Variation in the diet of Arctic Cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) in the Pacific Arctic and Bering Sea

Arctic cod, Boreogadus saida (Lepechin, 1774), is a nodal species in Arctic marine foodwebs as an important prey of many birds, marine mammals, and other fishes, as well as an abundant predator of zooplankton and epibenthic fauna. We examined the summer diet of Arctic cod across a latitudinal gradient extending from the southern limit of their distribution in the eastern Bering Sea to the northern margins of the eastern Chukchi Sea (ECS) continental shelf. Specimens were collected from demersal and pelagic trawls conducted between 1999 and 2012, and across a range of predator sizes (3–26 cm). Arctic cod diets vary with body size and between regions within the study area, and appear to vary between years in the eastern Bering Sea, indicating opportunistic feeding habits. Constrained Analysis of Principal Coordinates was conducted on ECS demersal samples and revealed consumption of fish and decapod crustacea were positively correlated with Arctic cod length while consumption of euphausiids and copepods had the opposite relationship. The demersal Arctic cod diet in the northern latitudes of the ECS was dominated by copepod consumption (47% by weight, %W), but copepods were less important (12–26%W) in the central and southern latitudes of the ECS and in the northern and eastern Bering Sea—areas where diets were more varied in their composition. High levels of variation in the diet of Arctic cod highlights the need to monitor Arctic cod diets to identify consistently dominant prey types and potential future changes to trophic relationships related to climate change or increasing anthropogenic activity.     

Temperature-dependent growth and behavior of juvenile Arctic cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) and co-occurring North Pacific gadids

The thermal sensitivity of Arctic fish species is poorly understood, yet such data are a critical component of forecasting and understanding ecosystem impacts of climate change. In this study, we experimentally measured temperature-dependent growth and routine swim activity in the juvenile stage of two Arctic gadids (Arctic cod, Boreogadus saida and saffron cod, Eleginus gracilis) and two North Pacific gadids (walleye pollock, Gadus chalcogrammus and Pacific cod, Gadus macrocephalus) over a 6-week growth period across five temperatures (0, 5, 9, 16 and 20 °C). Arctic cod demonstrated a cold-water, stenothermic response in that there was relatively high growth at 0 °C (0.73 % day^?1), near-maximal growth at 5 °C (1.35 % day^?1) and negative impacts on activity, growth and survival at 16 °C. In contrast, saffron cod demonstrated a warmer-water, eurythermic response, and temperature had a positive effect on growth and condition beyond 16 °C. However, despite these distinct thermal responses, walleye pollock and Pacific cod grew 2–3 times faster than Arctic gadids across a relatively broad temperature range above 5 °C. These results, coupled with possible northward expansion by both Pacific cod and walleye pollock, suggest Arctic cod are highly vulnerable to continued climate change in the Arctic, especially in coastal areas of the Beaufort and Chukchi Seas where temperatures already exceed 14 °C in the summer growth period.     

Borealization of nearshore fishes on an interior Arctic shelf over multiple decades

Borealization is a type of community reorganization where Arctic specialists are replaced by species with more boreal distributions in response to climatic warming. The process of borealization is often exemplified by the northward range expansions and subsequent proliferation of boreal species on the Pacific and Atlantic inflow Arctic shelves (i.e., Bering/Chukchi and Barents seas, respectively). But the circumpolar nearshore distribution of Arctic-boreal fishes that predates recent warming suggests borealization is possible beyond inflow shelves. To examine this question, we revisited two nearshore lagoons in the eastern Alaska Beaufort Sea (Kaktovik and Jago lagoons, Arctic National Wildlife Refuge, Alaska, USA), a High Arctic interior shelf. We compared summer fish species assemblage, catch rate, and size distribution among three periods that spanned a 30-year record (baseline conditions, 1988–1991; moderate sea ice decline, 2003–2005; rapid sea ice decline, 2017–2019). Fish assemblages differed among periods in both lagoons, consistent with borealization. Among Arctic specialists, a clear decline in fourhorn sculpin (Myoxocephalus quadricornis, Kanayuq in Iñupiaq) occurred in both lagoons with 86%–90% lower catch rates compared with the baseline period. Among the Arctic-boreal species, a dramatic 18- to 19-fold increase in saffron cod (Eleginus gracilis, Uugaq) occurred in both lagoons. Fish size (length) distributions demonstrated increases in the proportion of larger fish for most species examined, consistent with increasing survival and addition of age-classes. These field data illustrate borealization of an Arctic nearshore fish community during a period of rapid warming. Our results agree with predictions that Arctic-boreal fishes (e.g., saffron cod) are well positioned to exploit the changing Arctic ecosystem. Another Arctic-boreal species, Dolly Varden (Salvelinus malma, Iqalukpik), appear to have already responded to warming by shifting from Arctic nearshore to shelf waters. More broadly, our findings suggest that areas of borealization could be widespread in the circumpolar nearshore.     

