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.

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 Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Melle, W., Klevjer, T., Strand, E., Wiebe, P. H., Slotte, A., & Huse, G. Fine-scale observations of physical and biological environment along a herring feeding migration route. Deep-Sea Research Part II: Topical Studies in Oceanography, 180, (2020): 104845, doi:10.1016/j.dsr2.2020.104845.

Description: We observed herring horizontal and vertical distribution during feeding migration along a 128 km transect across the Arctic front of the Norwegian and Iceland seas, in early June, in relation to its physical, chemical and biological environment, distribution of prey organisms and pelagic and mesopelagic competitors. The Norwegian Spring Spawning herring is one of the largest and economically most important stocks of pelagic fish in the world and understanding what controls its feeding migration is, and has been for centuries, a major research question that also has major implications for management. High resolution ecosystem data were obtained by hull mounted multi-frequency acoustics and a towed platform undulating between 10 and 400 m equipped with multi-frequency acoustics, temperature, salinity and fluorescence sensors, an Optical Plankton Counter and a Video Plankton Recorder. Additional sampling was done by MOCNESS, Macroplankton trawl, and CTD equipped with water bottles for temperature, salinity, nutrients and chlorophyll at discrete stations along the transect. Biological characteristics and stomach content of the herring were obtained from samples at discrete trawl stations. The Arctic front proved to be an important transitional zone in zooplankton biomass, abundance and diversity. Phenology of phyto- and zooplankton also changed across the front, being somewhat delayed on the cold side. The herring were distributed all along the transect showing a shallow distribution on the warm side and both deep and shallow on the cold side, not clearly related to light and time of the day. The herring stomach content was higher on the cold side. There was no significant pattern in average age, weight, or body length of the herring along the transect. The herring were present and fed in the area of the transect during the time when the overwintering generation of Calanus finmarchicus dominated, before the development of the new generation of the year. We suggest that the phenology of C. finmarchicus can be an important driver of the herring feeding migration. While prey-availability was higher on the Arctic side of the front, light conditions for visual feeding at depth were probably better on the Atlantic side. The herring did not show classical dial vertical migration, but its prey did, and the herring's prey were probably available within the upper 100 m during the course of a 24 h cycle. With a general westward direction of migration, the herring along the transect moved towards lower temperatures and temperature did not seem to be a probable driver for migration. We conclude that fine-scale studies of herring migration and feeding can increase our understanding of the migratory processes and add to our understanding of large-scale distributional patterns, changes therein, and herring trophodynamics and ecological role. The fine-resolution parameters can also be important as input to ecosystem models.

Description: We would also like to acknowledge the funding from Euro-BASIN, EU FP7, Grant agreement No 264933, HARMES, Research Council of Norway project number 280546 and MEESO, EU H2020 research and innovation programme, Grant Agreement No 817669.

Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Melle, W., Klevjer, T., Drinkwater, K. F., Strand, E., Naustvoll, L. J., Wiebe, P. H., Aksnes, D. L., Knutsen, T., Sundby, S., Slotte, A., Dupont, N., Salvanes, A. G. V., Korneliussen, R., & Huse, G. Structure and functioning of four North Atlantic ecosystems - a comparative study. Deep-Sea Research Part II: Topical Studies in Oceanography, 180, (2020): 104838, doi:10.1016/j.dsr2.2020.104838.

Description: The epi- and mesopelagic ecosystems of four sub-polar ocean basins, the Labrador, Irminger, Iceland and Norwegian seas, were surveyed during two legs from Bergen, Norway, to Nuuk, Greenland, and back to Bergen. The survey was conducted from 1 May to 14 June, and major results were published in five papers (Drinkwater et al., Naustvoll et al., Strand et al., Melle et al., this issue, and Klevjer et al., this issue a, this issue b). In the present paper, the structures of the ecosystem are reviewed, and aspects of the functioning of the ecosystems examined, focusing on a comparison of trophic relationships in the four basins. In many ways, the ecosystems are similar, which is not surprising since they are located at similar latitudes and share many hydrographic characteristics, like input of both warm and saline Atlantic water, as well as cold and less saline Arctic water. Literature review suggests that total annual primary production is intermediate in the eastern basins and peaks in the Labrador Sea, while the Irminger Sea is the most oligotrophic sea. This was not reflected in the measurements of different trophic levels taken during the cruise. The potential new production was estimated to be higher in the Irminger Sea than in the eastern basins, and while the biomass of mesozooplankton was similar across basins, the biomass of mesopelagic micronekton was about one order of magnitude higher in the western basins, and peaked in the Irminger Sea, where literature suggests annual primary production is at its lowest. The eastern basins hold huge stocks of pelagic planktivore fish stocks like herring, mackerel and blue whiting, none of which are abundant in the western seas. As both epipelagic nekton and mesopelagic micronekton primarily feed on the mesozooplankton, there is likely competitive interactions between the epipelagic and mesopelagic, but we're currently unable to explain the estimated ~1 order of magnitude difference in micronekton standing stock. The results obtained during the survey highlight that even if some aspects of pelagic ecosystems are well understood, we currently do not understand overall pelagic energy flow in the North Atlantic.

Description: We greatly appreciate the Captain and crew of the R.V. G.O. Sars for their dedication and help during the BASIN survey. We also thank the technical support from the Institute of Marine Research that helped during the cruise and those that contributed to the processing and analysis of the data on land. The sampling, data analysis and reporting have been supported by IMR and University of Bergen through funding of ship time, laboratory costs and salaries of researchers through internally funded projects. We would also like to acknowledge the funding from Euro-BASIN, EU FP7, Grant agreement No 264933, HARMES, Research Council of Norway project number 280546 and MEESO, EU H2020 research and innovation programme, Grant Agreement No 817669. KD undertook this study as part of the Ecosystem Studies of Subarctic and Arctic Seas (ESSAS) programme.