title: Cycles and trends in cod populations.
abstract: Year-to-year fluctuations 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 fluctuations displayed by the coastal cod populations. Resonant effects may occur in many marine populations that exhibit this combination of traits.
authors.affiliation: Department of Biology, Division of Zoology, University of Oslo, PO Box 1050 Blindern, N-0316 Oslo, Norway. bjornsta@nceas.ucsb.edu
authors.forename: O N
authors.initials: ON
authors.lastname: Bjørnstad
authors.forename: J M
authors.initials: JM
authors.lastname: Fromentin
authors.forename: N C
authors.initials: NC
authors.lastname: Stenseth
authors.forename: J
authors.initials: J
authors.lastname: Gjøsaeter
mesh_terms: D000367:Age Factors; D000818:Animals; D005399:Fishes / Q000502:physiology*; D006801:Humans; D015233:Models, Statistical; D009010:Monte Carlo Method; D009664:Norway; D011157:Population Dynamics