Infection of Baltic herring (Clupea harengus membras) with Anisakis simplex larvae, 1992–1999: a statistical analysis using generalized linear models

Anisakis simplex larvae were counted in the body cavity of Baltic herring sampled in the southern Baltic in 1992-1993, 1995-1997 and 1999. Generalized linear models (GLM) were used to model the prevalence and intensity of infection as dependent on year, quarter, area, length, condition factor, sex, and stage of gonad development. A binomial error distribution was assumed for the prevalence and a negative binomial distribution for infection intensity. The model for prevalence explained 71% of the deviance and all factors and variables tested were highly significant. The model for infection intensity explained 26% of the deviance and significant effects were length, year, quarter, area and sex. Prevalence increased with both length and condition factor, while decreasing eastwards and being higher in coastal areas than in offshore waters. Highest values were observed in the 1st and 2nd quarters (spawning season), and almost no infection was observed in the 3rd quarter. Both prevalence and intensity were higher in males than females. Prevalence has been stable from 1993 onwards, while intensity increased sharply in 1997 and 1999.