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Populations of the macaroni penguin Eudyptes chrysolophus and the eastern rockhopper penguin E. filholi breeding at Marion Island (Prince Edward Islands) in the sub-Antarctic Indian Ocean decreased from 1994 to 2018. We examined their diets when rearing chicks during this period. There was substantial overlap in the diets of the two species, with crustaceans, dominated by the euphausiids Thysanoessa vicina and Euphausia vallentini, making up >80% of the diets by number and >60% by mass over the study period. The lanternfishes Krefftichthys anderssoni and Protomyctophum tenisoni were the most-commonly consumed fish in the diet of breeding macaroni penguins and dominated in their diet in three of the 25 years (1994, 1998 and 2014); they were also the most commonly consumed fish by eastern rockhopper penguins. Macaroni penguins consumed more of the amphipod crustacean Themisto gaudichaudii and the lanternfish Electrona carlsbergi as well as a greater diversity of and larger-sized fish than did eastern rockhopper penguins. The horsefish Zanclorhynchus spinifer was found in substantial amounts in the diet of eastern rockhopper penguins in 1996 and 1997. Despite annual variations in relative prey contributions to the diets, there were no significant long-term changes in the diet of either penguin species over the study period or when compared with an earlier assessment in 1982. We conclude that changes in the relative proportions of prey in the diets of these penguin species during breeding are unlikely to account for the recent declines in these penguin populations.

The results of the analysis of zooplankton assemblage state of the southern Barents Sea are presented. Zooplankton samples were collected during the cruise of the RV “Dalnie Zelentsy” in May 2016. Hydrological conditions were typical for Murmansk coastal water this season. A total of 47 zooplankton taxa were identified. Taxa number varied between stations, ranging 18–29, with copepods being a dominant group in zooplankton. The most frequent ones were Calanus finmarchicus, Metridia longa, Metridia lucens, Microcalanus spp., Oithona atlantica, Oithona similis, Pseudocalanus spp., copepod nauplii and ova, as well as cladoceran Evadne nordmanni, larvae of Echinodermata and Polychaeta, chaetognath Parasagitta elegans, and early stages of the euphausiids of the genus Thysanoessa. In populations of common copepod species Pseudocalanus spp. and Oithona similis, early age stages dominated, which indicated their continued reproduction. Total zooplankton abundance ranged from 748 to 6576 ind.·m−3, averaging 3012. Total zooplankton biomass varied from 17 to 157 mg of dry mass per m³, with a mean value of 83. The data obtained were comparable to those registered in Murmansk coastal water in July 2008 and were higher than those in August 2007. The authors suggest that it might be related to the differences in sampling seasons and hydrological conditions. Daily zooplankton production was estimated to be 0.49–4.04 mg of dry mass per m³, averaging (2.17 ± 0.17). These estimates were about twice as high as mean values, registered in Murmansk coastal water during summer period. This seems to be due to higher phytoplankton concentrations in spring. Total zooplankton stock for water area studied (25.8 thousand km²) was estimated to be 425,000 thousand tons of dry mass. Cluster analysis revealed four groups of stations that differ in relative abundance of Calanus finmarchicus, Copepoda nauplii, Oithona similis, larvae of Echinodermata, and appendicularian Fritillaria borealis. Spatial variation of zooplankton abundance was closely related to station location (latitude, longitude, and sampling depth), as well as bottom layer temperature and mean salinity at the station.

This study couples observations of krill (Thysanoessa inermis, Thysanoessa longicaudata, Thysanoessa rashii) from Tucker trawl nets and cameras, trying to test hypothesis that in the glaciated fjords of Svalbard most of the euphausiids biomass is located in near-bottom habitat and explains why in this region there are a substantial part of the krill population near the sea floor. Photographic material from the summers of 2013–2017 shows large numbers of near-bottom euphausiids (39% of the total krill biomass in Hornsund and 41% in Kongsfjorden), which reached a maximum density of 751 ± 224 indiv. m−3 in Kongsfjorden, 731 ± 198 indiv. m−3 in Hornsund, and 426 ± 124 indiv. m−3 in Adventfjorden. Regional distribution of near-bottom aggregations of krill seem to be associated with close proximity to the glacier front rather than with depth. The highest densities were located in the glacial bays. Where and why these aggregations occur is probably complicated and dependent on many environmental factors acting together. However, the dominant factors seem to be sedimentation and estuarine circulation. No krill aggregations were found during the winter cruise. The dominating species was T. inermis which made up 90% of the community. Other krill species—T. rashii and T. longicaudata, made up 6% and 4%, respectively. In the summer, aggregations of other macrozooplankton were also observed: amphipods of the genus Themisto and chaetognaths of the genera Eukrohnia and Parasagitta. Euphausiid densities in the water column (from Tucker trawl hauls) were an order of magnitude lower (0.33 indiv. m−3 for Kongsfjorden and 0.61 indiv. m−3 for Hornsund) than those of the near-bottom aggregations observed on cameras system. At most stations, the krill exhibited a behaviour, known as “nose diving” in the sediment, which is likely related to feeding. Observation of this phenomenon may indicate that krill (mostly T. inermis), found near the bottom of Spitsbergen fjords, is looking for food there. Near-bottom aggregations of zooplankton, mainly krill, are common in glacial bays and can be important in the function of the fjord ecosystem. Our research proves that the zooplankton biomass can be highly underestimated if only Tucker trawl sampling is done, due to neglecting the near-bottom layer in this type of method.

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.

This study sought to estimate the effect of an anthropogenic and climate-driven change in prey availability on the degree of individual and population specialization of a large marine predator, the fin whale (Balaenoptera physalus). We examined skin biopsies from 99 fin whales sampled in the St. Lawrence Estuary (Canada) over a nine year period (1998–2006) during which environmental change was documented. We analyzed stable isotope ratios in skin and fatty acid signatures in blubber samples of whales, as well as in seven potential prey species, and diet was quantitatively assessed using Bayesian isotopic models. An abrupt change in fin whale dietary niche coincided with a decrease in biomass of their predominant prey, Arctic krill (Thysanoessa spp.). This dietary niche widening toward generalist diets occurred in nearly 60% of sampled individuals. The fin whale population, typically composed of specialists of either krill or lipid-rich pelagic fishes, shifted toward one composed either of krill specialists or true generalists feeding on various zooplankton and fish prey. This change likely reduced intraspecific competition. In the context of the current “Atlantification” of northern water masses, our findings emphasize the importance of considering individual-specific foraging tactics and not only population or group average responses when assessing population resilience or when implementing conservation measures.

In polar ecology, zooplankton diets and survival rates vary according to the seasonality of solar radiation and oceanographic conditions. Each zooplankton species has evolved feeding strategies to survive in the diet-limited conditions of the “polar night.” Many zooplankton studies have reported seasonal adaptations in feeding activity during polar night based on their trophic niches. Nitrogen isotope analysis of amino acids has provided improved accuracy in estimates of trophic position (TP) in various marine species. In this study, field work was conducted in Kongsfjorden before (October 2017) and after polar night (April 2018). As representative zooplankton, an amphipod (Themisto abyssorum), euphausids (Meganycitiphanes norvegica and Thysanoessa sp.), a chaetognath (Parasagitta elegans), and copepods (Calanus spp. and Oithona similis) were collected. trophic position values of each taxon were estimated using the nitrogen isotope ratio of glutamic acid (δ15NGlu) and phenylalanine (δ15NPhe). Results showed that TP values of P. elegans were relatively constant, averaging 3.2 in both seasons, likely due to continuous feeding activity during polar night. Trophic position values were also constant for Calanus spp., ranging 2.5–3.0 in both seasons, due to their ability to utilize stored high-energy wax. In contrast, average TP values for O. similis, an omnivorous zooplankton, were 2.9 in October and 2.3 the following April. Trophic position values for O. similis before polar night can be attributed to the relatively high availability of algae during longer periods of daylight. We found that TP variation in zooplankton before and after polar night differed according to feeding activities in diet-restricted circumstances.

