Feeding ecology of Atlantic mackerel (Scomber scombrus) in the Norwegian Sea

Previous studies on Atlantic mackerel (Scomber scombrus) feeding during the highly migratory summer period are scarce. In this study we present detailed diet analyses and investigations of prey selection for Atlantic mackerel in relation to zooplankton distribution and hydrographical conditions in the Norwegian Sea in July 2004. A wide variety of prey organisms were found in the stomachs of mackerel in all water masses. According to dry weight was Limacina retroversa the principal prey in Coastal and Atlantic water masses. L. retroversa was presumably abundant in the Norwegian Sea and taken as secondary prey in the quest for more nutritious prey. Calanus finmarchicus was less important in all water masses than previously documented, also when combined with calanoidae copepod remainders, indicating that their descend to overwinter in deeper waters had already started in late July. Mackerel showed a size selective feeding behaviour and preferred larger prey species over smaller prey. Small copepods (< 1 mm) were numerous in the zooplankton samples in all water masses, especially Coastal and Atlantic, but were not present in the stomach samples. Furthermore, the three largest copepodite stages of C. finmarchicus (CIV – CVI) were selected over the three smallest stages (CI – CIII). Pronounced prey and size selection strongly suggests that mackerel performed particulate feeding. The potential feeding competition between mackerel and herring was also investigated. There was a clear difference in the diets of the two species. C. finmarchicus and L. retroversa were the main prey of mackerel, whereas herring preferred euphausiids and amphipods. These latter species normally swim in deeper waters and only enter the upper layers for feeding when protected by the dark. Moderate feeding by herring at the end of their feeding season and more active feeding by mackerel could explain the pronounced differences in the diet and lack of feeding competition between these two important planktivorous species

The diet of juvenile Atlantic mackerel (Scomber scombrus) feeding in new northern nursery areas along the Norwegian coast

The North-east Atlantic mackerel stock size increased substantially from 2006–2014 coinciding with high recruitment. This resulted in a pronounced northerly geographic expansion of mackerel, followed by an influx of juvenile mackerel into Norwegian waters. The objective of this work was to study the diet and feeding intensity of juvenile mackerel at the new nursing grounds along the Norwegian coast during the summer. Juvenile mackerel were feeding as far north as 70°N. Stomach content was analysed for the first time from co-occurring juvenile and adult mackerel at the same locations. Almost 80% of all juvenile mackerel had prey in their stomachs, and juveniles had similar stomach fullness as adult mackerel in the same areas. The juveniles preyed on a wide variety of prey species and seemed to utilize both passive filter feeding and active particulate feeding. The most abundant prey group was Appendicularia, accounting for 31% of the stomach content by weight. Juveniles fed on similar prey species as adults, but their diet niche differed somewhat as adult mackerel fed more on krill. Juvenile mackerel can thus successfully survive and feed on various prey in high latitudes and can potentially be a feeding competitor to other planktivorous fish species in the area.publishedVersio

Fish diversity data from the Barents Sea Ecosystem Survey 2004-2019

Assessing and improving the status of biodiversity are main targets of international agreements such as the Convention on Biological Diversity and the associated Aichi Biodiversity Targets for 2020. Here we present fish data from the Barents Sea Ecosystem Survey 2004-2019 on a format suitable for biodiversity assessments. The Barents Sea Ecosystem Survey is run jointly by the Institute of Marine Research (IMR, Norway) and the Polar Research Institute of Marine Fisheries and Oceanography (PINRO, Russia) - since 2019 - Polar Branch of Russian Federal Research Institute of Fisheries and Oceanography (VNIRO). We present a Barents Sea Fish Species Reference List that we use with the software StoX version 3.0 to translate and export data from raw data xlm -files in the NMDBiotic v3 format stored at IMR to a format suitable for diversity analysis. Finally, we present some example results based on the fish data from the Barents Sea ecosystem survey 2004-2019.publishedVersio

Arctic fishes in the Barents Sea 2004-2015: Changes in abundance and distribution

