Distribution and growth of blue whiting in the north-east Atlantic

Abstract

The distribution pattern of the blue whiting spawning stock has gradually changed in recent years, and concentrations are recorded more offshore and more to the south than before. The activity of the fleet with its shifting of fishing localities, gives knowledge of the blue whiting's migration route during the spawning season. Also during the summer season the distribution pattern has changed. The recordings of the highest concentrations have shifted from north and northwest of the warmer side of the Norwegian Sea to more southern parts. The acoustic estimates, which are considered as indices only, have decreased noticeably since the beginning of the 1980's. Some years they have been lower than the spawning stock assessments, and thus demonstrated that the total stock has not been properly covered during the feeding season. In the period from 1983 to 1987 the structure of the blue whiting stock has been dominated by the strong of 1982- and 1983-yearclasses. The growth of blue whiting, based on observed lengths as well as backcalculated lengths, is described for various years and yearclasses as occurring in various areas. Especially the 1982-and 1983-yearclasses when, as juveniles, being basis for the mixed industrial fishery in the North Sea. The growth pattern varied significantly among the various yearclasses, and the rate increased with increasing years up to the 1978-yearclass. Thereafter this effect was reversed up to the 1983-yearclass. After the age of 5 years the blue whiting growth pattern may diverse significantly from area to area, and off the Norwegian Coast blue whiting grow to a larger size than more offshore in the Norwegian Sea and west of the British Isles. However, while the growth in the Hebrides area was quite similiar to that in the Norwegian Sea, it differed slightly from the growth in the Porcupine bank area. This indicates influence of blue whiting from other areas not taking part in the migration back to the Norwegian Sea. Analysis of annual zone measurements also indicates differences in the stock.