Source at http://dx.doi.org/10.1002/lno.10519 Zooplankton vertical migration enhances the efficiency of the ocean biological pump by translocating carbon (C) and nitrogen (N) below the mixed layer through respiration and excretion at depth. We measured C and N active transport due to diel vertical migration (DVM) in a Svalbard fjord at 79°N. Multifrequency analysis of backscatter data from an Acoustic Zooplankton Fish Profiler moored from January to September 2014, combined with plankton net data, showed that Thysanoessa spp. euphausiids made up > 90% of the diel migrant biomass. Classical synchronous DVM occurred before and after the phytoplankton bloom, leading to a mismatch with intensive primary production during the midnight sun. Zooplankton DVM resulted in C respiration of 0.9 g m−2 and ammonium excretion of 0.18 g N m−2 below 82 m depth between February and April, and 0.2 g C m−2 and 0.04 g N m−2 from 11 August to 9 September, representing > 25% and > 33% of sinking flux of particulate organic carbon and nitrogen, respectively. Such contribution of DVM active transport to the biological pump in this high-Arctic location is consistent with previous measurements in several equatorial to subarctic oceanic systems of the World Ocean. Climate warming is expected to result in tighter coupling between DVM and bloom periods, stronger stratification of the Barents Sea, and northward advection of boreal euphausiids. This may increase the role of DVM in the functioning of the biological pump on the Atlantic side of the Arctic Ocean, particularly where euphausiids are or will be prevalent in the zooplankton community.

Mesopelagic sound scattering layers (SSL) are ubiquitous in all oceans. Pelagic organisms within the SSL play important roles as prey for higher trophic levels and in climate regulation through the biological carbon pump. Yet, the biomass and species composition of SSL in the Arctic Ocean remain poorly documented, particularly in winter. A multifrequency echosounder detected a SSL north of Svalbard, from 79.8 to 81.4°N, in January 2016, August 2016, and January 2017. Midwater trawl sampling confirmed that the SSL comprised zooplankton and pelagic fish of boreal and Arctic origins. Arctic cod dominated the fish assemblage in August and juvenile beaked redfish in January. The macrozooplankton community mainly comprised the medusa Cyanea capillata, the amphipod Themisto libellula, and the euphausiids Meganyctiphanes norvegica in August and Thysanoessa inermis in January. The SSL was located in the Atlantic Water mass, between 200–700 m in August and between 50–500 m in January. In January, the SSL was shallower and weaker above the deeper basin, where less Atlantic Water penetrated. The energy content available in the form of lipids within the SSL was significantly higher in summer than winter. The biomass within the SSL was >12-fold higher in summer, and the diversity of fish was slightly higher than in winter (12 vs. 9 species). We suggest that these differences are mainly related to life history and ontogenetic changes resulting in a descent toward the seafloor, outside the mesopelagic layer, in winter. In addition, some fish species of boreal origin, such as the spotted barracudina, did not seem to survive the polar night when advected from the Atlantic into the Arctic. Others, mainly juvenile beaked redfish, were abundant in both summer and winter, implying that the species can survive the polar night and possibly extend its range into the high Arctic. Fatty-acid trophic markers revealed that Arctic cod mainly fed on calanoid copepods while juvenile beaked redfish targeted krill (Thysanoessa spp.). The relatively high biomass of Arctic cod in August and of redfish in January thus suggests a shift within the SSL, from a Calanus-based food web in summer to a krill-based food web during winter.

We synthesize recent research on variation in annual production of copepods (Calanus spp.), euphausiids (Thysanoessa spp.), and juvenile walleye pollock (Gadus chalcogrammus) in the southeastern Bering Sea. We reach five conclusions: 1) the timing of the spring bloom is more important than the amount of annual primary production for the transfer of primary to secondary production (i.e., timing matters); 2) summer and fall, not just spring, matter: organisms must maximize energy intake devoted to somatic growth and storage of lipids and minimize energy expenditures during each season; 3) stored lipids are important for the overwinter survival of both zooplankton and age-0 walleye pollock; 4) variation in ice extent and timing of ice retreat affect the spatial distributions of phytoplankton, zooplankton, and age-0 walleye pollock; when these spatial distributions match in late-ice-retreat years, the annual production of copepods, euphausiids, and juvenile walleye pollock often increases (i.e., location matters); 5) if years with late ice retreat, which favor copepod, euphausiid, and juvenile walleye pollock production, occur in succession, top–down control increases. These conclusions help to explain annual variation in production of copepods, euphausiids and juvenile walleye pollock. Copepods and euphausiids often are more abundant in cold years with late ice retreat than in warm years with early ice retreat due to bloom timing and the availability of ice algae during years with late ice retreat. As a consequence, age-0 walleye pollock consume lipid-enriched prey in cold years, better preparing them for their first winter and their overwinter survival is greater. In addition, there is a spatial match of primary production, zooplankton, and age-0 walleye pollock in cold years and a mismatch in warm years.

Euphausiids are an important component of the eastern Bering Sea marine ecosystem. We synthesized information on the ecological roles of two species, Thysanoessa raschii, which predominates over the Middle and Inner Shelf Domains, and Thysanoessa inermis, which predominates over the Outer Shelf Domain. Although estimates of euphausiid biomass across the shelf are not well constrained, we estimated that, between April and July, 2004–2010, euphausiid biomass was 3.08–5.25 g C m−2 on the outer shelf and 1.95–3.92 g C m−2 on the middle shelf. Modeled estimates of euphausiid production, for spring and summer combined, varied between 0.043 g C m−2 d−1 and 0.051 g C m−2 d−1, depending on location, with a mean of 0.048 g C m−2 d−1. Recently reported field measurements of annual primary production over the southeastern Bering Sea in 2008–2009 vary between 0.06 and 6.65 g C m−2 d−1, with a mean of 1.262 g C m−2 d−1±2.049 g C m−2 d−1 in spring and summer combined, a level sufficient to support euphausiids, at least on an annualized basis. Walleye pollock (Gadus chalcogrammus, hereafter pollock) is the single most important consumer of euphausiids over the eastern Bering Sea shelf. We estimated that pollock consumed between 0.0042 and 0.019.7 g C m−2 d−1 of euphausiids, depending on year, with a mean of 0.011 g C m−2 d−1 in summer averaged over 1999–2009. This consumption is equivalent to between 17% and 29% of summer euphausiid production, depending on location. Over the period for which data were available (2004–2012), we observed a strong negative relationship between euphausiid biomass as determined in acoustic surveys and pollock biomass as estimated in the eastern Bering Sea pollock stock assessment (r2=0.82). During this time period, sea-surface temperature was the second strongest predictor of euphausiid biomass, (r2=0.63). However, for the period 2004–2010, bottom temperature (r2=0.94) was the strongest predictor, followed by pollock biomass from the pollock stock assessment (r2=0.82), and sea-surface temperature (r2=0.81). Mean pollock density in the acoustic surveys was not a powerful predictor of euphausiid biomass during either period. In spatially explicit multiple regression analyses for the periods 2004–2012 and 2004–2010 those formulations that included sea-surface and bottom temperatures as well as survey estimates of pollock had the greatest explanatory value. However, when either or both temperature terms were dropped, the explanatory value of the models dropped considerably. When pollock biomass was dropped from the models, there was little change in explanatory value compared to the full model. Euphausiid production and pollock consumption data coupled with a negative relationship between euphausiid biomass and stock assessment estimates of pollock biomass indicate a top-down predation effect. However, strong negative relationships between euphausiid biomass and water temperatures indicate the influence of a bottom-up mechanism. The apparent differences in these results may relate to the different spatial and temporal scales used to assess the pollock biomass used in the analyses. Alternatively, euphausiid biomass may be strongly controlled during a restricted portion of the year, such as spring, if critical food needs are not met in some years. We lack the data necessary to resolve these alternative hypotheses.