Diets and body condition of polar cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) in the northern Bering Sea and Chukchi Sea

To understand trophic responses of polar cod Boreogadus saida (a key species in Arctic food webs) to changes in zooplankton and benthic invertebrate communities (prey), we compared its stomach contents and body condition between three regions with different environments: the northern Bering Sea (NB), southern Chukchi Sea (SC), and central Chukchi Sea (CC). Polar cod were sampled using a bottom trawl, and their potential prey species in the environment were sampled using a plankton net and a surface sediment sampler. Polar cod fed mainly on appendicularians in the NB and SC where copepods were the most abundant in the environment, while they fed on copepods, euphausiids, and gammarids in the CC where barnacle larvae were the most abundant species in plankton samples on average. The stomach fullness index of polar cod was higher in the NB and SC than CC, while their body condition index did not differ between these regions. The lower lipid content of appendicularians compared to other prey species is the most plausible explanation for this inconsistency.     

Lipid composition and trophic relationships of krill species in a high Arctic fjord

Our study deals with the lipid biochemistry of the krill community in the ecosystem of the high Arctic Kongsfjord (Svalbard). During the last decades, Kongsfjord experienced a change in krill species composition due to recent increased advection of Atlantic water masses carrying characteristic boreal as well as subtropical-boreal euphausiids into the ecosystem. The lipid biochemistry and trophic relationships of the species recently inhabiting the Arctic water masses are scarcely known, although a change in a krill population may have a significant impact on the ecosystem. A comparison of nutrition and energy storage strategies, stable isotopes, lipid profiles and fatty acid compositions showed remarkable differences between the krill species. These reflected the diverse feeding behaviours and specific adaptations to the environments of their origin: the boreal Meganyctiphanes norvegica and subtropical Nematoscelis megalops appear more carnivorous and have significantly lower mean lipid contents (29 and 10 %, respectively) and a different energy storage pattern (triacylglycerols and polar lipids, respectively) than the arcto-boreal Thysanoessa inermis, which consists of up to 54 % of lipids mainly stored as wax esters (>40 %). These differences may have significant implications for the rapidly changing marine food web of Kongsfjord—especially for higher trophic levels relying on the nutritional input of animal lipids.     

Characterization of the abiotic drivers of abundance of nearshore Arctic fishes

Fish are critical ecologically and socioeconomically for subsistence economies in the Arctic, an ecosystem undergoing unprecedented environmental change. Our understanding of the responses of nearshore Arctic fishes to environmental change is inadequate because of limited research on the physicochemical drivers of abundance occurring at a fine scale. Here, high‐frequency in situ measurements of pH, temperature, salinity, and dissolved oxygen were paired with daily fish catches in nearshore Alaskan waters of the Beaufort Sea. Due to the threat that climate change poses to high‐latitude marine ecosystems, our main objective was to characterize the abiotic drivers of abundance and elucidate how nearshore fish communities may change in the future. We used generalized additive models (GAMs) to describe responses to the nearshore environment for 18 fish species. Relationships between abundance and the physicochemical environment were variable between species and reflected life history. Each abiotic covariate was significant in at least one GAM, exhibiting both nonlinear and linear associations with abundance. Temperature was the most important predictor of abundance and was significant in GAMs for 11 species. Notably, pH was a significant predictor of abundance for six species: Arctic cod (Boreogadus saida), broad whitefish (Coregonus nasus), Dolly Varden (Salvelinus malma), ninespine stickleback (Pungitius pungitius), saffron cod (Eleginus gracilis), and whitespotted greenling (Hexagrammos stelleri). Broad whitefish and whitespotted greenling abundance was positively associated with pH, while Arctic cod and saffron cod abundance was negatively associated with pH. These results may be a bellwether for future nearshore Arctic fish community change by providing a foundational characterization of the relationships between abundance and the abiotic environment, particularly in regard to pH, and demonstrate the importance of including a wider range of physicochemical habitat covariates in future research.     

