Effects of hatching time on year-class strength in Norwegian spring-spawning herring (Clupea harengus)

Husebø, Å., Stenevik, E. K., Slotte, A., Fossum, P., Salthaug, A., Vikebø, F., Aanes, S., and Folkvord, A. 2009. Effects of hatching time on year-class strength in Norwegian spring-spawning herring (Clupea harengus). – ICES Journal of Marine Science, 66: 1710–1717. Effects of mean hatching date, post-hatching temperature, wintering temperature of adults, spawning stock size, and percentage of recruit spawners on larval survival in Norwegian spring-spawning herring (Clupea harengus) during the period 1987–2004 were analysed. In the final model, only hatching date proved to be significant. However, hatching date was itself negatively correlated with wintering temperature and positively correlated with the percentage of recruit spawners. This suggests indirect effects on larval survival, whereby low percentages of recruit spawners and high temperatures during gonad development lead to early spawning. Early hatching could be favourable for survival by allowing the larvae to drift away from areas where potential predators concentrate in spring, before predation pressure increases. Indirect support for this hypothesis was found in the activity of the purse-seine fishery for immature saithe (Pollachius virens) along the Norwegian coast. This fishery starts as soon as the saithe aggregate into large schools, which is presumed to reflect their feeding activity. The commercial catch data indicated that the saithe became active 2 months earlier in the area south of 67°N, than in areas to the north. Both field data and larval drift models confirmed that the majority of the early hatched larvae had passed across this border by that time of year.