An Ichthyophonus hoferi epizootic in herring in the North Sea, the Skagerrak, the Kattegat and the Baltic Sea.
An epidemic caused by the internal parasite I. hoferi in herring (Clupea harengus) was recorded between 1991 and 1993 around the Danish coast. A surveillance programme from research vessels and commercial fishing boats was conducted in the North Sea, Skagerrak, Kattegat and Baltic Sea. 15 769 hearts of adult herring were examined for evidence of infection. The prevalence of infection decreased from 10.6 to 2.0% in the North Sea, from 8.0 to 5.6% in the Skagerrak, from 12.0 to 1.1% in the Kattegat and from 4.5 to 0.4% in the Baltic Sea. Prevalence rates in research vessel catches were significantly higher (by 1.8 times) than in commercial fishing boat catches. The results show a high degree of temporal and spatial variation in the prevalence of I. hoferi. Infected fish were significantly longer than unaffected fish in the North Sea and the Skagerrak. The majority of the infected fish were aged 2+, 3+ and 4+ years, except in the commercial catches from the North Sea where most of the affected fish were aged 5+, 6+ and 7+ years. Annual mortalities in the different areas, based on the observed prevalences was estimated at 12.8 to 36% in 1991, decreasing to a only a few percent in 1993. The spawning stock biomass of North Sea herring was reduced by 50% between 1990 and 1995. It is suggested that this reduction may have been due to a combination of increased fishing intensity and the general effect of the I. hoferi epizootic.