Feeding ecology of dominant groundfish in the northern Bering Sea

We investigated current diets of the six most abundant benthic fish in the northern Bering Sea. Our objective was to explore feeding strategies and potential competition with other top predators as ecosystem changes occur in the northern Bering Sea ecosystem. Our approach used stomach content data collected from field sampling during spring 2006 and 2007. Calanoid copepods and ampeliscid amphipods were important prey of Arctic cod (Boreogadus saida) but in different proportions depending upon fish size, feeding location, and local environmental conditions. Snailfish (Liparidae) occupied a broad niche and fed on a variety of benthic amphipods. Arctic alligatorfish (Ulcina olrikii) and Arctic staghorn sculpin (Gymnocanthus tricuspis) consumed ampeliscid amphipods predominantly. Shorthorn sculpin (Myoxocephalus scorpius) had a less-diverse diet, with snow crab (Chionoecetes opilio) most important by weight. Finally, all Bering flounder (Hippoglossoides robustus) sampled had empty stomachs. Our results indicate that ampeliscid amphipods, which have high biomass in the central region of the northern Bering Sea, are the most important prey for the dominant groundfish in the Chirikov Basin. Generally, all dominant benthic fish in the northern Bering Sea had narrow feeding niches, except snailfish. High diet overlap was found among many of the fish species, including Arctic cod and snailfish, snailfish and Arctic alligatorfish, and Arctic alligatorfish and Arctic staghorn sculpin. These findings are consistent with a relatively short food chain for benthic fish that are for the most part specialized feeders with narrow preferences for food and may be affected by changes in benthic prey distributions.     

Pelagic fish and zooplankton species assemblages in relation to water mass characteristics in the northern Bering and southeast Chukchi seas

This research explores the distributions and community composition of pelagic species in the sub-Arctic and Arctic waters of the northern Bering and central and southern Chukchi seas during September 2007 by linking pelagic zooplankton and fish assemblages to water masses. Juvenile saffron cod (Eleginus gracilis), polar cod (Boreogadus saida), and shorthorn sculpin (Myoxocephalus scorpius) were most abundant in warm, low salinity Alaska Coastal Water (ACW) of the central Chukchi Sea, characterized by low chlorophyll, low nutrients, and small zooplankton taxa. Adult Pacific herring (Clupea pallasii) were more abundant in the less stratified Bering Strait waters and in the colder, saltier Bering Shelf Water of the northern Bering and southern Chukchi seas, characterized by high chlorophyll, high nutrients, and larger zooplankton taxa. Juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon were most abundant in the less stratified ACW in the central Chukchi Sea and Bering Strait. Abundances of large zooplankton were dominated by copepods (Eucalanus bungii, Calanus glacialis/marshallae, Metridia pacifica) followed by euphausiids (juvenile Thysanoessa raschii and unidentified taxa), whereas small zooplankton were dominated by bivalve larvae and copepods (Centropages abdominalis, Oithona similis, Pseudocalanus sp.). Pelagic community composition was related to environmental factors, with highest correlations between bottom salinity and large zooplankton taxa, and latitude and fish species. These data were collected in a year with strong northward retreat of summer sea ice and therefore provide a baseline for assessing the effects of future climate warming on pelagic ecosystems in sub-Arctic and Arctic regions.     

