Analysis of variables associated with the Ichthyophonus hoferi epizootics in Norwegian spring spawning herring, 1992-2008

We present a long-term study of epizootics in a migratory fish species that shows major advancements in our understanding of marine disease ecology. On the individual macroscopic level, Ichthyophonus hoferi infections in herring significantly reduced body condition (p < 0.01) and reproductive capacity (gonad weight, p < 0.001). Infection prevalence increased with age (p < 0.001, R-adj(2) = 0.71). Prevalence in the catches strongly depended on target school size and was thus higher in trawl catches. We found strong seasonality in infection prevalence within years, with peaks in summer and winter. Summer peaks spatially formed an infectious belt'' off the west coast of Norway, consisting of infected fish unable to follow the migration routes to the feeding grounds. Prevalence varied also between years (p <= 0.001). Since 1992, it had declined from stock prevalence levels of 10%, down to almost extinction, with another major peak in 1999. The annual peaks could best be associated with strong year classes (Pearson's r = 0.56), a good overall body condition (r = 0.86), and plastic wintering behaviour.