O.N. Bjørnstad, J.-M. Fromentin, N.C. Stenseth, J. Gjøsæter: Cycles and trends in cod populations

Proceedings of the National Academy of Science USA (1999) 96:5066-5071

Year-to-year fluctuations in fish stocks are usually attributed to variability in recruitment, competition, predation, as well as changes in catchability. Trends in abundance, in contrast, are usually ascribed to human exploitation and large-scale environmental changes. In the present study we demonstrate, through statistical modeling of survey data (1921-1994) on the 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 also induced. The coupling of age-structure and variable recruitment should therefore be considered when explaining both short and long-term fluctuations displayed by the coastal cod populations. Resonant effects may occur in many marine populations that exhibit this combination of traits.

Key words: Age-structured dynamics, Gadus morhua, Markov Chain Monte Carlo estimation, Nonlinear Stochastic Dynamics, Stock recruitment, Time series.