Resource partitioning may limit interspecific competition among Arctic fish species during early life

Arctic cod (Boreogadus saida) strongly dominates the ichthyoplankton assemblages of High Arctic seas, hence competition with other native species seldom has been studied. Yet, interspecific competition could negatively impact the survival of early life stages of fishes in Arctic areas where higher diversity prevails. We surveyed the ichthyoplankton community of the Greenland Sea, in August–September 2017. Gadids (mostly Arctic cod, with a low number of ice cod Arctogadus glacialis) and non-gadids (bigeye sculpin Triglops nybelini and gelatinous snailfish Liparis fabricii) co-dominated age-0 fish assemblages. Here, we document their diet, prey selectivity, horizontal and vertical distributions as well as that of their prey to assess resource partitioning and the potential for interspecific competition. All fish species occupied the top 30 m of the water column, but Arctic cod occurred in highest abundances over the continental slope, whereas other species distributed almost exclusively over the continental shelf. A particle track analysis suggests that Arctic cod larvae could have hatched in the open waters of the Northeast Water Polynya, drifted with the East Greenland Current, and benefited from the high secondary production associated with these oceanographic features. The diet of gadids did not overlap significantly with the diet of non-gadids, but strong selectivity for Pseudocalanus spp. and Calanus spp. copepodites among the larvae suggests potential competition for these key prey items, although limited by size partitioning of the prey. We thus conclude that interspecific competition among early life stages of Arctic fishes is limited for now. However, changing conditions and the northward range expansion of boreal species following climate change could increase competition and, in turn, negatively affect the recruitment of Arctic ichthyoplankton

Resource partitioning may limit interspecific competition among Arctic fish species during early life

Arctic cod (Boreogadus saida) strongly dominates the ichthyoplankton assemblages of High Arctic seas, hence competition with other native species seldom has been studied. Yet, interspecific competition could negatively impact the survival of early life stages of fishes in Arctic areas where higher diversity prevails. We surveyed the ichthyoplankton community of the Greenland Sea, in August–September 2017. Gadids (mostly Arctic cod, with a low number of ice cod Arctogadus glacialis) and non-gadids (bigeye sculpin Triglops nybelini and gelatinous snailfish Liparis fabricii) co-dominated age-0 fish assemblages. Here, we document their diet, prey selectivity, horizontal and vertical distributions as well as that of their prey to assess resource partitioning and the potential for interspecific competition. All fish species occupied the top 30 m of the water column, but Arctic cod occurred in highest abundances over the continental slope, whereas other species distributed almost exclusively over the continental shelf. A particle track analysis suggests that Arctic cod larvae could have hatched in the open waters of the Northeast Water Polynya, drifted with the East Greenland Current, and benefited from the high secondary production associated with these oceanographic features. The diet of gadids did not overlap significantly with the diet of non-gadids, but strong selectivity for Pseudocalanus spp. and Calanus spp. copepodites among the larvae suggests potential competition for these key prey items, although limited by size partitioning of the prey. We thus conclude that interspecific competition among early life stages of Arctic fishes is limited for now. However, changing conditions and the northward range expansion of boreal species following climate change could increase competition and, in turn, negatively affect the recruitment of Arctic ichthyoplankton

Stomach contens of polar cod (Boreogadus saida) from the central Arctic Ocean

This file contains stomach content data from polar cod (Boreogadus saida) caught from underneath the sea ice of the central Arctic Ocean during spring and summer. The fish were collected during Polarstern expeditions PS80 (2012), PS92 (2015), PS106.2 (2017) and PS122 (2019/2020). The data was collected to gain knowledge on the diet of polar cod and the ice-associated food web of the central Arctic. The stomach contents of 86 individuals were analysed using a Discovery V8 stereomicroscope (Zeiss, Germany). All recognizable food items were counted and are listed in this dataset

Diet of polar cod (Boreogadus saida) from the central Arctic Ocean

This file contains stomach content data from polar cod (Boreogadus saida) caught from underneath the sea ice of the central Arctic Ocean during spring and summer. The fish were collected during Polarstern expeditions PS80 (2012), PS92 (2015), PS106.2 (2017) and PS122 (2019/2020). The data was collected to gain knowledge on the diet of polar cod and the ice-associated food web of the central Arctic. The stomach contents of 86 individuals were analysed using a Discovery V8 stereomicroscope (Zeiss, Germany). All recognizable food items were counted and are listed in this dataset. Fish parameters, such as length and weight, were recorded. In addition, the condition index of the fish was calculated. The stomach and stomach contens were also weighed and the degree of digestion of the contents were estimated. The dataset also contains length measurements performed on prey items found in the stomachs using an AxioCam HRc with AxioVision40 V 4.8.2.0 software (Zeiss, Germany), including Calanus spp., Tisbe spp, Themisto spp, and Apherusa glacialis

