The role of fecundity regulation and abortive maturation in the reproductive strategy of Norwegian spring-spawning herring (Clupea harengus)

Abstract

This study investigates the reproductive strat- egy, an important component in the estimation of stock reproductive potential, in Norwegian spring-spawning (NSS) herring (Clupea harengus), an iteroparous, extreme capital spawner, through the estimation of fecundity over a period of 3 years including two complete maturation cycles and three spawning seasons. NSS herring have an ‘opti- mistic’ strategy, with almost all adult herring caught in August being in the vitellogenic stage of ovary develop- ment, despite overwintering energy levels not being determined at this time. Fecundity in the summer, i.e., more than half a year before spawning in spring (February– April), was also much higher than could be supported by an individual’s concurrent energy levels. Consequently, fecundity was later reduced through atresia with the majority of this occurring before overwintering. The total reduction and the length of the time period in which the reduction took place appeared to vary between years. During the spawning season, atresia was mostly prevalent in small first-time spawners <180 g and several individuals aborted ovary development at this time. Final fecundity varied between years with a difference of up to 18% and was linked to annual variations in condition. In conclusion, this extensive field study has demonstrated that each indi- vidual herring can display a suite of size-specific repro- ductive tactics to fine-tune oocyte production in response to fluctuating levels of planktonic prey.