ALBERT

Description: Atlantic herring, Clupea harengus, have complex population structures and different populations can be found in fully marine, as well as nearly freshwater conditions. Mixing of populations is known, but the extent of connectivity is still unclear. Ripe spring spawning herring were collected in marine (salinity 35, Atlantic) and brackish water (salinity 6, Baltic Sea) conditions. One Atlantic herring female was crossed with one Atlantic and one Baltic male generating an F1-generation consisting of Atlantic purebreds and Atlantic/Baltic hybrids which were incubated and co-reared at two different salinities, 16 and 35 respectively, for three years. The F1-generation was repeatedly sampled for length measurements, vertebral counts and otoliths were also extracted for shape analysis. Atlantic purebreds grew better than Atlantic/Baltic hybrids at salinity 35, but not at salinity 16. In contrast, Atlantic/Baltic hybrids achieved larger size-at-age than the wild caught Baltic parental group. Mean vertebral counts and otolith aspect ratios were higher for Atlantic purebreds than Atlantic/Baltic hybrids, consistent with the parental groups. There were no differences in vertebral counts and otolith aspect ratios between herring with the same genotype but raised in different salinities. A Canonical Analysis of Principal Coordinates was applied to analyze the variation in wavelet coefficients that described otolith shape. The first discriminating axis identified the differences between Atlantic purebreds and Atlantic/Baltic hybrids, while the second axis represented salinity differences. These results demonstrate that otolith shape and vertebral counts have a significant genetic component and are therefore useful for studies on population dynamics and connectivity.

Description: 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.