Distribution and ecology of polar cod (Boreogadus saida) in the eastern Barents Sea: A review of historical literature

Strange attractor of a narwhal (Monodon monoceros)

Detecting structures within the continuous diving behavior of marine animals is challenging, and no universal framework is available. We captured such diverse structures using chaos theory. By applying time-delay embedding to exceptionally long dive records (83 d) from the narwhal, we reconstructed the state-space portrait. Using measures of chaos, we detected a diurnal pattern and its seasonal modulation, classified data, and found how sea-ice appearance shifts time budgets. There is more near-surface rest but deeper dives at solar noon, and more intense diving during twilight and at night but to shallower depths (likely following squid); sea-ice appearance reduces rest. The introduced geometrical approach is simple to implement and potentially helpful for mapping and labeling long-term behavioral data, identifying differences between individual animals and species, and detecting perturbations.

While animal-borne ocean sensors continue to advance and collect more data, there is a lack of an adequate framework to analyze records of irregular behavior. For example, in the Arctic—there sea-ice is declining but is fundamental for the life cycle of many endemic animals—near-surface dive records are usually ignored, and continuous data are reduced to a maximum depth or similar. Here, we propose to transform our way of thinking about animal motion underwater by turning to a chaos approach and using a flowing geometrical shape to understand the full diversity of behaviors on an example of a satellite-tagged narwhal. Our method may help to assess the susceptibility of narwhal and other animals to sea-ice loss and climate warming.

Successive extreme climatic events lead to immediate, large-scale, and diverse responses from fish in the Arctic

The warming trend of the Arctic is punctuated by several record-breaking warm years with very low sea ice concentrations. The nature and reversibility of marine ecosystem responses to these multiple extreme climatic events (ECEs) are poorly understood. Here, we investigate the ecological signatures of three successive bottom temperature maxima concomitant with surface ECEs between 2004 and 2017 in the Barents Sea across spatial and organizational scales. We observed community-level redistributions of fish concurrent with ECEs at the scale of the whole Barents Sea. Three groups, characterized by different sets of traits describing their capacity to cope with short-term perturbations, reacted with different timing and intensity to each ECE. Arctic species co-occurred more frequently with large predators and incoming boreal taxa during ECEs, potentially affecting food web structures and functional diversity, accelerating the impacts of long-term climate change. On the species level, responses were highly diversified, with different ECEs impacting different species, and species responses (expansion, geographical shift) varying from one ECE to another, despite the environmental perturbations being similar. Past ECEs impacts, with potential legacy effects, lagged responses, thresholds, and interactions with the underlying warming pressure, could constantly set up new initial conditions that drive the unique ecological signature of each ECE. These results highlight the complexity of ecological reactions to multiple ECEs and give prominence to several sources of process uncertainty in the predictions of climate change impact and risk for ecosystem management. Long-term monitoring and studies to characterize the vertical extent of each ECE are necessary to statistically link demersal species and environmental spatial-temporal patterns. In the future, regular monitoring will be crucial to detect early signals of change and understand the determinism of ECEs, but we need to adapt our models and management to better integrate risk and stochasticity from the complex impacts of global change.

Modelling the radioactive contamination of commercial fish species in the Barents Sea following a hypothetical short-term release to the Stepovogo Bay of Novaya Zemlya

A dynamic modelling of radionuclides accumulation in commercial species in the Barents Sea is performed for hypothetical SCR accident with the dumped submarine K-27 at the Stepovogo Bay. Box radioecological model is employed for calculating the radionuclides dynamics in water, bottom sediments and marine biota. The model takes into account the seasonal fish migrations in the Barents Sea. The model allows predicting the dynamic effects of the radionuclide transfer in fish in case of an accidental water contamination. Maximum ¹³⁷Cs activity concentrations in bottom sediments from the eastern part of the Barents Sea could be reached as late as 5-6 years after the accidental release. Based on the results of model calculations, assessment was made of doses to humans from consumption of seafood contaminated after a hypothetical SCR accident with K-27 at the Stepovogo Bay. The peak activity concentration of the released ¹³⁷Cs in fish from the Stepovogo Bay is calculated to be 109 Bq∙kg^-1, ⁹⁰Sr - 12 Bq∙kg^-1; both estimates are below the permissible activity concentrations of these radionuclides in commercial fish. Predicted maximum annual dose from consumption of fish from the Stepovogo Bay is 47 ± 18 μSv∙year^-1, the Barents Sea fish - less than 3∙10^-6 μSv∙year^-1.¹³⁷Cs is the major dose contributor; it provides more than 99% to the annual dose Radiation risks from consumption of the Barents Sea commercial fish are evaluated to be negligible in case of a hypothetical SCR accident with the submerged submarine K-27 in Stepovogo Bay of Novaya Zemlya.

