Trawl avoidance as a source of error in estimates of the prevalence of Icthyophonus hoferi disease in Norwegian spring spawning herring (Clupea harengus L.) in the feeding area

Summary: Results are presented from a survey in the Norwegian Sea in the summer 1993, where the abundance and distribution of herring was investigated, using a high resolution sonar and pelagic trawling at the surface. An index of avoidance was constructed by comparing the catch to be expected from the sonar registrations of the number of schools and the amount of scattered registrations, with the actual catch in each haul. This index was negatively correlated to the frequency of diseased herring in the catch. This indicates that diseased herring is less capable of avoiding the trawl. Application of this result for estimating the true disease prevalence is discussed