EPIZOOTIOLOGY OF ICHTHYOPHONUS-HOFERI IN HERRING POPULATIONS OFF THE SWEDISH WEST-COAST

During August-September 1991, Ichthyophonus hoferi caused a mass morta lity among the herring Clupea harengus population of the Skagerrak-Kat tegat. 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- a nd microscopically for signs of ichthyophonosis. Macroscopically visib le signs of ichthyophonosis were found in 269 of the 25 156 herring ex amined. Microscopic examination of different organs showed more than 4 times the prevalence than did macroscopic examinations. The ratio bet ween the results of the micro- and macroscopical examinations, i.e. si gns 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 f luctuated seasonally, probably due in part to the migration pattern of herring; the spring spawning population was assumed to be the most he avily 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 epizoo tic among herring stocks due to I. hoferi and those reported from the western North Atlantic are also discussed in the 'Discussion' section.