Summer-fall macrozooplankton assemblages in a large Arctic estuarine zone (south-eastern Barents Sea): Environmental drivers of spatial distribution

Macrozooplankton assemblages were studied during the summer and fall of 2012 in the Pechora Sea, a large coastal region of the south-eastern Barents Sea strongly affected by discharges of freshwater. Sampling was performed at 23 stations with 550-μm mesh IKS-80 nets. We focused on macrozooplankton and large (>1.5 mm) non-copepod zooplankton (larvae of benthic animals and medusae). A combination of multivariate cluster analysis, diversity indices and canonical correspondence analysis was used to relate the spatio-temporal patterns of the zooplankton assemblages to environmental factors. Summer macrozooplankton density varied from 3 to 928 ind. m^-3, averaging 131 ± 49 ind. m^-3, with medusae being the most numerous (mean ± SE: 67.9 ± 7.8% of the total abundance). The total macrozooplankton biomass ranged from 0.3 to 516 mg dry mass (DM) m^-3 with a mean value of 28 ± 25 mgDM m^-3. There was a clear decline in the total macrozooplankton abundance (1.1-6.0, 4.4 ± 1.6 ind. m^-3) and biomass (0.4-2.7, 1.2 ± 0.7 mgDM m^-3) in fall when only two taxa (Parasagitta elegans and Mertensia ovum) were identified. Cluster analysis revealed three groups of stations in summer. The clusters were similar in terms of the abundance in common taxa but differed by the total density, biomass and diversity. We found that the offshore zone was dominated by the euphausiids Thysanoessa spp. and the chaetognaths Parasagitta elegans while the coastal area was characterized by high density of hydromedusae and larvae of bottom animals (Hyas spp., Pagurus spp.). There was a significant decline in the total macrozooplankton abundance and biomass from summer to fall. Canonical correspondence analysis showed that during the summer and fall of 2012, 68% of macrozooplankton variability was explained by environmental factors with water temperature and longitude being the most important. The distribution of macrozooplankton taxa was also related to local circulation patterns and possibly to mesozooplankton as potential prey for carnivorous taxa. Our data may be useful for future monitoring in the coastal Arctic estuarine regions.

Distinct genetic clustering in the weakly differentiated polar cod, Boreogadus saida Lepechin, 1774 from East Siberian Sea to Svalbard

The cold-adapted polar cod Boreogadus saida, a key species in Arctic ecosystems, is vulnerable to global warming and ice retreat. In this study, 1257 individuals sampled in 17 locations within the latitudinal range of 75–81°N from Svalbard to East Siberian Sea were genotyped with a dedicated suite of 116 single-nucleotide polymorphic loci (SNP). The overall pattern of isolation by distance (IBD) found was driven by the two easternmost samples (East Siberian Sea and Laptev Sea), whereas no differentiation was registered in the area between the Kara Sea and Svalbard. Eleven SNP under strong linkage disequilibrium, nine of which could be annotated to chromosome 2 in Atlantic cod, defined two genetic groups of distinct size, with the major cluster containing seven-fold larger number of individuals than the minor. No underlying geographic basis was evident, as both clusters were detected throughout all sampling sites in relatively similar proportions (i.e. individuals in the minor cluster ranging between 4 and 19% on the location basis). Similarly, females and males were also evenly distributed between clusters and age groups. A differentiation was, however, found regarding size at age: individuals belonging to the major cluster were significantly longer in the second year. This study contributes to increasing the population genetic knowledge of this species and suggests that an appropriate management should be ensured to safeguard its diversity.

Midtrophic fish feeding modes at the poles: an ecomorphological comparison of polar cod (Boreogadus saida) and Antarctic silverfish (Pleuragramma antarctica)

The polar cod (Boreogadus saida) and the Antarctic silverfish (Pleuragramma antarctica) are pelagic fish endemic to the Arctic and Antarctica sea, respectively. Both species are abundant and play a central role as midtrophic wasp-waist species in polar ecosystems. Due to their biological and ecological characteristics (small size, complex life histories, relatively short generation cycles, movement capability, planktivorous diet, and importance as prey), the polar cod and the Antarctic silverfish are potentially good sentinels of ecosystem change. Changes in polar zooplankton communities are well documented. How changes impact ecosystems as a whole largely depend on the degree of diet specialization and feeding flexibility of midtrophic species. Here, we provide the ecomorphological characterization of polar cod and Antarctic silverfish feeding performances. A comparative functional ecology approach, based on the analysis of morpho-anatomical traits, including calculation of suction index and mechanical advantage in jaw closing, was applied to profile the feeding modes and flexibility of the two species. Ecomorphological evidence supports differences in food acquisition: the polar cod appears able to alternate particulate ram-suction feeding to a pump filter feeding, and the Antarctic silverfish results be both a particulate ram and a tow-net filter feeder. Both species exhibit opportunistic feeding strategies and appear able to switch feeding mode according to the abundance and size of the available prey, which is a clue of potential resilience to a changing environment.