Recruitment Dynamics of Spring-Spawning Herring in the Northwest Atlantic

Abstract

Recruitment patterns of the seven major spring-spawning herring populations in the Northwest Atlantic were found to be determined largely by annual variations in overwintering temperatures and salinities associated with the Labrador Current, and the patterns were therefore synchronous. Removal of the environmental component of recruitment variability revealed dome-shaped stock–recruit curves with sharply ascending left limbs and relatively high degrees of compensatory density dependence in recruitment as measured by the Ricker parameter, β. The degree of such density dependence was positively correlated with recruitment variability which was itself negatively associated with mean stock size, i.e. large herring stocks tended to have less recruitment variability but lower levels of density dependence in recruitment than smaller stocks. In addition, environmental variability was positively associated with recruitment variability, and for stocks of the same size, the more northern populations of spring-spawning herring, inhabiting a more variable environment, have a higher level of density dependence on recruitment. Simulation studies showed that larger herring stocks could sustain higher levels of fishing mortality than smaller herring stocks, due mainly to reduced recruitment variability. We suggest that the most appropriate fishing strategy for herring is to maintain fishing mortality at a level of F = 0.20 with the option of closure when parent stock size falls below the critical level. A surprising finding from our study was the high level of density dependence of recruitment in herring stocks which we attributed to several features: (1) a large egg size and the ability of herring populations to adapt to a wide range of spawning conditions, (2) a density-dependent mortality associated with egg hatching success, and (3) a strong homing tendency ensuring that changes in spawning population sizes are spread evenly throughout the spawning area, thereby optimizing larval production.