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
Original version
This report is not to be cited without prior reference to the authorsAbstract
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.