Spawning season fidelity in sympatric populations of Atlantic herring (Clupea harengus)

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Abstract
Considerable debate surrounds the issue of reproductive exchange between subunits of marine fish populations. Discrete population theories predict that spawning time and location are predetermined during larval life and that spawning season fidelity and natal homing maintain population integrity. Conversely, spawning assemblages may represent interconnected subgroups within metapopulations with social transmission of spawning behaviours facilitating mixing. This study examines spawning season fidelity in three subpopulations of Atlantic herring (Clupea harengus) with overlapping distributions. Autumn- and winter-spawned fish were collected from spawning and feeding grounds in the Irish and Celtic seas. Hatching season was determined using otolith microstructure and was compared with season of spawning as indicated by visual gonad inspection. Spawning assemblages in the Irish Sea consisted almost exclusively of autumn-spawned fish, despite extensive mixing with winter-spawned fish during the juvenile phase. The available evidence suggests that in Celtic herring, the season and location of spawning are not learned from repeat-spawning adults, but are predetermined, possibly through larval imprinting. The seasonal components showed distinct patterns of gonad development, and estimated rates of spawning season switching were below 10%. Although this exchange rate will facilitate some gene flow, mixing of subgroups is limited. This has important implications for management of seasonal spawning populations.
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