Euphausiid Biomass Research Articles

Feeding of walleye pollock and its feeding base in the Chukchi Sea in summer-autumn period

Data on feeding of walleye pollock <i>Gadus chalcogrammus</i> (3–70 cm long) in the Chukchi Sea in summer-autumn of 2017–2020 are presented. The food base includes zooplankton, benthic invertebrates and fish. The juveniles in the northwestern part of the sea consumed 0.033 t/km2 of prey per day, mainly fish (30.9 %), mysids (20.1 %) and amphipods (13.9 %), whereas the portion of copepods and euphausiids was 5.0–7.6 % of the diet. In the southwestern part of the sea, the juveniles consumption was much higher, as 0.200 t/km2 of prey per day, and included mainly euphausiids and copepods (24.9–51.1%). The adults with length 40–70 cm long dominated in the southwestern Chukchi Sea and consumed on average 13.2 t/km2 of prey per day, shared between fish (42 %), euphausiids (20 %), copepods (6 %), and decapods (11 %). Composition and structure of plankton was different between the main water masses on the shelf occupied the northern and the southern parts of the sea. In the northwestern Chukchi Sea, zooplankton distributed with the mean spatial density of 56.3 t/km2, formed mostly by chaetognaths (35.3 t/km2) and copepods (12.6 t/km2). In the southwestern Chukchi Sea, the mean spatial density of zooplankton was 49.2 t/km2, mostly euphausiids (17.3 t/km2), chaetognaths (15.8 t/km2) and copepods (9.2 t/km2). The total density was close to the level observed in the 2000s, but slight decreasing in biomass of chaetognaths and copepods was observed, mainly due to the recent lack of cold-water species <i>Calanus glacialis</i>, whereas the biomass of euphausiids, in particularly <i>Thysanoessa inermis</i>, had almost doubled. Biomass of the main groups of zooplankton prevailing in the diet in the Chukchi Sea in summer-autumn, as copepods and euphausiids, exceeded daily consumption of these groups by walleye pollock in 6–11 times that may indicate a sufficient food supply for this species. Besides, walleye pollock dwelled at the sea bottom and fed on shrimps, gammarids, bottom polychaetes, echiurids, mollusks, and also bottom fish. Total fullness of the pollock stomachs was in the range of 218–228 ‱ that also suggested successful feeding.

Interannual response of euphausiid community abundance during the anomalous warming period (2014–2016) in the Gulf of California

El Niño in the northeast Pacific and Gulf of California typically causes a decrease in euphausiid abundance (biomass), and modifies their latitudinal biogeographic distributions, but the effect of recent marine heat waves (or their combined effect with El Niño) are less understood. The interannual response of euphausiid community structure and abundance were investigated during the cold–warm transition period (June) of six years (2013–2019, except 2015) in the central Gulf of California, Mexico. Abundance in June 2013–2019 was compared with historical baseline for euphausiid abundance made in June 1957 from a California Cooperative Oceanic Fisheries Investigation cruise, just before El Niño 1957–1958. During 2014–2019 two anomalously warm events occurred: the marine heat wave of 2014 (MHW 2014) and El Niño 2015–2016 (May 2015–May 2016). We tested the hypothesis that the numerically dominant subtropical neritic species Nyctiphanes simplex and temperate oceanic Nematoscelis difficilis significantly decrease in abundance in response to the prolonged warming that occurred during 2014–2016 and that tropical euphausiid species could increase in relative abundance during this period. Ten euphausiid species were collected accounting for < 3% of total relative abundance of 34 zooplankton taxonomic groups collected during June 2013–2014, 2016, 2018 and between 7 and 22.4% during 2017 and 2019. N. simplex and Ne. difficilis accounted for > 80% of euphausiid total abundance during June 2013–2019, with considerably lower abundance during 2013, the MHW 2014, and El Niño 2015–2016 than in June 2017, 2018, and 2019. Mean euphausiid abundance in 2017 and 2019 was slightly higher than that recorded in June 1957. Tropical species increased their relative abundances during El Niño 2015–2016 event, but they were always < 15% of total euphausiid relative abundance during 2013–2019. We estimated that it took at least 3 years for the euphausiid species abundance in the gulf to slowly recover from the unfavorable conditions associated with MHW 2014, and El Niño 2015–2016 to attain highest mean abundance in 2019. Multi–variated ordination analyses showed that the prolonged warming and anomalously low Chl-a concentrations recorded in 2014–2016 negatively influenced euphausiid abundance, a proxy for euphausiid biomass available for pelagic predators in the Gulf of California.

