Characterization of herring populations west of the British Isles: an investigation of mixing based on otolith microchemistry

Herring along the west coast of the British Isles are managed and assessed as a series of discrete stocks. The relationship between the spawning components, mixed (feeding) aggregations, and juveniles in nursery areas for these stocks was modelled by discriminant analysis and integrated stock mixture analysis based on otolith elemental composition data. The relative elemental concentrations produced otolith signals corresponding to three main groupings of nursery-ground fish representing the Irish Sea, Scottish sea lochs, and the Minch. There were significant differences among spawning groups in the otolith concentrations of Li, Na, Mg, Mn, Sr, and Ba. Inclusion of length-at-age information improved the classification rates, ranging overall from 35 to 100%. Spawning groups consist of individuals from a number of different nursery areas and originate from several different management areas. Each of the mixed aggregations contained at least three spawning components. Results suggest that most west coast herring belong to interconnected populations subject to mixing and that populations are not discrete, so the current practice of assessments based on individual spawning components will probably not provide sufficiently robust information for management advice. The complexity of herring populations needs to be considered for both fisheries and coastal-zone management