ICES Journal of Marine Science

Karlsson, Konrad; Søreide, Janne E

doi: 10.1093/icesjms/fsae188pmid: N/A

As Arctic sea temperatures rise and sea ice declines, boreal species are becoming more abundant in these waters. Generally, both inter- and intra-species variations show larger body sizes at higher latitudes and in colder climates. Continued Arctic amplification may lead to shifts in the size and composition of marine plankton, with cascading effects throughout the ecosystem. This study examines the metabolic rates of three common zooplankton species, Calanus finmarchicus, C. glacialis, and Metridia longa, across different temperatures (0°C, 3°C, and 6°C) to understand these dynamics. Results showed a distinct decrease in aerobic scope with rising temperatures for all three copepod species, indicating potential fitness reductions in warmer waters. Larger copepods exhibited higher aerobic scopes than smaller ones at all temperatures; however, this advantage diminished at 6°C, suggesting that smaller body sizes may confer metabolic benefits at higher temperatures. Conversely, larger sizes are favored in colder waters. These findings help explain the increase of smaller boreal species in warming Arctic seas and why colder Arctic conditions favor larger individuals.

Chevrier, Thomas; Cowart, Dominique A; Nieblas, Anne-Elise; Charrier, Grégory; Bernard, Serge; Evano, Hugues; Brisset, Blandine; Chanut, Jérémie; Bonhommeau, Sylvain

doi: 10.1093/icesjms/fsae179pmid: N/A

Swordfish (Xiphias gladius) is of significant economic importance as it is the second most exploited billfish in the Indian Ocean. While the Indian Ocean Tuna Commission (IOTC) considers swordfish to be a single panmictic population in the Indian Ocean, several studies have examined the potential for spatial variations within this highly migratory species with conflicting results, including emerging evidence that population structuring does indeed exist within swordfish. These findings therefore raise questions about the current guidelines for management adopted by the IOTC. In the present study, we address questions about the genetic structuring of swordfish in the Indian Ocean through the analysis of three datasets: (i) neutral SNPs, (ii) with, and (iii) only SNPs under potential selection identified from 1694 swordfish originating from 24 distinct locations across the Indian Ocean. A discriminant analysis of principal components showed the presence of two swordfish subpopulations in the Indian Ocean in the north and the south and was confirmed by admixture methods. This genetic differentiation may be explained by a chromosomal inversion, indicating that both populations could be demographically connected but remain differentiated by this structural variant.

Kühn, Susanne; van Franeker, Jan A; Jensen, Jens-Kjeld; Olsen, Bergur; Danielsen, Jóhannis; Simonsen, Poul-Johannes

doi: 10.1093/icesjms/fsae175pmid: N/A

Seabird bycatch is common in longline fisheries and detrimental to some seabird populations. Simultaneously, the loss of longline bait to seabirds may pose a considerable economic loss to fishermen. Here, we study northern fulmars (Fulmarus glacialis), bycaught in longline fisheries close to the Faroe Islands between 2004 and 2021. Biological data on age and sex and the quantity of bait in the stomachs was assessed. Males were over-represented in the bycatch sample, as were adult birds, likely caused by sex- and age-specific foraging segregation. Overall, 71% of 820 fulmars contained bait, on average 2 items per bird (range: 0–17 items). No difference in the average number of bait items was detected between males and females or between adult and non-adult birds. During the breeding season, however, adults contained significantly more bait in their stomach than outside the breeding season. No such effect was found for non-adult birds. Together with a lower body condition during the breeding season, this may indicate higher food requirements by breeding birds, potentially increasing entanglement risks. Regular loss of bait reduces fishing efficiency, likely resulting in economic loss. Reducing seabird bycatch may therefore benefit fishermen as well as seabird populations.

