The effect of population structure on recruitment in herring

Abstract

Time of spawning of herring during a spawning season is size- and age-dependent. Thus the reproductive outputs of dominant year classes within a mature population occur in succession, appearing as “spawning waves” during the season. Larval cohorts stemming from these batches of eggs have been shown to exhibit a wide range of rates of growth and mortality, presumably a result of their experiencing different environmental conditions. It has been argued that this pattern of spawning is an effective risk-spreading tactic which ensures that an adequate proportion of the total larval population survives. Evidence to support this is found in Norwegian herring. Age-frequency distributions of spawning populations were examined and each was assigned a polymodality value on the assumption that the number of age peaks would represent the expected number of spawning waves. Better-than-average year classes were only associated with high polymodality values, and when the spawning population was unimodal recruitment was extremely poor. Thus recruitment would appear to be a probabilistic function of the number of dominant age classes within a spawning population.