Quantifying the effects of extreme events and oceanographic variability on the spatiotemporal biomass and distribution of two key euphausiid prey species

Euphausiids form a critical component of oceanic food chains and individual species vary in their responses to perturbation events. We aimed to establish if patterns of fine scale oceanographic variability and larger-scale climate events such as marine heatwaves (MHWs) could be linked with spring variability in the biomass of two key forage species in the northeast Pacific Ocean, Euphausia pacifica and Thysanoessa spinifera. To achieve this, we used long-term datasets from the west coast of Vancouver Island Canada, an important commercial fishing area, to first quantify interannual signals of variability in fine-scale oceanographic conditions using multivariate analysis. We then used geostatistical spatiotemporal modelling to quantify the effects on species-specific euphausiid biomass. Oceanographic data showed that the effects of warm events are not always observable, and effects vary across small spatial scales. Warming due to the 2014-2016 MHW was relatively mild on the continental shelf during spring (&amp;lt;1°C above climatology). Spring biomass of euphausiids, particularly E. pacifica, peaked in 2015, and all euphausiid groups analysed (E. pacifica, T. spinifera and total euphausiids) exhibited significant correlations with positive phases of the Pacific Decadal Oscillation. These results have implications for marine predators as euphausiids may act as system stabilisers in the northeast Pacific, thereby potentially increasing ecosystem resilience during extreme events.

Community structure and biomass of larval euphausiids and postlarval mysids in relation to anomalous warming events during a weekly time series (2014–2017) at Cabo Pulmo National Park, Mexico

Euphausiids and mysids are important components of pelagic ecosystems due to their high abundance and tendency to form dense aggregations, swarms, and schools, which concentrate biomass that attracts visual predators. Species richness, abundance, and carbon biomass of euphausiids and mysids were investigated from a weekly daytime surface zooplankton time series from 2014 to 2017, in relation to 15 environmental variables at Cabo Pulmo National Park (CPNP, 23°27′ N, 109°25′ W), Mexico. A marine heatwave (MHW; January–August 2014) and El Niño 2015–2016 (May 2015–May 2016) caused anomalously warm temperatures in relation to the 2002–2018 temperature time series. We tested the hypotheses that CPNP is dominated by a tropical euphausiid and mysid species assemblage and that abundance of both taxonomic groups decreased during the MHW 2014 and El Niño 2015–2016. Euphausiids (7.9 ind. m−3) had a higher mean abundance than mysids (3.4 ind. m−3). Eight euphausiid species, mostly of tropical-oceanic affinity, were collected (98 % larvae). Nyctiphanes simplex decreased and Euphausia distinguenda and Stylocheiron carinatum increased in abundance during El Niño 2015–2016. Five mysid species were collected (juveniles and adults). Mysidium rickettsi alone accounted for 99.98 % of total mysid abundance. Although the euphausiids collected were mainly larval stages, due to sampling in shallow coastal waters during the day, they contributed more biomass than juvenile and adult M. rickettsi in the pelagic habitat, though the abundance of this mysid in its epibenthonic habitat was likely underestimated. A canonical correspondence analysis of euphausiid and mysid abundance as a function of 10 environmental variables explained 77.4 % of the total variability, showing greater seasonal (warm vs cold) than interannual changes. The best correlated environmental variables with euphausiid and M. rickettsi abundance were net primary production (−0.419), zooplankton biovolume (0.3177) and the MEI.v2 (−0.278). We concluded that the most evident changes in community structure and abundance were seasonal (e.g. N. simplex dominated during the cold season while E. distinguenda dominated during the warm season) rather than interannual. Moreover, the abundance and biomass of euphausiids decreased during the MHW 2014 and El Niño 2015–2016; paradoxically the highest abundance and biomass of E. distinguenda and the mysid M. rickettsi occurred during El Niño 2015–2016. This implies higher available biomass for benthonic and nektonic predators in this neritic tropical reef ecosystem, contrasting with decreased abundance of euphausiids observed along the west coast of the Baja California Peninsula during this period.

