An ichthyophoniasis epizootic in Atlantic herring in marine waters around Iceland

A widespread ichthyophoniasis epizootic occurred in the Icelandic summer-spawning herring (Clupea harengus) stock during the years 2008-2014. The spatial and temporal variation in prevalence of heart lesions and the seasonal development of the disease in the stock were explored with an inspection of hearts. The year classes from 2004 to 2006 had generally the highest prevalence of heart lesions, varying from similar to 47%-50% in 2009 to similar to 31%-34% in 2014. Newly developed disease was apparently occurring in the autumns of 2008-2010 but not in the autumns thereafter. Analyses of the data strongly suggest that the disease caused mortality in 2009-2011 but was insignificant thereafter, and consequently, the epizootic waned after mid-2011. The analytical assessment model used for the stock gave the best fit to the data when applying disease mortality equivalent to 30% of the diseased herring for these 3 years. Thus, we conclude that the widespread ichthyophoniasis epizootic in the stock over these 6 years caused significant mortality in the stock during the first 3 years but less mortality than suggested in other publications.