The polar night in the Arctic is characterized by up to six months of darkness, low temperatures and limited food availability. Biological data on species composition and abundance during this period are scarce due to the logistical challenges posed when sampling these regions. Here, we characterize the plankton community composition during the polar night using water samplers and zooplankton net samples (50, 64, 200, 1500 μm), supplemented by acoustics (ADCPs, 300 kHz), to address a previously unresolved question–which species of zooplankton perform diel vertical migration during the polar night? The protist community (smallest plankton fraction) was mainly represented by ciliates (Strombidiida). In the larger zooplankton fractions (50, 64, 200 μm) the species composition was represented primarily by copepod nauplii and small copepods (e.g., Microcalanus spp., Pseudocalanus spp. and Oithona similis). In the largest zooplankton fraction (>1500 μm), the euphausiid, Thysanoessa inermis, was the most abundant species followed by the chaetognath Parasagitta elegans. Classical DVM was not observed throughout the darkest parts of the polar night (November–mid-January), although, subtle vertical migration patterns were detected in the acoustic data. With the occurrence of a more distinct day–night cycle (i.e., end of January), acoustical DVM signals were observed, paralleled by a classical DVM pattern in February in the largest fractions of zooplankton net samples. We suggest that Thysanoessa spp. are main responsible for the acoustical migration patterns throughout the polar night, although, chaetognaths and copepods may be co-responsible.

We studied seasonal and regional change in vertical distribution and DVM patterns of four euphausiid species (Euphausia pacifica, Thysanoessa inspinata, Thysanoessa longipes, and Tessarabrachion oculatum) from two years of surveys using MOCNESS above 1500 m depth across a transect in 3 regions of the northwestern (NW) Pacific, off east of Japan; Oyashio, Kuroshio, and Oyashio–Kuroshio Mixed Water Regions (MWR). The four euphausiid species exhibited a regional change in vertical distribution, i.e., slightly deeper in the MWR and much deeper in the Kuroshio region than in the Oyashio region. They found in higher and wider temperature ranges in the MWR than in the Oyashio region, which demonstrated that the four species were able to adapt to different temperatures in different regions. In the MWR and Oyashio regions, E. pacifica is a surface migrant (differences between day and night mean median depths, D-N, were ca. 300 m) and T. oculatum is a moderate subsurface migrant that performs short DVM in the upper mesopelagic zone (D-N ca. 100 m). The other two morphologically similar Thysanoessa species (T. inspinata and T. longipes) segregated vertically between E. pacifica and T. oculatum at night in the Oyashio region, suggesting vertical habitat partitioning with the former two species but not with themselves. However, a seasonal pattern was observed in the vertical distribution and DVM of T. longipes in the Oyashio region. It behaves as a surface migrant in May, whereas most of individuals were found in the mesopelagic layer in September. In contrast, T. inspinata did not exhibit a clear DVM throughout the year (i.e., a moderate subsurface migrant). This seasonal difference might be a strategy to minimize competition between related species. Among the four species, only E. pacifica was found in higher temperatures at night than during the daytime, and the highest temperatures at the median depth varied among species (from 7.5 °C to 13.7 °C) although the lowest temperature did not vary greatly (from 1.0 °C to 1.8 °C), which indicates high temperatures act as a limiting factor as opposed to low temperatures. Furthermore, the integrated chlorophyll a values exhibited significant negative correlation with median depths of only E. pacifica at night. These results indicate a strategy which makes E. pacifica the dominant species in the area, that is, it has a trade-off of long migrations and a warmer environment that accelerates metabolism, in return for obtaining a food-rich environment.

Euphausiid (krill) and amphipod dynamics were studied during 2006–2011 by use of plankton nets in Kongsfjorden (79°N) and adjacent waters, also including limited sampling in Isfjorden (78°N) and Rijpfjorden (80°N). The objectives of the study were to assess how variations in physical characteristics across fjord systems affect the distribution and abundance of euphausiids and amphipods and the potential for these macrozooplankton species to reproduce in these waters. The abundances of euphausiids and amphipods were higher in Kongsfjorden than in Rijpfjorden and Isfjorden, and the highest abundances were observed at the innermost stations of Kongsfjorden, where Thysanoessa inermis and Themisto libellula dominated. The Atlantic species Thysanoessa longicaudata, Meganyctiphanes norvegica and Themisto abyssorum dominated at the outside Kongsfjorden. Inter-annual and seasonal variability in abundances of euphausiids and amphipods were evident. The presence of ripe euphausiids outside Kongsfjorden indicates that they may reproduce in these areas. Mature individuals of T. abyssorum were recorded mainly outside Kongsfjorden, whereas no mature or ripe T. libellula were present in both the inner and outer parts of this fjord. If the warming trend persists, as seen during the last decade, this would favour the Atlantic/boreal euphausiid species, while Arctic species, such as the amphipod T. libellula, may decline. Euphausiids and amphipods are major food of capelin (Mallotus villosus) and polar cod (Boreogadus saida), respectively, in this region, and changes in prey abundance will likely have an impact on the feeding dynamics of these important fish species.

This study investigated spatial and temporal patterns in distribution, population structure and diet of Bolin's lanternfish Protomyctophum bolini, Tenison's lanternfish Protomyctophum tenisoni and gaptooth lanternfish Protomyctophum choriodon in the Scotia Sea using data collected by midwater trawl during spring, summer and autumn. Protomyctophum bolini was the most abundant species of the genus encountered throughout the Scotia Sea with the greatest concentrations occurring around the Antarctic Polar Front (APF). This species had a life cycle of 2+ years, but spatial differences in population structure were apparent as the I-group was absent from all regions south of the APF, suggesting that the species does not recruit in the Scotia Sea. Protomyctophum tenisoni occurred mostly in waters characteristic of the APF and was absent from the southern Scotia Sea. It had a limited size range, but there was clear size-related sexual dimorphism with males significantly larger than females. The species had a life cycle of c. 2 years, but the I-group (c. 1 year old, 1 November to 31 October the next year) occurred only in regions close to the APF suggesting that recruitment is restricted to these waters. A seasonal southward migration for P. choriodon is likely as the species occurred mostly to the south-west of South Georgia in summer, but extended to the sea-ice sectors in autumn. Protomyctophum choriodon had a life cycle of 4+ years in the Scotia Sea and the population was dominated by age classes >3 years old. Larval stages were absent during the surveys for all species. Diurnal variations in vertical distribution were apparent for all three species. Interspecific variations in diet were evident, but all species were primarily copepod feeders, with Metridia spp., Rhincalanus gigas and Calanus simillimus generally dominating their diet. Small euphausiids, principally Thysanoessa spp., were also an important component of their diets, particularly for P. choriodon which had the largest body size. The spatial and temporal variations in diet for both P. bolini and P. tenisoni were broadly consistent with underlying abundance patterns within the mesozooplankton community.

The high-Arctic Kongsfjord is influenced mainly by cold Arctic water but also by warmer Atlantic water masses. In recent years, the proportion of the Atlantic inflow from the south has increased. Concurrently, one temperate-boreal (Meganyctiphanes norvegica) and one subtropical-temperate (Nematoscelis megalops) krill species are now regularly found in the Kongsfjord, in addition to the previously prevailing arcto-boreal species Thysanoessa inermis and T. raschii. In view of the recent changes in these species’ biogeographic distributions, we compared their physiological tolerances. Using non-invasive optical oxygen sensors, respiration measurements served to characterize metabolic responses to temperature variations. Thysanoessa spp. appear more cold-stenotherm than the other 2 krill species: the upper level of respiratory capacity is reached at 12°C and they are less tolerant to decreasing oxygen concentrations. This finding is consistent with their arcto-boreal distribution. In contrast, M. norvegica and N. megalops showed a higher tolerance to temperature changes, a robust nutritional condition and sexual maturity. Such physiological plasticity may explain the recent northward expansion of their geographic range.