Borup, Melanie D; Brokensha, Luke N; Cataldo-Mendez, Camila; Corney, Inessa H R; Gibson, John A E; King, Sylvia R; Liu, Yunzhe; Maschette, Dale; Riengchan, Pimnara; Zhang, Haiting; Swadling, Kerrie M

doi: 10.1093/icesjms/fsae176pmid: N/A

Coastal Antarctic ecosystems feature long periods of sea ice cover, strong seasonality in primary production, and low biodiversity relative to marginal ice zone regions. There are few records of annual cycles of biodiversity and abundance of neritic Antarctic zooplankton, limiting the ability to predict how communities will respond to environmental change. Using a historical data set, we assess the annual cycle of primary production and zooplankton abundance in relation to the growth cycle of fast ice near Vestfold Hills, East Antarctica. Zooplankton abundance and biodiversity were high during the productive summer months, generally showing a lag behind peaks in chlorophyll a concentration. A total of 27 taxa were recorded from the water column, while only three species were recorded from the sea ice, apart from incidental occurrences. Copepods dominated the system throughout the year, especially in the non-productive season, with a small number of species contributing to overall abundance, including Paralabidocera antarctica, Oithona similis, and Oncaea curvata. The abundance of fauna living within the brine channel system was very high, although biodiversity was low relative to the water column. To survive the winter, the copepods employed a combination of life history strategies, including strong association with the fast ice habitat, synchronicity with the spring phytoplankton bloom, and dietary switching in winter. Complementary strategies, such as lipid storage and advection to and from deeper offshore waters, were also observed.

Biederbick, Johanna; Möllmann, Christian; Hauten, Elena; Russnak, Vanessa; Lahajnar, Niko; Hansen, Thomas; Dierking, Jan; Koppelmann, Rolf

doi: 10.1093/icesjms/fsae189pmid: N/A

Zooplankton in estuaries encounter complex physical and biogeochemical processes that affect the quantity, quality, and origin of their food sources. The knowledge about how zooplankton deal with highly variable organic matter sources is sparse. Here, we investigated the spatial and temporal patterns of zooplankton trophic dynamics and carbon sources in the intensively dredged, eutrophic Elbe estuary. For this purpose, we applied elemental and stable isotope analysis (δ13C and δ15N) on particulate organic matter (POM) and dominant meso- and macrozooplankton species, including ichthyoplankton, from five stations along the entire salinity gradient of the estuary in 2022. The δ13C values of POM (−29.2 to −23.0 ‰) indicated a mixture of riverine, terrestrial, and coastal carbon sources used by most taxa for their diet. Eurytemora affinis (−34.0 to −23.3 ‰) and Mesopodopsis slabberi (−22.2 to −20.0 ‰) exhibited a broader range in δ13C than POM, suggesting selective feeding on single POM components depending on the season. In winter and autumn, under high suspended matter loads and limited availability of high-quality autochthonous phytoplankton, zooplankton showed increased tendency for carnivory (higher δ15N values). Our study revealed a high trophic plasticity of estuarine Elbe zooplankton to buffer hydrological-related alterations in their food source by dietary niche partitioning and a flexible switch in their feeding behaviour.

Koch, Marie; Jungblut, Simon; Götze, Sandra; Bock, Christian; Saborowski, Reinhard

doi: 10.1093/icesjms/fsae181pmid: N/A

Subarctic fjord systems are facing a continuous temperature increase as well as more frequent and more intense marine heatwaves (MHWs). MHWs are periods of exceptionally high temperatures above the long-term average. In the Porsangerfjord (Northern Norway), the average water temperature ranges from 2.5°C in March to 10°C in August. MHWs frequently exceed the summer maximum by up to 6°C with so far unknown ecological effects. The green sea urchin, Strongylocentrotus droebachiensis, is a key grazer on habitat-forming kelp in Subarctic fjord systems. At high abundances, sea urchins transform productive kelp beds through pronounced grazing into depleted sea urchin barrens. In a laboratory experiment, we investigated the influence of acute temperature change on the grazing activity and the metabolic performance of the green sea urchin. Grazing rates increased continuously from 2 to 10°C but decreased again at 14°C. At 22°C, rapid decay of sea urchins were associated with changes of the metabolic energy state from aerobiosis to anaerobiosis. We propose two scenarios: Moderate warming might lead to the proliferation of the sea urchin population and increased grazing pressure on kelp forests, while extreme warming, as predicted by the end of the century, as well as the occurrence of extreme MHWs, may severely impair the sea urchin population and, in turn, might favour the proliferation of adaptable kelp species.

