The impact of long-term overfishing on the abundance dynamics of early- and late-maturing Northeast Arctic cod

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An algorithm is proposed for estimating age composition and abundance of cod spawning for the first time and repeat spawners in each calendar year. A fine structure of the Northeast Arctic cod spawning population was studied for a fairly long historical period from 1949 to 1997. It was shown that cod could be divided into at least two groups of spawners, early- and late-maturing individuals, which long-term abundance dynamics were different. Abundance of late-maturing cod displayed a marked decreasing trend over the entire time series, while the abundance of early-maturing cod has been steadily increasing since the early 1960’s. Structural change in the mature part of the population took place against the background of the long-term overfishing. A hypothesis is proposed about inter- and intra-annual qualitative heterogeneity of the spawners. Parameters of deterministic basis of the recruitment process (a sum of two Ricker’s stock-recruitment equations with additive independent variables and biomass of early- and late-maturing cod) vary in time. Contribution of early- and late- maturing fish as well as water temperature contribution to the recruitment to the commercial cod in 1976-1997 is shown.