Ecological considerations for species distribution modelling of euphausiids in the Northeast Pacific Ocean

Euphausiids are a keystone species in marine food webs of the Northeast Pacific Ocean, important for forage fish, commercial fish species and marine predators such as seabirds and whales. Ecosystem-based fisheries management relies on realistic lower-trophic-level information for forecasting under future changing climate scenarios. We synthesised information from quantitative modelling studies analysing the relationship between euphausiids and their environment for two species of euphausiids that dominate assemblages in this region — Euphausia pacifica and Thysanoessa spinifera. Studies analysed suggest that variables reflecting the physical and biological environment in situ, features reflecting the geomorphic marine landscape, and large-scale climate indices all significantly affect euphausiid biomass and distribution. Temperature was the most tested predictor variable in the euphausiid models reviewed; however, it was significant in fewer models than other variables tested. We review and compare model structures, predictor variable selection and temporal lag phases to develop recommendations for species distribution modelling of euphausiids in the Northeast Pacific Ocean. We believe the results from this review will be applicable globally across regions with similar climates where euphausiids are numerous and can be adapted for different species and environments.

Factors affecting the seasonal distribution and biomass of E. pacifica and T. spinifera along the Pacific coast of Canada: A spatiotemporal modelling approach.

Euphausiids are a keystone species in coastal food webs due to their high lipid content and seasonally high biomass. Understanding the habitat and environmental drivers that lead to areas of high biomass, or ‘hotspots’, and their seasonal persistence, will support the identification of important foraging regions for mid- and upper- trophic level predators. We quantify the distribution of hotspots of the two dominant species of euphausiid in the north-east Pacific Ocean: Euphausia pacifica and Thysanoessa spinifera, as well as euphausiid larvae (mixed species). The Canadian coast encompasses the northern California Current Ecosystem and the transition zone to the Alaska current, and is a highly productive region for fisheries, marine mammals, and seabirds. We used spatiotemporal modelling to predict the distribution of these three euphausiid groups in relation to geomorphic and environmental variables during the important spring-summer months (April through September) when euphausiid biomass is highest. We quantified the area, intensity, and persistence of biomass hotspots across months according to specific oceanographic ecosections developed for marine spatial planning purposes. Persistent hotspots of both adult species were predicted to occur along the 200 m depth contour of the continental slope; however, differences were predicted on the shallower Dixon shelf, which was a key area for T. spinifera, and within the Juan de Fuca Eddy system where E. pacifica hotspots occurred. The continental slope along the west coast of Vancouver Island was the only persistent hotspot region common between both adult species and euphausiid larvae. Larval distribution was more correlated with T. spinifera than E. pacifica biomass. Hotspots of adults were more persistent across months than hotspots of euphausiid larvae, which were seasonally patchy. The persistence of biomass hotspots of forage species through periods of low overall biomass could maintain trophic connectivity through perturbation events and increase ecosystem resilience to climate change.

Heatwave-induced synchrony within forage fish portfolio disrupts energy flow to top pelagic predators.

During the Pacific marine heatwave of 2014–2016, abundance and quality of several key forage fish species in the Gulf of Alaska were simultaneously reduced throughout the system. Capelin (Mallotus catervarius), sand lance (Ammodytes personatus), and herring (Clupea pallasii) populations were at historically low levels, and within this community abrupt declines in portfolio effects identify trophic instability at the onset of the heatwave. Although compensatory changes in age structure, size, growth or energy content of forage fish were observed to varying degrees among all these forage fish, none were able to fully mitigate adverse impacts of the heatwave, which likely included both top‐down and bottom‐up forcing. Notably, changes to the demographic structure of forage fish suggested size‐selective removals typical of top‐down regulation. At the same time, changes in zooplankton communities may have driven bottom‐up regulation as copepod community structure shifted toward smaller, warm water species, and euphausiid biomass was reduced owing to the loss of cold‐water species. Mediated by these impacts on the forage fish community, an unprecedented disruption of the normal pelagic food web was signaled by higher trophic level disruptions during 2015–2016, when seabirds, marine mammals, and groundfish experienced shifts in distribution, mass mortalities, and reproductive failures. Unlike decadal‐scale variability underlying ecosystem regime shifts, the heatwave appeared to temporarily overwhelm the ability of the forage fish community to buffer against changes imposed by warm water anomalies, thereby eliminating any ecological advantages that may have accrued from having a suite of coexisting forage species with differing life‐history compensations.