Length of prey items found in the stomach of polar cod (Boreogadus saida) from the central Arctic Ocean

This file contains length measurements performed on prey items found in the stomach content of polar cod (Boreogadus saida) caught from underneath the sea ice of the central Arctic Ocean during spring and summer. The fish were collected during Polarstern expeditions PS80 (2012), PS92 (2015), PS106.2 (2017) and PS122 (2019/2020). The data was collected to gain knowledge on the diet of polar cod and the ice-associated food web of the central Arctic. The stomach contents of 86 individuals were analysed using a Discovery V8 stereomicroscope (Zeiss, Germany). Length measurements were performed on prey items found in the stomachs using an AxioCam HRc with AxioVision40 V 4.8.2.0 software (Zeiss, Germany), including Calanus spp., Tisbe spp, Themisto spp, and Apherusa glacialis

Information on polar cod (Boreogadus saida) from the central Arctic Ocean, used for stomach content analysis

This file contains data on polar cod (Boreogadus saida) used for stomach content analysis. The individuals caught from underneath the sea ice of the central Arctic Ocean during spring and summer. The fish were collected during Polarstern expeditions PS80 (2012), PS92 (2015), PS106.2 (2017) and PS122 (2019/2020). The data was collected to gain knowledge on the diet of polar cod and the ice-associated food web of the central Arctic. Fish parameters, such as length and weight, were recorded for 86 individuals. In addition, the condition index of the fish was calculated. The stomach and stomach contents were also weighed and the degree of digestion of the contents were estimated

Go West: Sea-ice association of Polar cod and its prey in the western Arctic Ocean

Polar cod (Boreogadus saida), a key fish species in Arctic marine ecosystems, may be particularly susceptible to changing sea-ice habitats. It has been proposed that parts of the population get entrained with the growing sea ice in autumn, but how this happens and what proportion of the population becomes ice-associated is not known. The overall goal of the Go-West expedition was to test the hypothesis that entrainment of young Polar cod into the sea-ice habitat in the Chukchi and Beaufort seas during autumn is significant, and hence sea-ice association is an important survival strategy for Polar cod. During expedition SKQ201923S with RV Sikuliaq (06 November – 02 December 2019), we sampled Polar cod and its prey in the ice-water interface layer along with high resolution profiles of sea-ice and surface water properties with a Surface and Under-Ice Trawl (SUIT). We recorded backscatter of fish and zooplankton in the water column with the Sikuliaq’s EK80 echosounder, and sampled pelagic communities with two midwater trawls (Methot trawl and IKMT) and vertical zooplankton nets (CalVET and ring net). A conductivity-temperature—depth probe equipped with a rosette water sampler (CTD) sampled vertical profiles of temperature, salinity and fluorescence and was used to collect water samples for the analysis for chlorophyll a concentration, nutrient concentrations, trophic biomarkers and harmful algae (HAB). During four ice stations, we sampled the sea ice for the same parameters (except nutrients), and performed hyperspectral light measurements needed to derive ice algae biomass from hyperspectral profiles obtained from a sensor mounted on the SUIT. In addition, we performed respiration measurements on abundant zooplankton prey species of Polar cod. Altogether, we completed 11 SUIT stations (1 in open water, 10 under ice), 4 ice stations and 3 midwater trawls. All SUIT deployments were succesful, expanding Sikuliaq’s capability of advanced scientific operations in ice-covered waters. Polar cod were caught at all SUIT stations, totalling 153 fish. Most fish appeared to be first-year juveniles between 6 and 8 cm in size, pending age determinations. Fish abundance increased with increasing ocean depth, sea-ice draft and abundance of the ice amphipod Onisimus spp.. The mesozooplankton community in the upper 50 m was dominated by the copepod Metridia longa. Respiration experiments indicated that Metridia and, surprisingly, Calanus glacialis from shelf stations were in an active metabolic state. Chlorophyll a concentrations were low (< 0.5 mg m-3) in the water column. Conversely, visual inspection of ice core filters indicated that ice algal biomass had already begun to accumulate in the autumn sea ice. The trophic relationships between ice algae, zooplankton and sea-ice fauna and Polar cod will be analyzed in detail based on hundreds of biological samples, including diet and trophic biomarker samples. Investigations of otolith microchemistry and population genetics studies on each sampled fish will help elucidate their origins and migration patterns. Preliminary results of this expedition support our hypothesis that juvenile Polar cod associate with sea ice in autumn and show that prey is available to sustain them at the onset of winter