As in many high-latitude ecosystems, euphausiids (order Euphausiacea, 'krill') play a key role in the Barents Sea by channeling energy from primary producers to fish and other zoo- plankton predators. We used multifrequency acoustic data from several recent multidisciplinary surveys to describe the spatial distribution of backscatter likely to be from euphausiids. Spatial patterns in euphausiid backscatter observed in 2010, 2011, and 2012 were correlated with verti- cally integrated euphausiid biomass collected with plankton nets, and were also broadly consis- tent with the distribution of euphausiids expected from the literature. We used the high-resolution and broad-spatial coverage of our euphausiid backscatter data to update multiple regression models of baleen (fin, humpback, and minke) whale distribution to test the hypothesis that these animals aggregated where euphausiids were abundant. After controlling for physical environ- mental factors and the densities of capelin and several other potential prey taxa, we found that fin whale densities were positively and linearly associated with euphausiid backscatter, and higher than average densities of humpback whales were found in areas with high euphausiid back - scatter. No association was found between minke whales and euphausiids. Densities of all 3 whale species were also positively associated with capelin. For fin and humpback whales, the effects of capelin and euphausiids on whale densities appeared to be principally separate and additive, although there was some evidence for a stronger effect of euphausiids at low capelin densities. In terms of their preference for euphausiids and capelin, these whale species appeared to be flexible, opportunistic predators.

Concurrent measurements of predator and prey δ 15 N isotope values demonstrated that a cross-shelf isotopic gradient can propagate through a marine food web from forage species to top-tier predators and indicate foraging areas at a scale of tens of kilometers. We measured δ 13 C and δ 15 N in muscle tissues of thick-billed murres ( Uria lomvia ) and black-legged kittiwakes ( Rissa tridactyla ), and in whole body tissues of walleye pollock ( Gadus chalcogrammus ) and krill ( Thysanoessa spp), sampled across the continental shelf break in the Bering Sea in 2008 and in 2009. We found significant basin-shelf differences at fine scales ( 15 N among murres but not kittiwakes, and no such differences in δ 13 C in either seabird species at that scale. We then quantified the multi-trophic signal and spatial structure of a basin-shelf δ 15 Nitrogen gradient in the central and southern Bering Sea, and used it to contrast foraging patterns of thick-billed murres and kittiwakes on the open ocean. Seabird muscle δ 15 N values were compared to baselines created from measurements in krill and pollock tissues sampled concurrently throughout the study area. Krill, pollock, and murre tissues from northern, shallow, shelf habitat ( 15 N relative to samples taken from deeper habitats (>200 m) to the south and west. Krill δ 15 N baseline values predicted 35–42% of the variability in murre tissue values. Patterns between kittiwakes and prey were less coherent. The persistence of strong spatial autocorrelation among sample values, and a congruence of geospatial patterns in δ 15 N among murre and prey tissues, suggest that murres forage repeatedly in specific areas. Murre isotope values showed distinct geospatial stratification, coincident with the spatial distribution of three colonies: St. Paul, St. George, and Bogoslof. This suggests some degree of foraging habitat partitioning among colonies.

Although euphausiids of the genus Thysanoessa ('krill') are a key zooplankton taxon in the Bering Sea ecosystem, the processes controlling variation in the standing stock of these animals are not well understood. Both forcing by temperature ('bottom-up') and predation ('top- down') have been proposed. If strong top-down forcing were present, a negative relationship would be expected between the standing stock of euphausiids and that of walleye pollock Gadus chalcogrammus, their single most important predator in the Bering Sea. We developed multiple regression models using survey data collected over several years to test the hypothesis of a negative relationship between local (mesoscale) densities of these animals. We used the models to evaluate the relative importance of pollock biomass and water temperature in predicting euphau- siid biomass, and found that temperature was a far better predictor, a result that is not consistent with dominant top-down control of euphausiids by pollock predation.

The distribution, abundance, biomass, and community structure of macrozooplankton were investigated in relation to oceanographic conditions in the Barents Sea during summer 2013, using an IKS-80 net (0.5 m2 mouth opening, 550 μm mesh). Overall, we found 20 taxa (11 in Murmansk Coastal Water-MCW and in Atlantic Water-AW, 13 in Arctic Water-ArW, and 19 in Barents Sea Water-BSW). Total macrozooplankton abundance varied between 36 and 1116 ind. m−2 (0.2–4.6 ind. m−3) with a mean value of 238 ± 34 ind. m−2 (1.0 ± 0.1 ind. m−3) for the entire area. Although the catch was fairly diverse, a small number of species accounted for the majority of the total abundance and biomass. The most abundant taxa were Parasagitta elegans (MCW, AW, BSW, ArW), Thysanoessa spp. larvae (MCW, AW), Mertensia ovum (BSW), Aeginopsis laurentii (BSW), Dimophyes arctica (ArW), and Themisto libellula (ArW). Total macrozooplankton biomass ranged from 17 mg dry mass (DM) m−2 (0.1 mgDM m−3) to 6710 mgDM m−2 (23.1 mgDM m−3), averaging 1058 ± 240 mgDM m−2 (4.3 ± 0.9 mgDM m−3) across the study area, with Parasagitta elegans dominant by biomass. Cluster analysis revealed three communities (southern, central and northern) which demonstrated clear variations with latitude. Canonical correspondence analysis showed that location (latitude) and environmental conditions (mean temperature, salinity, mean phytoplankton abundance and biomass) explained 36% of the spatial variability in macrozooplankton communities. In BSW, there was a strong positive correlation between total macrozooplankton biomass and mesozooplankton stock suggesting an important role for food availability in controlling macrozooplankton variability. Predator–prey interactions also might play an important role in influencing the spatial distribution of macrozooplankton. The results may be useful as basis for future investigations of macrozooplankton in order to detect possible influence of changing environment on plankton communities of the Barents Sea during modern warming trend in the Arctic.

Capelin (Mallotus villosus) is the key forage fish species in the Newfoundland and Labrador Shelf ecosystem. Capelin stocks collapsed in the early 1990s, concurrent with declines in “northern” Atlantic cod, Gadus morhua. Neither has fully recovered yet. Changes in growth, condition, and behaviour accompanied capelin declines on the northern Grand Banks (NGB), and remain two decades later. Feeding, growth, and condition of NGB capelin were all lower when compared with capelin from the eastern Scotian Shelf (ESS), where abundance increased following predator declines. For age 2–5 capelin of both sexes, all but one of five comparable age–sex groups were significantly larger on the ESS (e.g. age 3 females average 169 mm on the ESS and 151 mm on the NGB). Neither temperature nor density-dependence explain these differences. However, dietary differences were prominent. ESS capelin had higher total fullness indices (TFIs) than NGB fish at all sizes [mean TFIESS = 1.43 (± 1.14), mean TFINGB = 0.48 (± 0.70)]. Euphausiids (especially Thysanoessa spp.) were a main dietary component on the ESS but not on the NGB. Stable isotope analyses (δ15N and δ13C) for NGB capelin also indicated few dietary euphausiids. Trophic fractionation of δ 15N was 4.740/00, suggesting NGB capelin were food limited. Capelin recovery on the Newfoundland and Labrador Shelf appears limited by bottom-up forcing, in particular lack of euphausiid prey.

Myctophid fish play an important role in the Southern Ocean pelagic food web. The lanternfish Krefftichthys anderssoni is one of the most common myctophids in the region, but its ecology is poorly known. This study examines spatial and temporal patterns in the species distribution of density, life cycle, population structure and diet using samples collected by mid-water trawl nets deployed in different seasons across the Scotia Sea. Virtually absent from the sea-ice zone, the species was most abundant in the northern Scotia Sea around the Georgia Basin at depths below 400 m that are associated with the Circumpolar Deep Water. The species migrated during night from waters deeper than 700 m to waters above the 400 m following their main prey species: the copepods Rhincalanus gigas and Calanoides acutus and euphausiids of the Thysanoessa genus. Larvae length distribution and post-larvae length-frequency analyses suggested a life cycle of ~ 3 years with spawning and recruitment strongly connected with APF and the South Georgia shelf. Our results show that species spatial distribution, population structure and diet changed both seasonally and ontogenetically. This study is the most comprehensive examination of the ecology of K. anderssoni in the Southern Ocean to date and contributes to resolving how pelagic food webs and ecosystems operate in the region.