Differing prey associations and habitat use suggest niche partitioning by fin and humpback whales off Kodiak Island

FinBalaenoptera physalusand humpbackMegaptera novaeangliaewhales share foraging areas and may compete for the same prey, but little is known about the extent to which they partition prey resources. Visual cetacean surveys and simultaneous acoustic-trawl surveys of prey were conducted around 2 submarine canyons off Kodiak Island, Alaska, in 2004 and 2006. Statistical models were used to examine the associations between sightings of fin and humpback whales and measures of their potential prey and environment. Observations and models indicate that fin whales were disproportionately abundant in areas with the highest observed euphausiid concentrations, while humpback whales were abundant at lower euphausiid concentrations and in areas where juvenile walleye pollock were abundant. Fin whales were abundant in the areas where euphausiid biomass was deepest and in the deepest areas surveyed (&gt;150 m depth). In contrast, humpback whales primarily occurred in shallower areas and near more shallowly distributed euphausiids. The different depth and prey affinities of fin and humpback whales suggest niche and habitat partitioning between these 2 co-occurring species. Abundance models built using acoustic estimates of prey density are a useful tool to further understanding of the abundance, distribution, and behavior of these animals.

Comparative morphology of Southern Ocean Euphausia species: ecological significance of sexual dimorphic features

Species of the genus Euphausia dominate the euphausiid biomass of the Southern Ocean, the three largest being Euphausia superba, E. triacantha and E. crystallorophias. We measured a number of morphological features to identify differences between, and within, these species to obtain ecological insights. Interspecifically, the greatest difference was carapace size, with that of E. superba being by far the largest and most variable. This likely reflects its prolific spawning capacity compared with other euphausiid species. E. triacantha exhibited an extended sixth abdominal segment that could facilitate greater levels of thrust in the tail flip escape response. The pleopods, which provide propulsion in forward swimming, were more than 50% larger in E. superba, indicating a greater capacity for directional movement at high velocities. E. crystallorophias had eyes that were almost double the size of those in E. superba and E. triacantha, which may help retain visual resolution within its under-ice habitat. Intraspecifically, we found the above morphological features differed little between sexes and developmental stages in E. crystallorophias and E. triacantha, but differed significantly in E. superba. Compared to females and juveniles, male E. superba had significantly larger eyes and pleopods, whilst the carapace in males became shorter as a proportion of body length during growth. These features indicate a greater capacity for searching and swimming in males, which, we hypothesise, increases their ability to locate and fertilise females. This morphological specialisation in male E. superba is indicative of comparatively greater inter-male competition resulting from its tendency to form large, dense swarms.

Basin-scale oceanographic processes, zooplankton community structure, and diet and reproduction of a sentinel North Pacific seabird over a 22-year period

