Introduction

Cod have provided one of the major North Atlantic fisheries for more than 500 years (Cushing, 1982, 1986; de La Villemarque, 1994) and have been more intensively studied than any other marine fish species. The existence of catch records going back several hundred years in some fisheries makes cod an excellent species on which to investigate the effects of environmental change. This was recognised in 1992, when the International Council for the Exploration of the Sea (ICES) established a Working Group on Cod and Climate Change to coordinate research and to encourage participation by a wider range of scientific disciplines than is normally involved in marine fisheries assessment. To date, insufficient attention has been paid to the application of results from biological oceanography and from experimental studies in assessing fish stocks and it is worth exploring some of the possibilities. ICES held a symposium on Cod and Climate Change in 1993, the proceedings of which (Jakobsson et al., 1994) provide an excellent coverage of recent research.

The total catch of cod rose from the early years of the 20th century to a peak of 3.9 million tonnes in 1968 and has declined steadily since then (Fig. 1). A number of stocks, which have separate spawning sites and different migration patterns, contribute to the total catch (Fig. 2). International fisheries assessments use data from commercial catches and research surveys to estimate stock abundance.