International audience; Recent studies have indicated a metabolic temperature sensitivity in both the arcto-boreal krill species Thysanoessa inermis and Thysanoessa raschii that may determine these species’ abundance and population persistence at lower latitudes (up to 40° N). T. inermis currently dominates the krill community in the Barents Sea and in the high Arctic Kongsfjord. We aimed to increase the knowledge on the upper thermal limit found in the latter species by estimating the CT50 value (19.7 °C) (critical temperature at which 50 % of animals are reactive) and by linking metabolic rate measurements with molecular approaches. Optical oxygen sensors were used to measure respiration rates in steps of 2 °C (from 0 to 16 °C). To follow the temperature-mediated mechanisms of passive response, i.e., as a proxy for molecular stress, molecular chaperone heat shock protein 70 (Hsp70) sequences were extracted from a transcriptome assembly, and the gene expression kinetics were monitored during an acute temperature exposure to 6 or 10 °C with subsequent recovery at 4 °C. Our results showed upregulation of hsp70 genes, especially the structurally constitutive and mitochondrial isoforms. These findings confirmed the temperature sensitivity of T. inermis and showed that the thermal stress took place before reaching the upper temperature limit estimated by respirometry at 12 °C. This study provides a baseline for further investigations into the thermal tolerances of arcto-boreal Thysanoessa spp. and comparisons with other krill species under different climatic regimes, especially Antarctica.

Seasonal variations in the diets of Pacific cod Gadus macrocephalus and walleye pol- lock Theragra chalcogramma were examined from fish collected during 5 sampling periods from August 1998 to June 1999 in the Kodiak Island area in the Gulf of Alaska. Both species were shown to be generalist predators, eating a wide variety of fish and invertebrates. Pollock, which are limited to pelagic prey, can be considered more specialized than cod. Cod consumed 78 prey items, and pollock consumed 45 prey items, with 28 items shared by both species. Individual pol- lock, however, typically concentrated on a single prey item, while individual cod stomachs con- tained a wider variety of prey. The principal prey of Pacific cod was Tanner crab Chionoecetes bairdi, comprising >28% of the cod diet by weight. The most common prey item for walleye pol- lock was the euphausiid Thysanoessa. Over the 5 sampling periods, the prey evenness and niche width occupied by the 2 species were similar, but seasonal differences were evident.

The morphological variability of four species of the euphausiid genus Thysanoessa from Kamchatka waters was studied. The neglecta form and 18 variants of the abdomen structure were found in Th. inermis. Th. raschii and Th. inermis exhibited four variants of eye shape; Th. longipes had three variants. In Th. longipes and Th. raschii, a tooth on the lateral edge of the carapace can be positioned closer to the middle. The most variable morphological features in Th. raschii were the location of a tooth on the carapace; in Th. longipes, the length of the second pair of thoracic legs and the position of a tooth on the carapace; in Th. inspinata, the length of the second pair of legs; in Th. inermis, the eye shape and the structure of the abdomen. A new key for the identification of euphausiids of the genus Thysanoessa that inhabit the Russian Far Eastern seas is proposed.

We analysed summer diet and fledging mass of macaroni penguins Eudyptes chrysolophus breeding at Bird Island, South Georgia, during the crèche period (January and February) between 1989 and 2010. Crustaceans were the main prey accounting, for over 90% of the diet by mass. Antarctic krill Euphausia superba was the main prey, in 17 out of 22 years. Amphipods Themisto gaudichaudii were the main prey in 1994 and 2009, fish in 2004, and the euphausiids Thysanoessa spp. and Euphausia frigida in 2000. There was no clearly dominant prey group in 1999. Prey diversity and the frequency occurrence of T. gaudichaudii both increased with a decreasing proportion of E. superba in the diet. There was strong evidence that macaroni penguins have a sigmoidal functional response, indicating that this kind of response should be accounted for when devising ecosystem-based management reference points for seabirds. The energy and mass of all euphausiids combined (rather than E. superba in particular) in the diet were the most reliable predictors of chick fledging mass; the correlation between model-predicted and observed values was 0.84. The gross energy content of individual meals was often above average in years when the diets contained fewer euphausiids, but fledging mass was always below average in these years. Although macaroni penguins are able to feed on a variety of prey types, chick growth was always severely impacted by a shortage of euphausiids due to higher energy or time costs associated with feeding on alternative prey types. Given their reliance on euphausiids, macaroni penguins would be particularly vulnerable to potential climate-driven declines in krill stocks.

541N 641N 1551W 1851W abstract Euphausiids (principally Thysanoessa spp.) are a key group of organisms in the Bering Sea ecosystem, linking production at lower trophic levels to top predators and important commercial fish stocks such as walleye pollock (Theragra chalcogramma). Here, we combine multifrequency acoustic survey methods, physics-based models of euphausiid backscatter, and net sampling to provide a means of monitoring the status and trends of euphausiid standing stock biomass on the Bering Sea shelf. Observations made using this approach during six summers (2004 and 2006-2010) indicate that standing stocks of euphausiids and pollock were inversely correlated over time as well as in space across the continental shelf. First-order calculations show that when pollock abundance was at its peak during these years, the pollock stock could have consumed 10-87% of the euphausiid standing stock between May and September. We hypothesize that predation by pollock is a significant top-down control on euphausiid standing stock in this system. Published by Elsevier Ltd.

Since the Discovery Investigations of the 1920s and 1930s, seawater temperatures have increased in the Atlantic sector by ∼1 °C; greater than the global mean rise. The aims of this paper were first to rescue the Discovery macrozooplankton data, second to provide quantitative “baseline” distribution maps, relating these to indices of temperature and food. Our third aim was to use the relationships we derived between abundance and temperature to project the potential affect of a 1 °C warming on the Discovery era distribution patterns. Based on the 1 m ringnet data retrieved from 615 stations (Nov–March), four taxa comprised >90% of the Antarctic macrozooplankton abundance: Rhincalanus gigas, Thysanoessa spp., Euphausia superba, and Chaetognaths. Most of the taxa, especially the more abundant ones, were warm water species penetrating into Antarctica and thus total macrozooplankton abundance decreased about 100-fold from 50°S to 70°S. While temperature correlated best with distribution at this large scale, food availability (proxied by a present-day satellite-based Chlorophyll a climatology) had a secondary effect, with the major euphausiids Euphausia superba and Thysanoessa spp. concentrated in high chl a areas. A modelled uniform 1 °C temperature rise produced a poleward shift for all taxa, but the Antarctic continent blocked this re-adjustment for the high latitude species, constricting their predicted range. More widespread polar/sub-polar species were predicted to increase their penetration into Antarctica by 4–12° in latitude, whereas the poleward shift in potential range of sub-Antarctic taxa were limited by the steep temperature gradient across the Antarctic Polar Front (APF). However, within the Scotia Sea the relatively warm temperatures of the northern Antarctic Zone, abundant food due to iron fertilisation and intense eddy activity provide a “gateway” for northern species to penetrate south of the APF. Our model predictions, based on measured distributional ranges and observed temperature increases, provide a yardstick with which to compare modern day data compilations and assess the potential effects of future temperature increases.