Climate-driven variation in the distribution, abundance and phenology of low trophic-level organisms can have cascading effects within marine food webs, and understanding these complex ecosystem dynamics is of vital importance in an age of dramatic environmental change. We tested the hypothesis that marine conditions associated with the state of the Pacific Decadal Oscillation (PDO) exert primary influence on the biogeographic affinities of the zooplankton community, and thus the composition of nestling diets and reproduction of a zooplanktivorous seabird, Cassin’s Auklet (Ptychoramphus aleuticus). The 22-year study (1996–2017) took place on Triangle Island, British Columbia, Canada, located at the northern extreme of the California Current System (NCCS) and site of the world’s largest breeding colony. Based on annual zooplankton tows in the vicinity of Triangle Island, the biomass of ‘Subarctic’ zooplankton species (copepods) was elevated in years in which the PDO was in its negative (cold-water) state. The biomass of ‘Subarctic’ zooplankton was unrelated to both the El Niño-Southern Oscillation (ENSO) and North Pacific Gyre Oscillation (NPGO) indices. Principal Components Analysis (PCA) showed that in those PDO-negative years, diets fed to nestling auklets included more copepod biomass and less euphausiid biomass. Virtually all (>99%) of the copepod biomass consisted of just one species, Neocalanus cristatus, one of the North Pacific Ocean’s diagnostic ‘Subarctic’ copepods. PCA also identified a secondary effect of the El Niño-Southern Oscillation (ENSO) on diets, in that auklet parents delivered more larval fish and less copepods-plus-euphausiids to nestlings in years in which ENSO was in a strongly positive state (i.e., El Niño). The growth rates of nestling auklets, indexed by their mean mass at 25 days of age, was higher in years in which they received more copepods-plus-euphausiids rather than larval fish, and in years in which they received more copepods rather than euphausiids. A previous study in the Central California Current System found that ENSO (to the mid-1990s) and then the NPGO (since the mid-1990s) exerted primary influence on productivity in Cassin’s Auklets. In combination, the two studies illustrate how basin-scale climatic processes can non-homogeneously influence productivity in a single species across oceanic domains. For Cassin’s Auklet, the contrast can be attributed to latitudinal differences in the prey base (more copepod-based in the north, more euphausiid-based in the south), and linked to ocean transport, temperature, and phenology. This correlational study provides mechanistic insight into ecological functioning in the offshore waters of the NCCS, with application to marine systems elsewhere. We highlight the particular importance of the ‘Subarctic’ copepods to a zooplanktivorous predator in the NCCS, and the value of seabirds as indicators in this rich marine ecosystem.

Unusually low euphausiid biomass during the warm years of 2014–2016 in the transition zone of the California Current

Euphausiids represent a significant part of the total zooplankton biomass and are a key link in the trophic web of the California Current System (CCS). During 2014–2016, the northeast Pacific experienced warming caused by two successive events, the Blob and the 2015–2016 El Niño, which affected the whole ecosystem. Here we present evidence for a strong impact of the 2014–2016 warm events on euphausiids in the Baja California region, namely two species (Euphausia pacifica and Nematoscelis difficilis) that are dominant in the CCS transition zone. Both species decreased approximately 95% in relation to the previous years (1999–2010). A similar decrease was observed for E. pacifica during the 1997–1998 El Niño, but the decrease in N. difficilis was higher in 2014–2016 and unprecedented during the 1997–2016 period. Biomass of the coastal species Nyctiphanes simplex showed negative anomalies during the Blob but not during the 2015–2016 El Niño. The tropical species Euphausia eximia and Euphausia recurva increased in biomass during the Blob in 2014 relative to the previous years, and even more during the 2015–2016 El Niño. The strong increase of E. eximia, E. recurva, and N. simplex during El Niño may have been due to upwelling activity in winter 2015–2016. Canonical correspondence analysis (CCA) was performed to assess the relation between the euphausiid community and environmental variables. The most influential variables were temperature and chlorophyll, and to a lesser extent salinity. The subarctic E. pacifica showed covariance with chlorophyll while a set of tropical species did with thermohaline conditions. However, the variance accounted for was low (20% taking the first three axis). N. difficilis did not show significant scores in the CCA suggesting that other variables such as availability of prey could be important for this carnivore species.

An assessment of climate change vulnerability for Important Bird Areas in the Bering Sea and Aleutian Arc.

