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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 25 156 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 lulled 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.

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ABSTRACT. Very high frequencies, up to 80%, of fin erosion of perch Perca fluviatiljs and ruffe Gymnocephalus cernua have been found in the area of a pulp mill releasing bleached kraft mill effluents, situated at the Swedish coast of the Gulf of Bothnia. In the effluent area, which was studied between 1984 and 1988, a gradient was established of both acute and healed fin erosion, showing decreasing frequencies with increasing distance from the pulp mill outlet. No fish from a reference area were found with these fin damages. Acute fin erosion was not found after 1984 which was congruent with a distinct decrease in discharge from the pulp mill. However, healed stages of fin erosion, i e. regenerated fin rays resulting in a very specific scar on the fins, were still present with highest frequencies closest to the mill. The study clearly indicates that fin erosion of perch and ruffe in the effluent area was caused by the bleached kraft mill effluents from the pulp rndl.