Establishment of new wintering areas in herring co-occurs with peaks in the 'first time/repeat spawner' ratio

Norwegian spring spawning (NSS) herring migrate to wintering areas in late summer and remain there for about 5 mo in dense aggregations. First-time spawning cohorts typically adopt the wintering area of the spawning stock. However, over the last 50 yr the NSS herring stock has occupied 7 discrete wintering grounds, spanning from the high seas to narrow fjords, which shows that there is considerable plasticity in choice of wintering area. We show that 5 cases of establishment of new wintering areas take place when abundant first-time spawning cohorts take up the adult migration pattern. The ratio in abundance between age 4 and age 5+ herring is on average 14 in the years when new wintering areas are established compared to 0.3 in years without changes. The wintering areas vary greatly with regards to temperature and distance to the feeding and spawning grounds, features that heavily affect energy expenditure during wintering and migration. Rather than being strictly optimized, the establishment of new wintering areas seems to be governed by general school cohesion mechanisms when naive first-time spawners flood the population and the social learning process is disrupted.