Recently available downscaled ocean climate models for the Bering Sea and Aleutian Arc offer the opportunity to assess climate vulnerability for upper trophic level consumers such as marine birds. We analyzed seasonal and annual spatial projections from three climate models for two physical climate variables (seawater temperature and sea ice) and three forage variables (large copepods, euphausiids, and benthic infauna), comparing projected conditions from a recent time period (2003–2012) to a future time period (2030–2039). We focused the analyses on core areas within globally significant Important Bird Areas, and developed indices of the magnitude of projected change and vulnerability agreement among models. All three climate models indicated a high degree of change for seawater temperature warming (highest in the central and eastern Aleutian Islands) and ice loss (most significant in the eastern Bering Sea) across scales, and we found those changes to be significant for every species and virtually every core area assessed. There was low model agreement for the forage variables; while the majority of core areas were identified as climate vulnerable by one or more models (72% for large copepods, 73% for euphausiids, and 94% for benthic infauna), very few were agreed upon by all three models (only 6% of euphausiid-forager core areas). Based on the magnitude-agreement score, euphausiid biomass decline affected core areas for fulmars, gulls, and auklets, especially along the outer shelf and Aleutian Islands. Benthic biomass decline affected eiders along the inner shelf, and large copepod decline was significant for storm-petrels and auklets in the western Aleutians. Overall, 12% of core areas indicated climate vulnerability for all variables assessed. Modeling and interpreting biological parameters to project future dynamics remains complex; the strong signal for projected physical changes raised concerns about lagged responses such as distribution shifts, breeding failures, mortality events, and population declines.

The pelagic ecosystem in the Northern California Current off Oregon during the 2014-2016 warm anomalies within the context of the past 20 years.

A warm anomaly in the upper ocean, colloquially named "the Blob," appeared in the Gulf of Alaska during the calm winter of 2013-2014, spread across the northern North Pacific (NP) Ocean, and shifted eastward and onto the Oregon shelf. At least 14 species of copepods occurred which had never been observed in shelf/slope waters off Oregon, some of which are known to have NP Gyre affinities, indicating that the source waters of the coastal "Blob" were likely of both offshore (from the west) and subtropical/tropical origin. The anomalously warm conditions were reduced during strong upwelling in spring 2015 but returned when upwelling weakened in July 2015 and transitioned to downwelling in fall 2015. The extended period of warm conditions resulted in prolonged effects on the ecosystem off central Oregon, lasting at least through 2016. Impacts to the lower trophic levels were unprecedented and include a novel plankton community composition resulting from increased copepod, diatom, and dinoflagellate species richness and increased abundance of dinoflagellates. Additionally, the multiyear warm anomalies were associated with reduced biomass of copepods and euphausiids, high abundance of larvaceans and doliolids (indictors of oligotrophic ocean conditions), and a toxic diatom bloom (Pseudo-nitzschia) throughout the California Current in 2015, thereby changing the composition of the food web that is relied upon by many commercially and ecologically important species.

Use of acoustic estimates of euphausiid biomass in trophic dynamics and ecosystem models of the Georges Bank/Gulf of Maine Region from 1999 to 2012

Euphausiids are a key link between primary production and higher-level predators in the Gulf of Maine, but are not well sampled during standard fisheries surveys. Multifrequency acoustic data may provide useful estimates of euphausiid distribution and biomass, as long as automated classification of acoustic backscatter is reliable and robust. Estimates of euphausiid biomass in the Georges Bank region of the Gulf of Maine were derived from annual acoustic/midwater trawl surveys from 1999 through 2012. Acoustic data were collected continuously with Simrad EK500 and EK60 echo sounders operating at 18, 38, and 120 kHz. Four different methods were used to classify euphausiids from the acoustic data: multifrequency single beam imaging, “dB-differencing” of 120- and 38-kHz volume backscatter, multifrequency z-score, and a multifrequency index. Scattering model predictions of euphausiid target strength and biological metrics were used to scale acoustic data to biomass for each classification method. Biomass estim...

