Biological processes and environmental factors regulating the dynamics of the Norwegian Skagerrak cod populations since 1919

Time series of 0- and 1-group cod sampled in the period 1919 to 1994 at 38 stations along the Norwegian Skagerrak coast were investigated using spatial and temporal analyses. Both groups displayed similar spatial structure and had similar spatial distributions. Spatial heterogeneity occurred on a mesoscale (differences between fjords) and on a local scale (both groups significantly more abundant at sheltered stations in the inner fjord). Temporal fluctuations exhibiting a cyclic component at around 2-2.5 years were spatially structured on a local scale. The cycle in abundance appears to be a result of biotic interaction, such as competition for space and food and/or cannibalism between cohorts. Similar long-term trends were also observed, although those for the 0-group were more pronounced. In contrast to the 2-2.5 years' cycle, long-term trends were related to events taking place on a scale equal to or larger than the Norwegian Skagerrak. The causes of these fluctuations are complex and probably result from factors such as biotic interactions, changes in seagrass coverage, and fishing. (C) 2000 International Council for the Exploration of the Sea.