AN ICHTHYOPHONUS-HOFERI EPIZOOTIC IN HERRING IN THE NORTH-SEA, THE SKAGERRAK, THE KATTEGAT AND THE BALTIC SEA

An epizootic caused by the internal parasite Ichthyophonus hoferi in h erring Clupea harengus was recorded from 1991 to 1993 in the waters ar ound Denmark. A surveillance programme from research vessels and comme rcial fishing boats was conducted in the North Sea, Skagerrak, Kattega t and Baltic Sea. A total of 15769 hearts of adult herring were examin ed macroscopically for evidence of infection. The prevalence of the in fection in this period decreased from 10.6 to 2.0% in the North Sea, f rom 8.0 to 5.6% in the Skagerrak, from 12.0 to 1.1% in the Kattegat an d from 4.5 to 0.4% in the Baltic Sea. Prevalence rates in research ves sel catches were significantly higher (by 1.8 times) than in commercia l fishing boat catches. The results show a high degree of temporal and spatial variation of the prevalence of I. hoferi. Infected fish were found to be significantly longer than unaffected fish in the North Sea and the Skagerrak. The majority of the infected fish belonged to the age groups 2+, 3+ and 4+ except in the commercial catches from the Nor th Sea where most of the affected fish belonged to the age groups 5+, 6+ and 7+. Annual mortality in the different areas, based on the obser ved prevalences was roughly estimated at 12.8 to 36% in 1991, decreasi ng to a few percent in 1993. The spawning stock biomass of North Sea h erring was reduced by 50% during the period 1990 to 1995. This reducti on may have been due to a combination of increased fishing intensity a nd the general effect of the I. hoferi epizootic.