Cостояние планктонного сообщества в прикурильских водах при современном возобновлении массовых нагульных северных миграций субтропических рыб

State of plankton community was monitored in the Pacific waters at Kuril Islands in July-August 2016, in conditions of mass feeding migrations of subtropical fish. Structure of the plankton community corresponded to its summer season: the large-sized fraction of zooplankton prevailed by biomass (74-86 %), the portions of medium- and small-sized fractions were 3-13 % and 11-15 %, respectively. The dominant groups in the large-sized fraction were copepods ( Neocalanus plumchrus, Metridia okhotensis, Metridia pacifica, Eucalanus bungii ), arrowworms ( Sagitta elegans ) and hyperiids ( Themisto pacifica ). Copepoda nauplii and copepodites of I-II stage, Euphausiacea furcilia and caliphyptophytes, and juvenile hyperiids were found in the small-sized and medium-sized fractions. Total biomass of the large-sized fraction was significantly higher in summer 2016 than in July-August 2015, obviously because of earlier phase of seasonal succession under colder thermal regime. In 2016, the biomasses of copepods and sagittas were at the mean long-term levels, and the biomass of euphausiids was above the norm. The biomasses of the small- and medium-sized fractions were above the norm, too. The total stock of plankton in the surveyed area in July-August 2016 was assessed in 92926 . 10 3 t. High biomass of zooplankton in this area was favorable for feeding of subtropical fish.

Dynamics of zooplankton production parameters in the north-western Bering Sea in the present period

On the basis of plankton surveys, conducted in the north-western Bering Sea in 2010-2015, the production characteristics of zooplankton were calculated and analyzed. Zooplankton was divided into two trophic groups: predatory (mainly chaetognaths and hyperiids) and non-predatory (copepods and euphausiids) zooplankton. Considerable variations of zooplankton production parameters were observed in autumn period. This is due to the inter-annual dynamics of regional patterns of the size, taxonomic and trophic structure of zooplankton.The sharp decline in the copepod biomass after 2011 and the significant reduction in the biomass of euphausiids in 2013 were characteristic for the considered period in this region. All these features of the dynamics of copepods and euphausiids had an influence on the value of zooplankton production. The highest non-predatory zooplankton production was in 2010-2011 (up to 5081 mg/m 3 ); the lowest, in 2012-2014 (up to 2716 mg/m 3 ). Production of predatory zooplankton was the most significant in 2013 in the waters of the eastern part of the Gulf of Anadyr; it was associated with the presence of Themisto libellula . In general, non-predatory zooplankton in the north-western Bering Sea produced from 14.8 to 53.1 million tons during autumn; predatory zooplankton, from 5.0 to 14.2 million tons.

The Barents Sea euphausiids: methodological aspects of monitoring and estimation of abundance and biomass

Abstract Sampling of euphausiids is difficult because of their intermediate size between macrozooplankton and micronekton. The Barents Sea is one of the few marine areas where there have been long-term studies of euphausiids. We have examined three monitoring datasets on euphausiids and consider likely sources of errors associated with the sampling. Results indicated a high degree of patchiness in the distribution of euphausiids, even at the largest scale of sampling with a pelagic trawl. This indicates that euphausiids may occur in large, but infrequent, swarms that have a low probability of being sampled by small nets. The mean biomass of euphausiids sampled with MOCNESS was 2 g wet weight m−2 integrated over the water column, which is an underestimate due to avoidance of large individuals. The mean biomass obtained with pelagic trawl in the upper 60 m of water at night during an autumn survey was 10 g wet weight m−2. The plankton net on bottom trawl collected mean and median density of euphausiids (0.1–0.2 g wet weight m−3) near bottom during a winter survey similar to the values found with pelagic trawl in the upper layer during autumn. The mean density for the autumn survey showed an increase from 2000 to 2011, while the winter survey showed generally a decrease from 2000–2007 to 2011. The increase in the autumn series coincided with a general warming trend presumably with a larger influx of euphausiids with Atlantic water, notably of Meganyctiphanes norvegica. In contrast, the decline during winter may reflect a decrease, particularly of Thysanoessa raschii in the southeastern Barents Sea in the most recent years. Improvements in sampling gears combined with more and better use of acoustical and optical technologies offer great promise for improved monitoring and quantification of the roles of euphausiids in the Barents Sea ecosystem.

