The complex population structure of Atlantic herring (Clupea harengus) was studied in the northeast Atlantic. Biological and environmental data from 1970-2015 of 13 local, coastal and oceanic areas of the North Sea, Skagerrak, Kattegat and western Baltic were analyzed. The aim was to identify distinct populations by comparing historical data on the temporal and spatial variation in phenotypic characteristics as well as to indicate mixing of populations in time and space. Our data indicated clear variation in biological characteristics such as mean vertebral counts (VS), growth and maturity ogives among herring caught in the defined areas. Generalized adaptive models demonstrated temporal, as well as intra-annual, dynamics of VS to be a population specific trait in this study. High variability of VS was observed and it was not affected by environmental factors. The variability can be explained by variation in presence/absence of herring populations in certain areas. The three main populations identified within this paper represented the three managed stocks in this area: Norwegian spring spawners (NSS), western Baltic spring spawners (WBSS) and North Sea autumn spawners (NSAS). In addition, several local populations have been identified in fjords or lakes along the coast. Our data could not demonstrate direct mixing of local populations with the three main populations. However, local populations are included in the management of the three stocks, without knowing the extent of mixing. Our results clearly highlighted the importance of recognizing and understanding herring dynamics and mixing of populations as this is a challenge for the management of herring.

Supplement to: Berg, Florian; Slotte, Aril; Johannessen, Arne; Kvamme, Cecilie; Clausen, Lotte Worsøe; Nash, Richard DM (2017): Comparative biology and population mixing among local, coastal and offshore Atlantic herring (Clupea harengus) in the North Sea, Skagerrak, Kattegat and western Baltic. PLoS ONE, 12(10), e0187374