Riaz, Javed; Büring, Tobias; van der Grient, Jesse; Winter, Andreas; Lee, Brendon; Brickle, Paul; Baylis, Alastair M M

doi: 10.1093/icesjms/fsae161pmid: N/A

Direct interactions between marine mammals and commercial fisheries are a worldwide conservation challenge. Observer programmes remain the most effective and reliable method for collecting data on these interactions. In the Falkland Islands—home to globally significant seal populations and commercial squid fisheries, seal-fishery interactions have escalated in recent years, prompting management concerns. Complete observer coverage within the squid fishery presents a valuable opportunity to investigate the nature, extent, and drivers of these interactions. Integrating multi-year observer records with extensive ancillary (i.e. vessel logbook and oceanographic) datasets, we examine the operational and environmental factors influencing the occurrence of seal-fishery interactions. Our findings show interactions most frequently occur in the main squid fishing grounds during trawls associated with high catch quantities. Assessment of long-term catch data (both finfish and squid) also suggests the increase in seal-fishery interactions may be caused by collapses in dominant finfish stocks over the past 20 years, constricting foraging resources available to seals. Taken together, our findings indicate resource competition may be a mechanism of interactions. To help mitigate this issue, we advocate for the development of ecosystem-based fisheries management, which considers the trophic effects of fishing practices and the energetic requirements of local marine predator populations.

Bolstad, Geir H; Diserud, Ola H; Paterson, Rachel A; Ulvan, Eva M; Karlsson, Sten; Ugedal, Ola; Næsje, Tor F

doi: 10.1093/icesjms/fsae192pmid: N/A

Sea trout, the anadromous phenotype of brown trout (Salmo trutta), is severely affected by high salmon lice infestation pressure that occurs in areas of Atlantic salmon (S. salar) aquaculture. Yet, and in contrast to Atlantic salmon, there is currently no effect indicator implemented for sea trout for regulating the Norwegian aquaculture industry. Here, we present such an indicator centred on the concept of fitness. Our numerical example, based on previous published estimates of reduced survival and reduced marine feeding time of sea trout due to spillback of salmon lice from aquaculture, demonstrates that the indicator is readily implementable. There are several benefits of our indicator: (i) multiple effects of salmon lice on sea trout can be implemented in a single biologically meaningful indicator; (ii) the indicator can be customized and simplified depending on data availability and knowledge; (iii) the simplifications provide a conservative yet effective indicator; (iv) the indicator is on the same scale as the already implemented Atlantic salmon indicator, making it possible to use the same management threshold values; and (v) the changes in fitness measured by the indicator can provide scenarios for the evolution of anadromy.

Champagnat, Juliette; Brown, Elliot John; Rivot, Etienne; Le Pape, Olivier

doi: 10.1093/icesjms/fsae178pmid: N/A

Spawning grounds and juvenile habitats are critical essential fish habitats (CEFH) facing multiple anthropogenic pressures. Yet the impacts of these pressures on fish population dynamics are poorly quantified. In this study, we address the impact of hypoxia on CEFH and its consequences on the productivity of exploited marine fish populations. Our approach relies on an age- and stage-structured simulation model, including habitat effects on egg, juvenile, or spawner mortalities to integrate the impact of hypoxia on spawning and juvenile habitats. We applied this framework in the Baltic Sea (BS) to explore population responses of cod, plaice, and herring, which face hypoxic conditions in their CEFH. All scenarios lead to modifications in fishing mortality, population size, and yield at maximum sustainable yield, but with different magnitudes depending on the impacted life stage. Perturbations of spawning habitats that result in higher mortality of eggs had the least impact, while the more direct effect on spawners themselves had the largest impact. Within each scenario, the responses also depended upon species and life history traits, with contrast in steepness and lifespan that explain most of the differences between species. Cumulative impacts of pressures in both CEFH were also investigated and revealed mainly nonadditive responses. Our framework is a generic tool, transferable to a large panel of case studies, to quantify the relative impact of CEFH degradation/restoration for the dynamics of marine exploited populations.

doi: 10.1093/icesjms/fsae197pmid: N/A