Fluctuations in year-class strength of cod and pollack in southeastern Norwegian coastal waters during 1920-1969

Abstract

TVEITE, S. 1971. Fluctuations in year-class strength of cod and pollack in southeastern Norwegian coastal waters during 1920-1969. FiskDir. Skr. Ser. HavUnders., 16: 65-76.

From 1920 a small beach seine has every September-October (except 1940-1944) been worked at selected localities along the Norwegian Skagerack coast. The catch per beach seine haul of 0-group cod agree fairly well with catch per trap per week of the same year-class as I-, II-, III- and IV-group and may thus be used as estimate of the strength of each year-class. There are two marked periods with rich year-classes, one before 1930 and one after 1950. In the intermediate period the only rich year-classes were 1938 and 1945. The fluctuations are generally similar in all districts. There has been increase in relative abundance in the most open areas compared to the more landlocked ones. By the present method no effect of the liberation of cod larvae has been revealed. The catch per beach seine haul of 0-group pollack is less reliable as estimates of year-class strength. Two rich periods, however, one before 1931 and one after 1953, are shown by the material. In the last period the average relative number is below that of the first period. Again, 1938 and 1945 are the only years in the intermediate period which gave rich broods. Covariation between year-class strength of pollack and cod excist to some extent, but not all years with rich year-classes of one of the species showed a similar year-class strength of the other species.