Publication Abstract

Changes in the North Sea cod stock during the twentieth century

N. Daan, H.J.L. Heessen and J.G. Pope

The development of the North Sea cod stock since the beginning of the last century is described on the basis of available yield and catch-per-unit-of-effort data. Given the data limitations, the analysis is not based on a strictly formal model, but rather is an attempt to capture the main trends in stock sizes and recruitment levels to match the realized catches and estimated fishing mortalities, The data indicate a very marked increase in recruitment during the period 1960-1990 which cannot be explained by density-dependent effects. Also, there is as yet no clue in any environmental signal providing a likely explanation. The hypothesis that the apparent increase in the carrying capacity of the North Sea for juvenile cod is caused by overexploitation of the pelagic fish species cannot be discounted. The recruitment pulse has had only a minor effect on stock biomass. because the expected increase has been counteracted by increased exploitation of the cod stock. The data suggest that the present spawning stock is at an all-time low.

Reference:

N. Daan, H.J.L. Heessen and J.G. Pope, 1994. Changes in the North Sea cod stock during the twentieth century. ICES Marine Science Symposia, 198: 229-243.