Duration and intensity of spawning in nerring Clupea harengus as related to the age structure of the mature population

Abstract

Herring Clupea harengus often spawns in waves which result in a succession of larval cohorts. First-time (recruit) spawners generally spawn later than repeat spawners and it is suggested that spawning proceeds consecutively through year classes from oldest to youngest due to differential maturahon. Further, it is suggested ~t 1s the large reproductive contribution of dominant year classes that gives the appearance of spawning waves; thus the number and relative size of these waves relates directly to the age structure of the reproductive population. In this paper historical data on the Norwegian spring-spawning herring stock are reworked in support of the hypothesis. An age-specific progression through annual maturation stages is demonstrated. This relation gives mean time of spawning by age which, combined with information on the age frequency distribution and age-specific fecundity, allows prediction of the duration of the spawning season and the intensity of spawning over time. Predictions agree well with observations made at actual spawning sites. Furthermore, it is probable that recruitment is, in part, dependent on the age structure of the mature population. The higher the number of age classes in the reproductive populahon the greater appears the likelihood of good recruitment.