Seabirds are effective samplers of the marine environment, and can be used to measure resource partitioning among species and sites via food loads destined for chicks. We examined the composition, overlap, and relationships to changing climate and oceanography of 3,216 food loads from Least, Crested, and Whiskered Auklets (Aethia pusilla, A. cristatella, A. pygmaea) breeding in Alaska during 1994–2006. Meals comprised calanoid copepods (Neocalanus spp.) and euphausiids (Thysanoessa spp.) that reflect secondary marine productivity, with no difference among Buldir, Kiska, and Kasatochi islands across 585 km of the Aleutian Islands. Meals were very similar among species (mean Least–Crested Auklet overlap C = 0.68; Least–Whiskered Auklet overlap C = 0.96) and among sites, indicating limited partitioning of prey resources for auklets feeding chicks. The biomass of copepods and euphausiids in Least and Crested Auklet food loads was related negatively to the summer (June–July–August) North Pacific Gyre Oscillation, while in Whiskered Auklet food loads, this was negatively related to the winter (December–January–February) Pacific Decadal Oscillation, both of which track basin-wide sea-surface temperature (SST) anomalies. We found a significant quadratic relationship between the biomass of calanoid copepods in Least Auklet food loads at all three study sites and summer (June–July) SST, with maximal copepod biomass between 3–6°C (r 2 = 0.71). Outside this temperature range, zooplankton becomes less available to auklets through delayed development. Overall, our results suggest that auklets are able to buffer climate-mediated bottom-up forcing of demographic parameters like productivity, as the composition of chick meals has remained constant over the course of our study.

Distribution and population structure of Thysanoessa inspinata were investigated off northeastern Japan and compared with those of Euphausia pacifica and other Thysanoessa species. Thysanoessa inspinata were distributed as widely as E. pacifica, but their abundance in the transitional water mass area (TWA) was conspicuously lower than in the cooler Oyashio water mass area (OWA). Another three Subarctic species of Thysanoessa occurred restrictively in the OWA, but rarely occurred during summer―autumn when sea surface temperature increased up to nearly 20°C. Thysanoessa inspinata was distributed in surface waters at night in the OWA in spring, but in the middle layers during summer―autumn; in contrast, E. pacifica occurred near the surface throughout the year. Euphausia pacifica was > 7 times more abundant than T. inspinata in the OWA, whereas T inspinata was >90% of the total numbers of Thysanoessa spp. Temperature tolerances may strongly affect the degree of dominance of euphausiids and explain why the TWA is the southern limit of T inspinata distribution, perhaps by restricting its access to the food-rich surface waters. Growth of males of T. inspinata was slower than that of females. Evidence from this study corroborates that E. pacifica is the key euphausiid species for food of surface-migratory Pacific saury during summer―autumn.

The abundance and population structure of Euphausia pacifica and Thysanoessa inspinata were investigated during the period 9 March–29 April 2007 at a single station in the Oyashio region. Water temperature and chlorophyll a concentration varied from 1.1 to 6.1 1C and from 0.02 to 6.3 mg m � 3 , respectively, primarily reflecting the effects of varying mixtures of the water masses prevailing in this region. The abundance of euphausiids, collected by oblique hauls with Bongo nets (0–200 m) at night, fluctuated from 41 to 1040 individuals m � 2 for E. pacifica and from 50 to 186 individuals m � 2 for T. inspinata. Correlation analyses revealed that the variations in the abundance of E. pacifica were related to water temperature and chlorophyll a, while abundance of T. inspinata only varied with temperature. Adult males and females (with and without spermatophores) were the major components of both euphausiid stocks (4 90% of the total). Modal size (¼total length or TL) analysis revealed that one group with large sizes (14–18 mm TL for E. pacifica, 16–18 mm TL for T. inspinata) occurred throughout the study period, often accompanied by one discrete group of smaller individuals (o 11 mm TL for both euphausiids), suggesting the distribution of the same euphausiid populations across the heterogeneous water masses. Adult males were significantly smaller than adult females in T. inspinata, but this was not the case in E. pacifica. Tracing the progressive increase in the mean sizes of the larger size groups, growth rates were estimated to be 0.082 mm day � 1 for E. pacifica and 0.022 mm day � 1 for T. inspinata. The difference in the growth rates of the two euphausiids is discussed in light of species-specific differences in the allocation of energy to reproduction and somatic growth during the present study period.

The abundance, biomass, diversity and community structure of euphausiids in the Southern Ocean along the 140°E meridian from latitudes 47°S–65°S were examined by RMT8 from 0 to ca. 1000 m during the 2001/2002 austral summer. Both total abundance and biomass of euphausiids were much higher in the south of the southern branch of the Polar Front (PF-S) located at 60°S–61°S than in the northern part, corresponding with higher chlorophyll a concentrations. Eleven species of euphausiids were collected from the research areas: 2 species of Thysanoessa, 6 of Euphausia, and a single species each of Thysanopoda, Bentheuphausia and Nematoselis. In Euphausia and Thysanoessa, clear horizontal distribution patterns enabled the classification of each species into one of 3 different types: species occurring only at stations north of the PF, species occurring in a broad distribution from south of the PF-S to the south of the Southern Boundary (SB) of the Antarctic Circumpolar Current, and species occurring only south of the SB. The latitudinal zonations occurrence and peak abundance were evident for all species. Hierarchical cluster analysis by species and station resulted in the identification of 2 distinct communities of euphausiids: community I comprised the three stations north of the PF where both euphausiid abundance and biomass were relatively low, and 2 species, Thysanopoda acutifrons and Thysanoessa gregaria dominated, community II comprised the southern five stations where abundance and biomass were high and Euphausia frigida, E. superba and Thysanoessa macrura were numerically dominant. Among the five oceanographic fronts within the research area, the PF (either south or north) appears to be the important biogeographical boundary separating abundance, biomass, and the community structure of euphausiids.

Morphology and biometry of eggs spawned by females of Euphausia pacifica, Thysanoessa spinifera and Thysanoessa inspinata were compared with eggs collected along the Oregon coast to identify the eggs in preserved samples to species level and to infer species spawning areas and intensity of spawning events in the field. The average chorion diameter (CD) and embryo diameter (ED) were significantly larger for E. pacifica than for T spinifera. Euphausia pacifica eggs usually have a significantly greater perivitelline space, and the chorion is firm, transparent, smooth, elastic and completely spherical, while T spinifera embryos are not completely spherical and typically are soft and sticky with particles attached. Eggs of the twitching stage embryo of T. spinifiera have an elliptical shape, while those of E. pacifica are spherical to very slightly elliptical even after hatching as nauplii. The CD and ED of T inspinata eggs were smaller than those of the other two species and they were transparent and spherical with a non-sticky chorion. Biweekly time series of eggs (identified to species), of nauplii + metanauplii and of ripe females collected along the Newport hydrographic line (44°40'N) during 1970―1972 show that E. pacifica and T spinifera spawn mainly < 40 km from the coast from March to October, but E. pacifica also spawns regularly at oceanic locations. A meta-analysis of the egg CD and ED values of 38 euphausiid species around the world indicates that these variables alone cannot be used to identify eggs to species, excepting specific regions where one or two broadcast spawners dominate the euphausiid assemblage.