Allometric relationships of ecologically important Antarctic and Arctic zooplankton and fish species

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Schaafsma, F. L., David, C. L., Kohlbach, D., Ehrlich, J., Castellani, G., Lange, B. A., Vortkamp, M., Meijboom, A., Fortuna-Wunsch, A., Immerz, A., Cantzler, H., Klasmeier, A., Zakharova, N., Schmidt, K., Van de Putte, A. P., van Franeker, J. A., & Flores, H. Allometric relationships of ecologically important Antarctic and Arctic zooplankton and fish species. Polar Biology 45, (2022): 203–224, https://doi.org/10.1007/s00300-021-02984-4.Allometric relationships between body properties of animals are useful for a wide variety of purposes, such as estimation of biomass, growth, population structure, bioenergetic modelling and carbon flux studies. This study summarizes allometric relationships of zooplankton and nekton species that play major roles in polar marine food webs. Measurements were performed on 639 individuals of 15 species sampled during three expeditions in the Southern Ocean (winter and summer) and 2374 individuals of 14 species sampled during three expeditions in the Arctic Ocean (spring and summer). The information provided by this study fills current knowledge gaps on relationships between length and wet/dry mass of understudied animals, such as various gelatinous zooplankton, and of animals from understudied seasons and maturity stages, for example, for the krill Thysanoessa macrura and larval Euphausia superba caught in winter. Comparisons show that there is intra-specific variation in length–mass relationships of several species depending on season, e.g. for the amphipod Themisto libellula. To investigate the potential use of generalized regression models, comparisons between sexes, maturity stages or age classes were performed and are discussed, such as for the several krill species and T. libellula. Regression model comparisons on age classes of the fish E. antarctica were inconclusive about their general use. Other allometric measurements performed on carapaces, eyes, heads, telsons, tails and otoliths provided models that proved to be useful for estimating length or mass in, e.g. diet studies. In some cases, the suitability of these models may depend on species or developmental stages.The Netherlands Ministry of Agriculture, Nature and Food Quality (LNV) funded this research under its Statutory Research Task Nature & Environment WOT-04-009-047.04. This research was further supported by the Netherlands Polar Programme (NPP), managed by the Dutch Research Council (NWO) under project nr. ALW 866.13.009 (ICEFLUX-NL). The study is associated with the Helmholtz Association Young Investigators Group ICEFLUX: Ice-ecosystem carbon flux in polar oceans (VH-NG-800) and contributes to the Helmholtz (HGF) research Programme Changing Earth – Sustaining our Future, Research Field Earth & Environment, Topic 6.1 and 6.3. NZ was supported by the GEOMAR project CATS: The Changing Arctic Transpolar System (BMBF-FK2 CATS). Contributions by KS were funded by the UK’s Natural Environment Research Council MOSAiC-Thematic project SYM-PEL: “Quantifying the contribution of sympagic versus pelagic diatoms to Arctic food webs and biogeochemical fluxes: application of source-specific highly branched isoprenoid biomarkers” (NE/S002502/1). BAL was further supported by the Norwegian Polar Institute and funding to M. Granskog from the Research Council of Norway to projects CAATEX (280531) and HAVOC (280292). DK was further funded by the Research Council of Norway through the project The Nansen Legacy (RCN # 276730) at the Norwegian Polar Institute. GC was further funded by the project EcoLight (03V01465) as part of the joint NERC/BMBF programme Changing Arctic Ocean. AVdP received support from Belspo in the framework the EU Lifewatch ERIC (Grant agreement FR/36/AN3) and the FEDTwin. Expedition Grant Numbers: ARK XVII/3 (PS80), AWI-PS81_01 (WISKY), ANT-XXIX/9 (PS82), AWI-PS89_02 (SIPES), AWI_PS92_00 (TRANSSIZ) and AWI_PS106/1_2-00 (SIPCA)