Modeling Pulse Disturbance Impact on Cod Population Dynamics: The 1988 Algal Bloom of Skagerrak, Norway

This study focuses on the population dynamic effects of a pulse disturbance (a toxic algal bloom in the early summer of 1988) on cod (Gadus morhua) populations along the Skagerrak coast of Norway. For this purpose, we applied "intervention analysis" (sensu G. E. P. Box and G. C. Tiao). For the marine system studied, we estimated that the 1988 bloom killed 42-81% of the 0-group (6-mo-old) cod before the fall of the same year (with an average effect estimated to be ∼60%). The algal bloom had no detectable direct harmful effect on the 1-group cod (during the same fall) but was found to have a delayed effect (from 58% to 99%) on the survival of the 0-group (in 1988) to the 1-group (in 1989). We concluded that the bloom had no long-term effect on the cod. Our analysis is presented as a case study using intervention analysis within the field of ecology. Assuming the availability of both a credible ecological model and a long-term observational time series (as was the case for our marine system), we suggest that intervention analysis provides a valuable tool for simultaneously studying the unperturbed dynamical structure and the impact of the environmental disturbances.