Chapter 2 - Repeated Incidents of Abrupt and Persistent Recruitment Failures in Gadoids in Relation to Increasing Eutrophication, 1919–2001

Abstract

Management, surveillance, and studies of ecosystems and their constituent species are generally based on the assumption of simple dose-response relationships: Gradual environmental changes or perturbations are expected to cause corresponding changes in the abundance of affected species. However, a unique time series (since 1919) of 0-group gadoid abundance data from the Norwegian Skagerrak coast shows repeated incidents of abrupt and persistent recruitment collapses in gadoid fishes, which are attributed to gradually increasing nutrient loads. After the collapses, there has been no sign of recovery, nor have other fishes replaced the gadoids. A number of candidate causes of the recruitment collapses are unlikely, such as overfishing, hypoxia, habitat degradation through changes in bottom vegetation, and direct impact of contaminants on fish and spawning products. It is suggested that nutrients, which have a direct impact on phytoplankton, have caused shifts in the plankton community that have deprived young-of-the-year gadoids of their natural prey.