The purpose of the study is to analyze the state of the Barents Sea euphausiids populations in the warm period (2000–2005) based on the study of their structure dynamics and distribution under the influence of abiotic and biotic factors. For estimation of their aggregations in the bottom layer, the traditional method was used with the help of the modified egg net (0.2 m 2 opening area, 564 μm mesh size). The net is used for collecting euphausiids in the autumn–winter period when their activity is reduced, which results in high-catch efficiency. The findings confirmed the major formation patterns of the euphausiids species composition associated with climate change in the Arctic basin. As before, in the warm years, one can see a clear-cut differentiation of space distribution of the dominant euphausiids Thysanoessa genus with localization of the more thermophilic Thysanoessa inermis in the north-west Barents Sea and Thysanoessa raschii in the east. The major euphausiids aggregations are formed of these species. In 2004, the first data of euphausiids distribution in the northern Barents Sea (77–79°N) were obtained, and demonstrated extremely high concentrations of T. inermis in this area, with the biomass as high as 1.7–2.4 g m −2 in terms of dry weight. These data have improved our knowledge of the distribution and euphausiids abundance during periods of elevated sea-water temperatures in the Barents Sea. The oceanic Atlantic species were found to increase in abundance due to elevated advection to the Barents Sea during the study period. Thus, after nearly a 30-year-long absence of the moderate subtropical Nematoscelis megalops in the Barents Sea, they were found again in 2003–2005. However in comparison with 1960, the north-east border of its distribution considerably shifted to 73°50′N 50°22′E. The portion of Meganyctiphanes norvegica also varied considerably—from 10% to 20% of the total euphausiids population in the warm 1950s–1960s almost to complete disappearing in 1970–1990s. The peak of this species’ occurrence (18–26%) took place in the beginning of warm period (1999–2000) after a succession of cold years. The subsequent reduction of the relative abundance of M. norvegica to 7% might have been mostly caused by fish predation during a period of low population densities of capelin. This high predation pressure may therefore have been mediated both by other pelagic fishes (i.e. herring, blue whiting, polar cod) but also by demersal fishes such as cod and haddock. Similar sharp fluctuations in the capelin stock (the major consumer of euphausiids) created marked perturbations in the food web in the Barents Sea in the middle 1980s and the early 1990s.

Across the California Current System (CCS), euphausiid crustaceans are important prey for many vertebrate predators, including the seabird Cassin's auklet Ptychoramphus aleuticus. How- ever, the effects of ocean climate on euphausiid biology, and their consequences for predators, remain poorly understood. Over a 13 yr period (1996 to 2008), Euphausia pacifica, Thysanoessa spinifera and T. inspinata cumulatively averaged about 35% of the annual biomass in the nestling diets of auklets at Triangle Island, but there was marked inter-annual variation. Whereas ocean cli- mate had little influence on the amounts of E. pacifica or T. inspinata delivered, diets included more T. spinifera if spring sea-surface temperatures in the previous year had been lower. Cold conditions might facilitate the production of a strong annual cohort, thus increasing adult biomass in the follow- ing year. Within seasons, the amount of E. pacifica in diets declined with date, and the decline was consistent across both warm- and cold-water years. In contrast, diets were especially rich in both Thysanoessa spp. late in the provisioning period in warm-water years, when the harvest of Neo- calanus cristatus declined dramatically due to a temporal mismatch between the auklet predator and this copepod prey. Both the annual nestling survival rate and the mean fledging masses of Cassin's auklets were tightly correlated to the amount of N. cristatus in their diets, and for fledging mass there was a further small, additive effect of increased amounts of T. inspinata. Results of the present study add new insight into effects of ocean climate on euphausiid biology in the northern CCS, and their potential consequences for population processes of an important euphausiid predator.

A series of oblique hauls with Bongo nets (0-1000 m) was made during the period of August 2002 through August 2004 in the Oyashio region, western subarctic Pacific, to investigate abundance, biomass and life cycle patterns of the three predominant euphausiids (Euphausia pacifica, Thysanoessa inspinata and T. longipes). While the three euphausiids occurred throughout the entire study period, E. pacifica was the most abundant (1,120 indiv. m � 2 , or 832 mg C m � 2 ), followed by T. inspinata (163 indiv. m � 2 , or 144 mg C m � 2 ) and T. longipes (73 indiv. m � 2 , or 75 m gCm � 2 ). Judging from the occurrence of females with spermatophores and furcilia larvae, the spawning was consid- ered to take place twice a year (April-May and August) for E. pacifica, year-round (peak season: March-May) for T. inspinata and in spring (March-May) for T. longipes. The population structure in terms of size (� total length) fre- quency distributions of the three euphausiids was characterized by the frequent co-occurrence of 2-3 cohorts in the same samples. The maximum size of males and females found were 21 mm and 24 mm, respectively, for E. pacifica, 18 mm and 23 mm, respectively, for T. inspinata, 27 mm and 31 mm, respectively, for T. longipes. Tracing the sequence of cohorts, the life spans of E. pacifica, T. inspinata and T. longipes were estimated to be 17-26 months, 17-19 months and 29-31 months, respectively. These results are compared with reports of the same species in other habitats in the light of regional variations.

The horizontal and vertical distributions of larvae and reproductive timing of euphausiids were investigated in the Ross Sea and its adjacent waters during austral summer 2004–2005. Occurrences of larvae of Euphausia frigida and E. triacantha were confined to the northern oceanic area where the Upper Circumpolar Deep Water prevailed, although their juvenile and adult stages extended the distribution further southward to the area where cooler Lower Circumpolar Deep Water prevailed. Larvae of Thysanoessa spp. were widely distributed within the oceanic to slope areas but did not occur on the shelf as juvenile or adult stages. Eggs and larvae of E. superba occurred with gravid females along the slope, but no juveniles occurred concurrently. Thus the slope does not appear to be a nursery ground for this species. The distribution of E. crystallorophias larvae was mostly confined to the shelf in the juvenile and adult stages. The onset of deepening was from early and later frucilia stages onward for E. triacantha and E. frigida, respectively. However, Thysanoessa spp. were concentrated increasingly within the surface layers from furcilia I onward. The onset of recruitment to calyptopis I appeared to be earlier in the more northern species with the exception of E. crystallorophias, which recruited before E. superba. The intense spawning of E. crystallorophias and E. superba coincided with a period of development of a coastal polynya. Generally, the onset of spawning of euphausiids appeared to be related to the latitudinal distribution and timing of sea-ice melting. Relationships between surface temperatures and development and recruitment of larvae of euphausiids are discussed.

Ontogenetic, inter-annual and regional variations in diet were investigated for mackerel icefish, Champsocephalus gunnari, in three successive summer seasons around South Georgia. Stomach contents from 2239 C. gunnari (130–560 mm total length) were examined. A bootstrapping technique was used to calculate confidence intervals for an index of relative importance of prey categories (% IRIDC). Diet varied significantly between years and age classes but there was little regional difference in diet. In general, diet was dominated by krill, Euphausia superba and by the amphipod Themisto gaudichaudii. Smaller (younger) fish tended to prey on a higher proportion of T. gaudichaudii and small euphausiids such as Thysanoessa sp. and took smaller quantities of E. superba. In a season of poor krill availability (summer of 2003–2004) the proportion of krill in the diet, stomach fullness and fish condition (indicated by length–weight relationships) were significantly lower than in the other summer seasons. A large reduction (>80%) in the estimated annual (2005) biomass of the C. gunnari stock directly followed the season of poor krill availability. This decline was largely because of mortality of 2+ and 3+ fish, which were more krill dependent than 1+ fish. Younger fish appear to have survived, leading to an increase in the estimated population biomass in 2006.

The krill Thysanoessa inermis and Thysanoessa longicaudata and the hyperiid amphipods Themisto abyssorum and Themisto libellula are important components of the pelagic ecosystem in the Marginal Ice Zone (MIZ) of the northwestern Barents Sea. High abundance of krill (336 no. m−2) and amphipods (107 no. m−2) was recorded at a station close to 81.4°N, north of Spitsbergen. Statistical splitting of length frequency distributions gave 3–4 cohorts for T. inermis and up to three cohorts for T. longicaudata. In the current study, up to three and five cohorts were recorded for T. abyssorum and T. libellula, respectively, with the majority belonging to 0-group. The distribution of Atlantic/boreal species of krill and amphipods seemed to be mainly restricted to the deeper Atlantic waters (>200 m). The cold waters in the MIZ seem to act as a barrier towards the reproduction of several dominant Thysanoessa species. Thus, the advection processes from the south appear to be important for maintaining the krill population in the high-arctic waters. In contrast to krill, specifically, the cold-water amphipod, T. libellula, seems to breed successfully in the MIZ.

