Year-to-year f luctuations in fish stocks are usually attributed to variability in recruitment, competition, predation, and changes in catchability. Trends in abundance, in contrast, are usually ascribed to human exploitation and large-scale environmental changes. In this study, we demonstrate, through statistical modeling of survey data (1921-1994) of cod from the Norwegian Skagerrak coast, that both short-and long-term variability may arise from the same set of age-structured interactions. Asymmetric competition and cannibalism between cohorts generate alternating years of high and low abundance. Intercohort interactions also resonate the recruitment variability so that long-term trends are induced. The coupling of age-structure and variable recruitment should, therefore, be considered when explaining both the short-and long-term f luctuations displayed by the coastal cod populations. Resonant effects may occur in many marine populations that exhibit this combination of traits.Most commercial fish stocks vary greatly in abundance and exhibit a combination of short-term (year-to-year) fluctuations and longer term trends. Different factors are usually thought to give rise to these two types of changes in population size. Trends are usually related to external forcing on the populations, especially from human exploitation (1, 2) and large-scale environmental variation, such as changes in atmospheric and oceanographic circulation (3) or temperature (4). The yearto-year variation, in contrast, is usually explained by two different classes of hypotheses, both of which are internal to the populations in the sense of affecting vital rates, either in a density-independent or density-dependent manner. First, variable (stochastic) recruitment caused by changes in winds, currents, temperature, or resources can generate substantial variation in juvenile survivorship (3-9). For instance, the relative timing of the spring algal bloom and egg hatching is a critical determinant of food availability to the larvae and postlarvae. Because the spring bloom is itself determined by environmental conditions, this ''match-mismatch'' hypothesis offers an explicit mechanism for the effect of climate on year class strength (6, 10, 11). Second, density-dependent interactions between and within cohorts (in the form of cannibalism and competition) can generate population fluctuations (12)(13)(14). Such interactions can lead to cycles or quasiperiodic oscillations (15)(16)(17). Fromentin et al. (18,19) described patterns of abundance in a long-term survey of cod (Gadus morhua L.) from the Norwegian Skagerrak coast. They demonstrated both 2-to 3-year cycles and long-term trends in these time series. The cycles were hypothesized to arise from interactions within and among cohorts (18,20). The long-term trends appeared as a regional feature but were not clearly linked to any large-scale changes in either zooplankton abundance or climatic variables (19). In the present study we built a dynamic model for the Skagerrak cod survey data to ...

Time series from an extensive research survey of juveniles of cod Gadus rnorhua, p~l i a~k Pollachius pollachius and whiting Merlangius rnerlangus 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 hetween fjords) and at a local scale (differences hetween stations within a fjord) for the 3 gadoids. However, the magnitude of the spatial heterogeneity differed hetween species. Cod and whiting, which were more abundant in sheltered areas, showed higher spatial variahility 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 oi the species. AJl3 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.