to be tempered by commercial reality. Similarly, the interruption of disease cycles on individual farm sites or in larger areas, through management policies such as fallowing, has both financial implications and consequences on the number of sites required. Many significant infections have responded well to treatment with chemicals. In general, such an approach to disease control should be considered to be a temporary way of achieving respite and that longer term solutions should lie in achieving enhanced resistance in the farmed fish or in management approaches.

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