Ichthyophonus hoferi disease in the herring in Norwegian waters

After the discovery of I. hoferi disease in Norwegian spring spawning herring in summer 1991, a program for systematic surveillance of the disease in the herring in Norwegian waters was developed. The programme, which aims primarily at monitoring the prevalence of the disease, is described. Macroscopic lesions in the heart emerged as the standard diagnostic criterium. The results so far indicate that the situation for Norwegian spring spawning herring is not alarming, altohough it still deserves attention. For the North Sea herring, the situation is less clear. The diseased herring seem to be less prone to follow normal scooling behavior, which causes sampling problems that must be taken into account when interpreting prevalence results