The Barents Sea is one of nine shelf ecosystem survey bordering the Arctic Basin. The Arctic region is warming faster than the rest of the world. In the Barents Sea the years since 2000 were the warmest since the onset of regular measurements (1900, Bochkov 1982, ICES 2016) and paleo-records based on foraminifera even suggest that the Atlantic Water flowing into to the Barents Sea area was at its warmest for the last 2000 years (Spielhagen et al. 2011). As a result the extent of Arctic Water with sub-zero temperatures and sea ice is shrinking in the Barents Sea (e.g. ICES 2016). The changes in hydrographic conditions change the conditions for the poorly known Arctic fish fauna in the region. Here we present for the first time results on trends in abundance and distribution of demersal Arctic fishes in the northern Barents Sea.publishedVersio

Quantifying diets for small pelagic fish: effects of weight versus occurrence methods and sampling effort

In the Norwegian Sea, assessment of diet composition relies on annual cruise activity combined with visual identification and weight determination of prey from fish stomachs. This weight-based method is labour intensive, and suggestions to reduce cost include moving from the weight-based to the occurrence-based method and/or reducing sampling effort. Studies have suggested that the occurrence-based method may be more robust while providing similar results as the weight-based method. Here we re-analyse data from >14.000 stomachs for herring, mackerel, and blue whiting. We compare diet composition estimates and quantify the uncertainty using both methods. We also quantify the impact of reducing sampling effort (number of trawl stations and fish per station) on the diet uncertainties. Our results confirm that occurrence-based estimates are more precise than those based on weight. In addition, they are better at capturing year-to-year fluctuations. The occurrence-based method provides similar results to the weight-based method. Differences between methods arise primarily from disparities in the mean weight of prey in stomachs. Decreasing the number of stations sampled leads to increased uncertainty, while reducing the sampling effort from 10 to 5 fish per station has little impact on uncertainty estimates. These results provide quantified insights to guide future diet monitoring programmes.publishedVersio

Arctic fishes in the Barents Sea 2004-2015: Changes in abundance and distribution

The Barents Sea is one of nine shelf ecosystem survey bordering the Arctic Basin. The Arctic region is warming faster than the rest of the world. In the Barents Sea the years since 2000 were the warmest since the onset of regular measurements (1900, Bochkov 1982, ICES 2016) and paleo-records based on foraminifera even suggest that the Atlantic Water flowing into to the Barents Sea area was at its warmest for the last 2000 years (Spielhagen et al. 2011). As a result the extent of Arctic Water with sub-zero temperatures and sea ice is shrinking in the Barents Sea (e.g. ICES 2016). The changes in hydrographic conditions change the conditions for the poorly known Arctic fish fauna in the region. Here we present for the first time results on trends in abundance and distribution of demersal Arctic fishes in the northern Barents Sea.publishedVersio

Follow-up fishery survey in the Hywind Tampen offshore wind power field during construction and early operation

The aim of the cruise was to follow up the pre-construction studies in Hywind Tampen in 2022. In march 2023, when this cruise was carried out, seven of the eleven planned turbines were in operation. As in 2022, the experiments were based on a gradient fish capture study using bottom gillnets at pre-determined stations. The catches were sampled and analysed to obtain information about species richness, abundance, maturity stage, sex, condition and diet and how these vary with distance to the windfarm. In addition, acoustic data were collected.Follow-up fishery survey in the Hywind Tampen offshore wind power field during construction and early operationpublishedVersio

The diet of juvenile Atlantic mackerel (Scomber scombrus) feeding in new northern nursery areas along the Norwegian coast

The North-east Atlantic mackerel stock size increased substantially from 2006–2014 coinciding with high recruitment. This resulted in a pronounced northerly geographic expansion of mackerel, followed by an influx of juvenile mackerel into Norwegian waters. The objective of this work was to study the diet and feeding intensity of juvenile mackerel at the new nursing grounds along the Norwegian coast during the summer. Juvenile mackerel were feeding as far north as 70°N. Stomach content was analysed for the first time from co-occurring juvenile and adult mackerel at the same locations. Almost 80% of all juvenile mackerel had prey in their stomachs, and juveniles had similar stomach fullness as adult mackerel in the same areas. The juveniles preyed on a wide variety of prey species and seemed to utilize both passive filter feeding and active particulate feeding. The most abundant prey group was Appendicularia, accounting for 31% of the stomach content by weight. Juveniles fed on similar prey species as adults, but their diet niche differed somewhat as adult mackerel fed more on krill. Juvenile mackerel can thus successfully survive and feed on various prey in high latitudes and can potentially be a feeding competitor to other planktivorous fish species in the area.publishedVersio