Spatial patterns of the temporal dynamics of three gadoid species along the Norwegian Skagerrak coast

Time series from an extensive research survey of juveniles of cod Gadus morhua, pollack Pollachius pollachius and whiting Merlangius merlangus sampled from 1919 to 1994 at 38 stations along the Norwegian Skagerrak coast were investigated. Spatial and temporal analyses were performed to study the spatial pattern of the temporal dynamics of the 3 fish species. Spatially consistent variations were detected in abundance, year-to-year fluctuation as well as in periodicity. The spatial heterogeneity occurred at a mesoscale (differences between fjords) and at a local scale (differences between stations within a fjord) for the 3 gadoids. However, the magnitude of the spatial heterogeneity differed between species. Cod and whiting, which were more abundant in sheltered areas, showed higher spatial variability than pollack, which was more abundant in exposed locations. in this way, the spatial pattern of abundance appeared to be linked to the scale of variability of the species. All 3 species exhibit periodic fluctuations of 2 to 2.5 yr in their optimal habitats, which probably resulted from intrinsic interactions in age-structured populations, such as density-dependent competition and cannibalism. In addition, all the species exhibited long-term trends possibly due to extrinsic forces, such as environmental changes or anthropogenic influences.