ABSTRACT: During August-September 1991, Ichthyophonus hoferi caused a mass mortality among the herring Clupea harengus population of the Skagerrak-Kattegat. This was the first record of a mass mortality of herring due to I. hoferi along the eastern coast of the North Atlantic and the first record of I. hoferi from the Skagerrak and Kattegat area. From August 1991 to December 1994 herring were sampled and examined both macro- and microscopically for signs of ichthyophonosis. Macroscopically visible signs of ichthyophonosis were found in 269 of the 25156 herring examined. Microscopic examination of different organs showed more than 4 times the prevalence than did macroscopic examinations. The ratio between the results of the micro- and macroscopical examinations, i.e. signs of passive/signs of active phase of infection, varied with the age of the fish as well as with the season. The prevalence of infection fluctuated seasonally, probably due in part to the migration pattern of herring; the spring spawning population was assumed to be the most heavily infected population of the area. The number of herring killed as a result of infection is estimated to be more than 300 million, i.e. more than 10% of the population. Differences between the recent epizootic among herring stocks due to I. hoferi and those reported from the western North Atlantic are also discussed in the 'Discussion' section.

KEY WORDS: Epizootiology · Herring · Clupea harengus · Ichthyophonus hoferi · Skagerrak · Kattegat

Published in DAO Vol. 27, No. 3 (1996) on January 12
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1996