Long term changes of growth and maturation in the Norwegian spring spawning herring

Abstract

Two periods with great differences i n year class abundance and recruitment where chosen for growth and maturation studies, 1950 - 1960 and 1973 - 1983. In the first period, the mean year class strength at 3 years was 6.8 x 109 individuals while in the second period it was 0.4 x 109 ind. The growth in each period as observed by the length at age, is compared to the year class strength and the abundance of the stock at the time. Variations in growth are observed and before maturation strong year classes has remarkably less growth than weak year classes. The latter period is therefore characterized by far better growth, especially in the early years of life , than the former. According to growth rates of the various year classes, the age at maturity is substantially higher in the first period than i n the latter. However, the length at maturity is found to be fairly constant showing the maturation to be size dependent rather than dependent on the age.