Divergent origins of sympatric herring population components determined using genetic mixture analysis

The origin and reproductive interactions of sympatric, spatially separated spawning components of Atlantic herring Clupea harengus have received long-standing interest. In the west- ern Baltic most herring spawn in spring, with smaller components spawning in winter. We used microsatellite DNA analysis and a novel Bayesian genetic mixture analysis approach to compare the genetic relationships of 2 western Baltic winter-spawning aggregations with those of their sympatric spring-spawning components, and combined information for genetic markers and morphological traits (otolith-determined hatching time and growth relationships) to test alternative hypotheses for the origin of winter spawners. We show that genetic relationships between sympatric components differ greatly between the 2 locations; the results indicate that winter spawning has arisen via 2 fundamentally different processes: (1) as a result of 'spawning-time switching' in a local spring- spawning component and (2) via 1 or more founder events from an extant winter-spawning popula- tion into an area otherwise dominated by spring spawners.