Genetically different Atlantic herring Clupea harengus spawning waves

Putative spawning waves of Atlantic herring Clupea harengus were collected from 4 locations and genetically compared using 9 microsatellite loci. Shelf-scale (100s km) spatial differences were measured (max FST = 0.01, p < 0.001) and 1 sub-annual temporal difference (FST = 0.0058, p = 0.001) was revealed between spawning fish (predominantly comprising the same year-class) collected, 6 d apart, from the same location (Devastation Shoal, coastal Nova Scotia). Herring in the second spawning collection at Devastation Shoal had a greater average length-at-age within the year-class (t-test, p < 0.001). The genetic and morphometric differences between the Devastation Shoal collections are assumed to reflect a replacement period of spawning fish of approximately 6 d or less (at least at this location). We offer 3 explanations to account for the observations: (1) genetically distinct sympatric subpopulations or spawning waves; (2) sub-annual (d) genetic patchiness; and (3) transient use of spawning grounds which may indicate that the assumption of philopatry (natal spawning-site fidelity) in herring is invalid.