An estimate of the total biomass of fish in the North Sea

Biomass estimates are made by combining ICES working group VPA biomass estimates of 11 commercial fish species with catch rates in the ICES International Young Fish Surveys (IYFS) in the first quarter of the year and in the English Groundfish Surveys (EGFS) in the third quarter of the year. Data from 1983, 1984, and 1985 are used.The total biomass of fish is calculated as the mean of the three years. In the first three months of the year the mean total biomass was 8.6 million tonnes, of which the 11 commercial species constituted 5.9 million tonnes. In the third quarter it was 13.1 million tonnes, of which the 11 commercial species constituted 8.5 million tonnes. The difference between the first and third quarters was mainly caused by the migration of 0.5 million tonnes of Western stock mackerel and 1.6 million tonnes of horse mackerel into the North Sea during the third quarter. Comparisons are made between the productivity of the North Sea and other shelf regions in temperate latitudes.