Macromolecular production of phytoplankton in the northern Bering Sea, 2007

Photosynthetic carbon allocations into different macromolecular classes provide important clues regarding physiological conditions of phytoplankton and the nutritional status of potential grazers. The productivity experiments for photosynthetic carbon allocations were conducted at three light depths (100, 30, and 1 %) for nine different stations in the northern Bering Sea as an important gateway into the western Arctic Ocean, using the ¹³C isotope tracer technique to determine the major controlling factors and physiological conditions of phytoplankton. The photosynthetic carbon allocations into different macromolecular classes [Low molecular weight metabolites (LMWM), lipids, proteins, and polysaccharides] of primary producers were determined based on the productivity experiments. LMWM and polysaccharides had similar vertical patterns whereas lipids and proteins had reverse vertical patterns at all the stations, which is consistent with other results under different light depths. The overall average allocations were 37.9 (SD = ± 18.8 %), 26.6 (SD = ± 17.4 %), 26.5 (SD = ± 20.7 %), and 9.1 % (SD = ± 7.8 %), for LMWM, lipids, proteins, and polysaccharides, respectively. Based on a general pattern of macromolecular production in the northern Bering Sea, phytoplankton was in a physiologically transitional phase from an unlimited status to a nitrogen-deficient condition during our cruise period, 2007. However, more in situ field measurements for macromolecular production under a variety of environmental conditions will improve the understanding of the physiological responses of phytoplankton to the ongoing environmental changes in the Arctic Ocean.     

Lipid composition of the three co-existing <Emphasis Type="Italic">Calanus</Emphasis> species in the Arctic: impact of season,location and environment

Arctic species of Calanus are critical to energy transfer between higher and lower trophic levels and their relative abundance, and lipid content is influenced by the alternation of cold and warm years. All three species of Calanus were collected during different periods in Kongsfjorden (Svalbard, 79°N) and adjacent shelf during the abnormally warm year of 2006. Lipid composition and fatty acid structure of individual lipid classes were examined in relation with population structure. Wax esters dominated the neutral lipid fraction. Phosphatidylcholine (PC) dominated the structural lipids followed by phosphatidylethanolamine (PE). PC/PE ratios of 3–6 suggested an increase in PC proportions compared to earlier studies. Depending on the time scale, fatty acids of wax esters illustrated either trophic differences between fjord and offshore conditions for C. hyperboreus and C. finmarchicus or trophic differences related to seasonality for C. glacialis. Similarly, seasonality and trophic conditions controlled the changes in fatty acids of triglycerides, but de novo synthesis of long-chain monoenes suggested energy optimization to cope with immediate metabolic needs. Polar lipids fatty acid composition was species specific and on the long-term (comparison with data from the past decade) composition appears related to changes in trophic environment. Fatty acid composition of PC and PE indicated relative dominance of 20:5n-3 in PC and 22:6n-3 in PE for all three species. The combination of PE and PC acyl chain and phospholipid head group restructuring indicates an inter-annual variability and suggests that membrane lipids are the most likely candidate to evaluate adaptive changes in Arctic copepods to hydrothermal regime.     

Dietary characteristics of co-occurring polar cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) and capelin (<Emphasis Type="Italic">Mallotus villosus</Emphasis>) in the Canadian Arctic,Darnley Bay

Reduction in sea ice due to climate change is expected to have a negative impact on habitat availability for Arctic marine fishes and induce range expansion of species from southern environments. Such an effect will likely be observed in the abundance of polar cod, Boreogadus saida (Lepechin, 1774), as well as interspecific interactions of this intermediate-level trophic taxon, particularly in more southerly fringing seas in the Arctic. Polar cod and capelin, Mallotus villosus (Müller, 1776), are pelagic, planktivorous forage fishes, which occupy similar dietary niches and are the primary prey of marine predators. Co-occurring polar cod and capelin were collected at seven stations in Darnley Bay, NT, during August 2013. Standard length (SL), used as a proxy for age, suggested that polar cod (mean ± 1 SD: 71.1 ± 10.3 mm) were predominantly age 1+ and capelin (96.2 ± 13.4 mm) were mostly age 2+. Stomach content analyses indicated that both species feed extensively on calanoid copepods (Calanus hyperboreus, C. glacialis, Metridia longa) and amphipods (Themisto libellula). There was high dietary overlap between capelin and polar cod, evidenced by Schoener’s index (0.80). Additionally the quantity of dietary items, biomass and energetic content consumed differed among size classes in both capelin (SL, 70.5–132.0 mm) and polar cod (SL, 42.1–114.4 mm). This study illustrates that the diets of these sympatric forage fishes in an Arctic ecosystem are very similar, indicating a high potential for interspecific competition as the sub-Arctic capelin expands its range into Arctic regions with climate change.     

