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 spatia l and temporal analyses. Both groups displayed similar spatial structure an d had similar spatial distributions. Spatial heterogeneity occurred on a me soscale (differences between fjords) and on a local scale (both groups sign ificantly more abundant at sheltered stations in the inner fjord). Temporal fluctuations exhibiting a cyclic component at around 2-2.5 years were spat ially structured on a local scale. The cycle in abundance appears to be a r esult of biotic interaction, such as competition for space and food and/or cannibalism between cohorts. Similar long-term trends were also observed, a lthough 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 s cale equal to or larger than the Norwegian Skagerrak. The causes of these f luctuations are complex and probably result from factors such as biotic int eractions, changes in seagrass coverage, and fishing. (C) 2000 Internationa l Council for the Exploration of the Sea.