"Spring rise" of whaleworm (Anisakis simplex; Nematoda, Ascaridoidea) third-stage larvae in some fish species from Norwegian waters.

The seasonal variation in the infection of saithe (Pollachius virens), cod (Gadus morhua), and redfish (Sebastes marinus) from a coastal area of central Norway with Anisakis simplex third-stage larvae (L3) was studied over a period of 1 year. In all three host species there was an increase in the abundance of the parasite in spring, with a peak appearing in March and April. Cod displayed the most distinct seasonal variation, showing a clear abundance peak in April. The abundance peak in April for redfish was not as pronounced. In saithe the abundance over the seasons was less prominent, with a maximum appearing in March and another increase in abundance occurring during July. It is apparent that the causes behind the pattern of infection observed in this study are complex. However, possible explanatory mechanisms are discussed. Arguments are propounded to suggest that the increased supply of parasite eggs from northward-migrating whales in addition to the general spring bloom of plankton constitute the most important factors governing the phenomenon of "spring rise" in A. simplex L3 in the study area.