Under-ice distribution of polar cod <Emphasis Type="Italic">Boreogadus saida</Emphasis> in the central Arctic Ocean and their association with sea-ice habitat properties

In the Arctic Ocean, sea-ice habitats are undergoing rapid environmental change. Polar cod (Boreogadus saida) is the most abundant fish known to reside under the pack-ice. The under-ice distribution, association with sea-ice habitat properties and origins of polar cod in the central Arctic Ocean, however, are largely unknown. During the RV Polarstern expedition ARK XXVII/3 in the Eurasian Basin in 2012, we used for the first time in Arctic waters a Surface and Under Ice Trawl with an integrated bio-environmental sensor array. Polar cod was ubiquitous throughout the Eurasian Basin with a median abundance of 5000 ind. km^?2. The under-ice population consisted of young specimens with a total length between 52 and 140 mm, dominated by 1-year-old fish. Higher fish abundance was associated with thicker ice, higher ice coverage and lower surface salinity, or with higher densities of the ice-amphipod Apherusa glacialis. The fish were in good condition and well fed according to various indices. Back-tracking of the sea-ice indicated that sea-ice sampled in the Amundsen Basin originated from the Laptev Sea coast, while sea-ice sampled in the Nansen Basin originated from the Kara Sea. Assuming that fish were following the ice drift, this suggests that under-ice polar cod distribution in the Eurasian Basin is dependent on the coastal populations where the sea-ice originates. The omnipresence of polar cod in the Eurasian Basin, in a good body condition, suggests that the central Arctic under-ice habitats may constitute a favourable environment for this species survival, a potential vector of genetic exchange and a recruitment source for coastal populations around the Arctic Ocean.     

Fatty acid and stable isotope characteristics of sea ice and pelagic particulate organic matter in the Bering Sea: tools for estimating sea ice algal contribution to Arctic food web production

We determined fatty acid (FA) profiles and carbon stable isotopic composition of individual FAs (δ¹³C_FA values) from sea ice particulate organic matter (i-POM) and pelagic POM (p-POM) in the Bering Sea during maximum ice extent, ice melt, and ice-free conditions in 2010. Based on FA biomarkers, differences in relative composition of diatoms, dinoflagellates, and bacteria were inferred for i-POM versus p-POM and for seasonal succession stages in p-POM. Proportions of diatom markers were higher in i-POM (16:4n-1, 6.6–8.7 %; 20:5n-3, 19.6–25.9 %) than in p-POM (16:4n-1, 1.2–4.0 %; 20:5n-3, 5.5–14.0 %). The dinoflagellate marker 22:6n-3/20:5n-3 was highest in p-POM. Bacterial FA concentration was higher in the bottom 1 cm of sea ice (14–245 μg L^?1) than in the water column (0.6–1.7 μg L^?1). Many i-POM δ¹³C_FA values were higher (up to ~10 ‰) than those of p-POM, and i-POM δ¹³C_FA values increased with day length. The higher i-POM δ¹³C_FA values are most likely related to the reduced dissolved inorganic carbon (DIC) availability within the semi-closed sea ice brine channel system. Based on a modified Rayleigh equation, the fraction of sea ice DIC fixed in i-POM ranged from 12 to 73 %, implying that carbon was not limiting for primary productivity in the sympagic habitat. These differences in FA composition and δ¹³C_FA values between i-POM and p-POM will aid efforts to track the proportional contribution of sea ice algal carbon to higher trophic levels in the Bering Sea and likely other Arctic seas.     

