Recolonisation of spawning grounds in a recovering fish stock: recent changes in North Sea herring

SUMMARY: There is evidence that the importance of the different spawning grounds of North Sea autumn-spawning her-ring has changed. It has been hypothesised that as herring stocks collapse, the diversity of spawning sites also collapses. This was found to be the case in the North Sea autumn-spawning herring, which collapsed in the late 1970s. The ICES Interna-tional Herring Larval Survey has been carried out since 1972 and covers most of the potential and historic spawning grounds of herring. The recovery of the stock did take place as predicted in terms of biomass, and re-colonisation of old spawning sites also did occur. We show that, despite the delayed response in re-colonisation of the southern spawning areas, there is almost no change in the number of spawning locations where the highest abundances of larvae (top 50%) are found from collapse to recovery (approximately 9 sites). It was a change in these core sites and the spread to other locations with lower larval abundance that caused the spread of herring spawning. We show that larval surveys are a useful tool for describing the dynamics of sub-stock structure in heterogeneous populations such as herring