Two components of Northeast Atlantic herring within the same school during spawning: Support for the existence of a metapopulation?

Abstract

It has been hypothesized that some stocks of Atlantic herring are subject to genetic exchange and therefore should fit the definition of a metapopulation with subunits. Genetic exchange requires spatial overlap of individuals from different populations during spawning. We investigated a local herring population that is assumed to feed and spawn within the semi-enclosed marine ecosystem of Lindspollene in west Norway, which is connected with the outer fjord only by a narrow sill. Acoustic recordings and gillnet samples demonstrated the mixed presence of two components within a single school throughout the spawning period 2005-2007. Members of the two components were of about the same size and in a similar stage of gonad development at all times, but the age composition was completely different. These observations represent the first documented case of different components being present in a spawning school and provide strong indications of regular interbreeding. Our data suggest that the two components represent autochthonous Linds herring and allochthonous herring of coastal/oceanic origin, thereby supporting the metapopulation concept. Two alternative explanations, based on one migratory and one resident component of a single Linds stock, are also discussed. © 2009 International Council for the Exploration of the Sea.