Distinct patterns of Arctic cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) presence and absence in a shallow high Arctic embayment,revealed across open-water and ice-covered periods through acoustic telemetry

With climate change resulting in unpredictable sea ice conditions between years, it is crucial to gain a more comprehensive understanding of the subsequent effects on Arctic marine ecosystems. Arctic cod (Boreogadus saida) play a key role in the Arctic marine food web, serving as a food source that is estimated to contribute up to 75 % of energy transfer to higher trophic levels. To investigate Arctic cod residency and distribution in Resolute Bay (74°44′N, 095°04′W), 85 individuals from four locations in the bay were captured, measured, weighed, implanted with acoustic tags and subsequently tracked on an acoustic array of 49 receivers. Two main periods of residence in the bay were identified, the first in open water and the second under ice cover, and both concluded with a collective mass departure of fish. A generalised linear mixed model was used to investigate the influence of variables on Arctic cod presence/absence in the bay, indicating that ingress and egress were influenced by environmental changes, particularly those associated with the transition from open-water to the ice-covered period. Timing and distribution, during the study period, appeared to be influenced by a combination of physiological acclimation, and a balance between resource availability and refuge from predators. Receiver site Residence Index (RI) analysis revealed strong site fidelity of fish towards the northern areas of the bay, and this behaviour was consistent between tagging groups and individuals, indicating that the majority of tagged cod were representative of a single school. This study represents the first employment of acoustic telemetry to monitor the movements of individual Arctic cod over 9 months, incorporating both open-water and ice-covered periods.     

Vertical segregation of age-0 and age-1+ polar cod (<Emphasis Type="Italic">Boreogadus saida</Emphasis>) over the annual cycle in the Canadian Beaufort Sea

The offshore marine ecosystem of the Canadian Beaufort Sea faces the double pressure of climate change and industrialization. Polar cod (Boreogadus saida) is a pivotal forage species in this ecosystem, accounting for 95 % of the pelagic fish assemblage. Its vertical distribution over the annual cycle remains poorly documented. Hydroacoustic records from 2006 to 2012 were analysed to test the hypothesis that age-0 polar cod segregate vertically from larger congeners. Trawls and ichthyoplankton nets validated the acoustic signal. Fish length, weight, and biomass were estimated from new regressions of target strength and weight on standard length. Polar cod were vertically segregated by size in all months, with small age-0 juveniles in the epipelagic (<100 m) layer and larger age-1+ deeper in the water column. From December to March, the biomass of age-1+ peaked in a mesopelagic layer between 200 and 400 m. With increasing irradiance from April to July, the mesopelagic layer deepened and extended to 600 m. Starting in July, age-0 polar cod formed an epipelagic scattering layer that persisted until November. From September onward, age-0 left the epipelagic layer to join small age-1+ in the upper mesopelagic layer. Low biomass in the mesopelagic layer from February to September likely resulted from large polar cod settling on the seafloor to avoid diving marine mammals. Longer ice-free seasons, warmer sea-surface temperatures, or an oil spill at the surface would likely impact epipelagic age-0, while mesopelagic age-1+ would be vulnerable to an eventual oil plume spreading over and above the seafloor.     

Ecology of Arctic cod <Emphasis Type="Italic">Boreogadus saida</Emphasis> (Gadiformes,Gadidae) and its fishery potential in Kara Sea

According to the bottom trawl-survey data, 97% of the ichthyomass in the southwestern region of the Kara Sea are composed of the Arctic cod Boreogadus saida; its stock is significantly higher than the previously registered resources. The Arctic cod is most unevenly distributed across the water area and capable to form the high-density aggregations, which can be caught by the targeted trawls. A wide range of the age composition (0+?6+), the size-age composition, and the growth rates of the Arctic cod in the trawl catches in the Kara Sea, which are different from those in the fish in the adjacent Barents Sea, can indicate their assignment of the Arctic cod in these seas to different populations.     