Euphausiids in the eastern Bering Sea: A synthesis of recent studies of euphausiid production, consumption and population control

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 3.08–5.25gCm−2 on the outer shelf and 1.95–3.92gCm−2 on the middle shelf. Modeled estimates of euphausiid production, for spring and summer combined, varied between 0.043gCm−2d−1 and 0.051gCm−2d−1, depending on location, with a mean of 0.048gCm−2d−1. Recently reported field measurements of annual primary production over the southeastern Bering Sea in 2008–2009 vary between 0.06 and 6.65gCm−2d−1, with a mean of 1.262gCm−2d−1±2.049gCm−2d−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 0.019.7gCm−2d−1 of euphausiids, depending on year, with a mean of 0.011gCm−2d−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.

Summer diet composition of walleye pollock and predator–prey relationships with copepods and euphausiids in the eastern Bering Sea, 1987–2011

The composition of walleye pollock diets from the eastern Bering Sea continental shelf was determined from 25 years of sampling during summer surveys from 1987 through 2011. Substantial differences in the stomach contents of walleye pollock were found among the sizes and geographic strata that correspond to geographic distribution of the prey. With increasing pollock size, copepods decreased in importance in middle and outer shelf areas while mysids decreased in importance in the inner shelf. Euphausiids increased in importance with increasing pollock size in southeastern areas of the shelf, and fishes and shrimp increased in importance with increasing pollock size in northeastern areas of the middle and outer shelf. The biomass-weighted average diet composition of eastern Bering Sea pollock in each year׳s survey indicated perennial but variable importance of euphausiids and copepods as prey. An index of partial fullness indicated an interannual pattern of below-average consumption of copepods by the surveyed pollock from 1993 to 2004, but during this period the amount of euphausiids consumed continued to fluctuate about a mean that was similar to years surveyed before and after that period. The summer feeding success, as indicated by average stomach fullness, of intermediate sizes of pollock appears to be closely related to the consumption of copepods (especially for pollock 30–39cm fork length (FL)) and prey other than euphausiids (especially for pollock 40–49cmFL). Length-specific predator–prey relationships with copepods and euphausiids correspond to patterns in pollock feeding migrations. Interannual trends in the biomass of copepods in the EBS are reflected most closely in the diet of 20–29cmFL pollock, and trends in the biomass of euphausiids in the EBS are reflected most closely in the diet of the largest pollock (60+cmFL). Climate-mediated changes in the zooplankton community will likely have differential impacts across the demographic spectrum of pollock in the EBS.

Baleen whale abundance and distribution in relation to environmental variables and prey density in the Eastern Bering Sea

The Bering Sea is one of the most productive marine ecosystems in the world and an important habitat for various marine mammal species. Once abundant in this region, most baleen whale species were severely depleted by commercial whaling in the 19th and early 20th centuries. Since their protection in mid-20th century, baleen whale populations have been recovering and reoccupying their historical habitats. These species can consume large amounts of their prey and thus can modify the local structure of ecosystems. Characterizing the extent to which environmental conditions and prey density influence baleen whale abundance in the Eastern Bering Sea is essential to improve our understanding of ecosystem dynamics and to predict how these species might respond to ecosystem variability associated with climate changes. In this study, physiographic, oceanographic, and biological datasets from 2008 to 2010 were combined to model the habitat characteristics of fin whales, humpback whales, and minke whales in the EBS in early summer (June and July) using generalized additive models (GAMs). The explained deviances of the best-supported models were 54.9%, 20.6%, and 68.3% for minke, fin and humpback whales, respectively. Minke and fin whales had similar distribution patterns in the EBS but their abundance was predicted by different explanatory variables. Euphausiid and pollock biomasses, and depth were important predictors of minke whale numbers, while distance to shore, euphausiid biomass, distance to the 200m isobath, and chlorophyll-a concentration better explained fin whale abundance. Humpback whales showed a preference for shallow, coastal waters north of the Alaska Peninsula. For this species, sea surface temperature, depth, chlorophyll-a concentration and euphausid biomass were important predictors of abundance. This study is the first to provide a habitat baseline for baleen whales in the EBS based on a quantitative assessment of the relationship between whale abundance, environmental variables, and density of euphausiids and age-1 pollock in early summer. Because this study was conducted during a cold temperature regime in the Bering Sea, additional research is needed to assess how whales respond to environmental variables and prey biomass in years with warm conditions.