In the present study, we question whether the mixing of autumn spawning herring with Norwegian spring spawning herring (Clupea harengus L.) in feeding and wintering areas is a result of year class twinning, that is, that they hatched in spring as products of the spring spawning stock, but ended up spawning in autumn. The otolith microstructure of the summer feeding autumn spawners (July 2001) was similar to the wintering autumn spawners (January 2002), stable and low in comparison with the sympatric spring spawners caught in the same two seasons. Hence, the otolith microstructure of the analysed larval herring seemed representative of their own spawning season. Data from 1982 to 2003 demonstrated that the fraction of autumn spawners by year class has been quite low at 1–14%, but relatively high in some of the weak year classes, indicating a different recruitment success between the spawning groups. Length at age and somatic weight at length did not differ between the spawning groups. In comparison, the neighbour North Sea autumn spawning herring were significantly smaller at the same age.