Spatial and ontogenetic patterns of Pacific cod (<Emphasis Type="Italic">Gadus macrocephalus</Emphasis> Tilesius) predation on octopus in the eastern Bering Sea

In 2012, the North Pacific Fishery Management Council adopted a consumption-based stock assessment method to determine catch limits for the non-target, multi-species octopus complex in the Bering Sea-Aleutian Islands (BSAI) fishery management area. The method uses Pacific cod (Gadus macrocephalus) diet data as a basis for estimating octopus complex natural mortality and minimum biomass. To enhance understanding of the predator-prey interaction between Pacific cod and octopus, we examined patterns of octopus consumption by Pacific cod using long-term stomach contents data from the eastern Bering Sea continental shelf, a large, ecologically unique subarea of the BSAI. Generalized additive modeling of octopus presence/absence in Pacific cod diets revealed distinct spatial, ontogenetic and seasonal consumption patterns. Prey octopus frequency of occurrence (FO) generally increased with bottom depth, latitude and Pacific cod fork length, and FO in the southern BSAI was lower during winter and spring than during summer. Prey octopus FO patterns may reflect overall consumption patterns and likely indicate long-term distribution patterns of small-sized (<1 kg) octopus on the EBS shelf, although we could not visually distinguish between octopus species using prey remains. Multi-species beak length-to-body mass regressions developed from three octopus species allowed reasonable estimation of prey octopus mass, and we found Pacific cod fork length was positively correlated with prey octopus mass, suggesting predator-prey interactions are sensitive to predator and prey size composition. Pacific cod consumed octopus with estimated masses ranging from 0.000017 kg to 4.62 kg, while octopus taken during concurrent bottom trawl surveys range from 0.05 kg to greater than 25 kg. Based on our findings, we expect the consumption-based stock assessment underestimates BSAI octopus complex biomass because it cannot account for larger octopus, such as the 10–20 kg Enteroctopus dofleini which dominate incidental take in BSAI Pacific cod pot fishery.     

Pre-winter distribution and habitat characteristics of polar cod (Boreogadus saida) in southeastern Beaufort Sea

Polar cod was shown to form dense under-ice winter aggregations at depth in the Amundsen Gulf (southeastern Beaufort Sea). In this paper, we verify the premises of the aggregation mechanism by determining the distribution and habitat characteristics of polar cod prior to the formation of winter aggregations. Multifrequency split-beam acoustic data collected in October–November 2003 revealed that polar cod split into two distinct layers. Age-0 polar cod formed an epipelagic layer between 0 and ~60 m depth without any clear large-scale biomass trend. In contrast, adult polar cod tended to distribute into an offshore mesopelagic layer between ~200 and 400 m that shoaled into a denser (1–37 g m^?2) benthopelagic layer on sloping bottoms (between 150 and 600-m isobaths) along the Mackenzie shelf and into the Amundsen Gulf basin. Concentrations peaked in the Amundsen Gulf where estimated total biomass reached ~250 kt. Both age-0 and adult polar cod distributed in the warmer waters (>?1.4 °C). We hypothesise that polar cod concentration over slopes is governed by the combined actions of (1) local currents concentrating both depth-keeping zooplankton and polar cod at the shelf-break and basin slopes and (2) trophic association with these predictable topographically trapped aggregations of zooplankton prey. During freeze-up, these slope concentrations of polar cod are thought to constitute the main source of the observed dense under-ice winter aggregations. The hypothesis of active short-distance displacements combined with prevailing mean currents is retained as the likely aggregation mechanism.     

Lipid content in 19 brackish and marine microalgae: influence of growth phase, salinity and temperature

Algal lipids provide essential fatty acids for higher trophic levels in the marine food web, and understanding the fatty acid composition in phytoplankton is critical for evaluating its value as a diet. Nineteen microalgal species, mainly originating from the Baltic Sea, covering major algal classes were grown in different growth conditions. Samples were taken during both the exponential and stationary growth phases and analysed regarding their fatty acid methyl esters and free fatty acids. Our results show that across all screened species, total fatty acids increased significantly from exponential to stationary growth phase. Furthermore, it was observed that warm-water species contained more lipids and differed in their lipid profile as compared with the cold-water species. Brackish water species also showed a slightly higher lipid content than the marine species, but their lipid profile was not significantly different. Plotting changes in lipids against changes in cell nitrogen revealed a significant dependency between decrease in cell nitrogen and increase in lipids across all tested species.