Length and age at maturity of Norwegian spring-spawning herring for the year-classes 1959 -61 and 1973 -78

The age and length at maturity is calculated for the six year-classes, 1973-78. The age at maturity is also calculated for the year-classes 1959-61. The two periods are caracterized by great differences in stock abundance and recruitment. The proportion of mature herring in an age group is estimated as it's proportion in the spawning stock while maturing (at 3, 4 and 5 years) relative to it's proportion of mature fish in the spawning stock when fully recruited (mean proportion at 6, 7 and 8 years). The length at maturity for an age group is calculated as follows. The length distributions of the recruiting yearclasses while still maturing is back calculated from the length distribution of the fully recruited age group (at 6, 7 or 8 years). To find the length at maturity, the relative proportions of mature fish in the early age groups are used directly in these length distributions assuming the maturation to be length dependent. According to the growth rates of the different year-classes, the age at maturity vary from 3 to 5 years in the investigated period. However, the length at maturity is found to be fairly constant