The horizontal and vertical distribution and population structure of euphausiids in the Ross Sea and its adjacent waters were investigated during the summers of 2004/2005 using stratified towed samples. Nine species of euphausiids occurred in the survey area. Among them, Euphausia triacantha was dominant in biomass north of the southern boundary of the Antarctic circumpolar current (SB). Thysanoessa spp. was widely distributed north of the continental slope, while E. superba was distributed from the SB to the slope, where it showed the highest biomass. Juvenile E. superba was distributed offshore near the SB and remained at the surface, but gravid females were dominant in the slope and mainly occurred in the middle layers (400–600 m). Adult and juvenile E. crystallorophias were found at 200–300 m in the colder water of the continental shelf. In general, the peak biomass of euphausiids was found in the mid layers of the Ross Sea area. The life span and the number of spawns for major species are also discussed.

The food habits of the dominant myctophid Stenobrachius leucopsarus were examined in the central basin of the Bering Sea in relation to oceanographic conditions, in summer 2002 and 2003 and spring 2006. S. leucopsarus exhibited an ontogenetic and seasonal dietary shift. In spring, small fish (≤40 mm) preyed mainly on Neocalanus flemingeri/plumchrus whereas large fish fed mainly on Neocalanus cristatus. In summer, small fish preyed mainly on Metridia pacifica whereas large fish fed mainly on euphausiids (Thysanoessa spp.). In the summer of 2003, when water temperature in the epipelagic layer (≤100 m) was warmer, reflecting the prevalence of the Alaskan Stream, small-sized S. leucopsarus showed a higher stomach content index, perhaps reflecting the greater abundance of M. pacifica. Thus, the present study shows that the physical variability in the epipelagic layer affects not only diets but also feeding performance of micronekton.

The vertical distribution of dominant euphausiid species was investigated in the offshore waters of north- eastern Japan in April and September 1998 and November 1999 using MOCNESS-I net samples. In total, 32 eu- phausiid species in eight genera were identified in the study area. Subarctic Euphausia pacifica and Thysanoessa in- spinata occurred over a wide range, although mainly in the Oyashio and cooler transitional areas, and the former made a relative contribution of 78% to the total numerical abundance of euphausiids. At night, these species tended to stay near the surface in the Oyashio area and in deeper layers in warmer areas. During the daytime, they tended to stay in deeper layers in the warmer areas, although E. pacifica stayed in deeper layers even in the Oyashio area where the chlorophyll a concentration was low. T. inspinata did not perform clear diel vertical migration. Except for these subarc- tic species, five warm-water species of Euphausia were abundant among the epipelagic euphausiids. At night, these species tended to stay at a wide range of depths from the surface to subsurface layers in the warmer Kuroshio area, but were concentrated near the surface in the transitional area. During the daytime, they stayed in deeper layers in the Kuroshio area. Diel vertical migration of euphausiids seems to be strongly affected by water temperature, but other fac- tors such as phytoplankton abundance may also be important. The possible effects of diel vertical migration of eu- phausiids on euphausiid fisheries are discussed.

Fecal pellets of zooplankton can be important carriers of organic matter from surface to deep waters of the oceans, The summer cruise ANTARES 4 gave us the opportunity to collect zooplankton from the frontal subantarctic zone (station 7 or s-ant) and adjacent subtropical waters (station 8 or s-trop). Triglyceride fatty acid profiles were used to assess the trophic position of two copepods (Metridia lucens and Pleuromamma spp.), three euphausiids (Euphausia spinifera, Nematoscelis megalops, and Thysanoessa sp) and a decapod Acanthephyra sp. The euphausiid E. spinifera was studied at the two sites and its reaction to contrasting trophic environments and hydrology was highlighted by variations in its lipid composition. Most species showed various degree of omnivory: E. spinifera from subantarctic showed the highest degree of herbivory on phytoplankton (high proportion of 16:4(n-1), 18:5(n-3), 16:1(n-7)), while the same species at the subtropical station, as well as Thysanoessa sp and M lucens from the subantarctic station displayed more omnivorous feeding characteristics, partly linked to the abundance of microzooplankton cells. Acanthephirasp, N. megalops and Pleuromammasp from station 8 (s-trop) appear to be essentially carnivorous (high 18:1 n-9/18:1n-7 ratio, high levels of 20: 1 (n-9) or 22: 1 (n-11)). The trophic position influenced the composition of feces, with higher PUFA percentages in fecal pellets from carnivorous species. Hence, the surface faunal assemblage and the season controlled not only the quantity but also the quality of the food supply as PUFA exported towards the benthic domain. (c) 2007 Elsevier B.V. All rights reserved.

Seasonal changes in distribution and abundance of euphausiids off south‐eastern Hokkaido (41°−43°N), Sanriku (38°−41°N), and Joban (36°−38°N) were investigated using cylindrical‐conical nets every two months from March 1997 to February 1998. Twenty‐six species of seven genera of euphausiids occurred during the survey. Among them, subarctic‐transitional Euphausia pacifica was the most abundant throughout the year in coastal waters, as their relative contribution to the total abundance of euphausiids was 89–92%. This species occurred in each coastal water throughout the survey and was abundant from winter to early summer (February–June) off Sanriku and Joban and in autumn in south‐eastern Hokkaido. Thysanoessa inspinata occurred off south‐eastern Hokkaido and Sanriku throughout the survey, mainly in spring (April) but rarely occurred off Joban. Three other subarctic Thysanoessa species occurred mainly off south‐eastern Hokkaido from winter to spring. Conversely, warm‐ and transitional‐water epipelagic species occurred exclusively off Sanriku and Joban in autumn. The characteristics of seasonal distributional patterns of euphausiids are discussed in relation to the spatial and temporal changes of oceanographic conditions and several predators off north‐eastern Japan.

The species composition of euphausiids was investigated in relationship to the hydrographic conditions in the North Atlantic cold-core rings (CCR) and adjacent waters to elucidate species succession in evolving water masses. Using data, dating back to the 1970s, from as many CCRs as possible and selecting typical cases where no major physical perturbations occurred, a general pattern of euphausiid succession and change in vertical distribution in rings with time was obtained. This pattern was related to the general distribution of euphausiids in the northwestern North Atlantic Ocean, aiming at providing basic information on probable response of North Atlantic marine ecosystem to global warming. Of the 34 euphausiid species identified, 5 were cold-water species, 17 were warm-water species, 6 were wide-ranging warm-water species, 1 was transitional, 4 were cosmopolitan and the remaining was Thysanoessa parva . Among cold-water species, Euphausia krohni and Nematoscelis megalops were dominant in CCRs. E. krohni became rare in rings older than 6 months, whereas N. megalops survived longer, being abundant in some rings of 9 months or older, by staying within its preferred temperature range as the CCR elevated isotherms sank to depths where they are normally found in the Sargasso Sea and because it is an omnivore–carnivore. Among warm-water species, epipelagic species appeared first in rings, corresponding to the physical change occurring most rapidly in the surface layers. Mesopelagic species appeared later. Cold-water species made up 65–85% of the total euphausiid population in number in younger rings (1–5 months old), while warm-water species contributed only 2–7%. Wide-ranging warm-water species made up about up to one fourth of the total in rings 5 and 7 months old. Warm-water species, mainly E. brevis , increased in older rings (9 months old or older) and made up 50% of the total in the oldest ring. The contribution of cold-water species decreased to 14% in older rings. T. parva made up 26–38% of the total in rings 6 months or older. CCR populations can be characterized by high species number, but intermediate evenness between the Slope Water and northern Sargasso Sea. In CCRs, only a limited number of species were dominant even if there were more species present in rings as old as 9–12 months than in the northern Sargasso Sea. In rings older than 9 months, euphausiids showed two peaks in their vertical distribution: a shallow daytime peak at about 400 m and a nighttime peak in the upper 100 m consisting of warm-water species (mainly E. brevis ) and a deeper persistent peak at 800 m or deeper consisting of the species N. megalops and T. parva . This shallow peak in CCRs is shallower than that in the surrounding northern Sargasso Sea, and the deep peak is rarely observed in these waters.