The flow of Pacific-origin water across the Chukchi Sea shelf impacts the regional ecosystem in profound ways, yet the two current branches on the eastern shelf that carry the water from Bering Strait to Barrow Canyon - the Alaskan Coastal Current (ACC) and Central Channel (CC) Branch - have not been clearly distinguished or quantified. In this study we use an extensive collection of repeat hydrographic sections occupied at three locations on the Chukchi shelf, together with data from a climatology of shipboard velocity data, to accomplish this. The data were collected predominantly between 2010 and 2020 during the warm months of the year as part of the Distributed Biological Observatory and Arctic Observing Network. The mean sections show that mass is balanced for both currents at the three locations: Bering Strait, Point Hope, and Barrow Canyon. The overall mean ACC transport is 0.34 +/- 0.04 Sv, and that of the CC Branch is 0.86 +/- 0.11 Sv. The dominant hydrographic variability at Bering Strait is seasonal, but this becomes less evident to the north. At Barrow Canyon, the dominant hydrographic signal is associated with year-to-year variations in sea-ice melt. Farther south there is pronounced mesoscale variability: an empirical orthogonal function analysis at Bering Strait and Point Hope reveals a distinct ACC mode and CC Branch mode in hydrography and baroclinic transport, where the former is wind-driven. Finally, the northward evolution in properties of the two currents is investigated. The poleward increase in salinity of the ACC can be explained by lateral mixing alone, but solar heating together with wind mixing play a large role in the temperature evolution. This same atmospheric forcing also impacts the northward evolution of the CC Branch.

The physical environments of high-latitude systems are rapidly changing. For example, the Chukchi Sea has experienced increased water temperatures, advection from the Bering Sea, declines in sea-ice concentration, earlier spring ice retreat, and delayed fall ice formation. This physical restructuring is expected to impact ecosystem structure and function. In this study, a series of bio-oceanographic research surveys were conducted in the summers of 2010, 2011, and 2012 to characterize the physical environment and to examine the influence of physical forcing on zooplankton community distribution and abundance. Results revealed yearly advection from the Bering Sea influenced zooplankton community structure, but this influence became less apparent in the northeastern Chukchi due to changes in current speeds and patterns. Decreased advection and later ice retreat in colder years resulted in zooplankton communities that exhibited more diversity, had higher abundances of the lipid-rich copepod Calanus glacialis , and were less closely related to water masses advected from the south. These findings suggest more localized processes are influencing zooplankton community structure in the Chukchi Sea. Increased inflow of water into the Chukchi is predicted with increased warming in the Arctic and changes in food-web structure and function are likely to result.

The oceanography of the Arctic is changing, with the potential to restructure the function and production of its ecosystems. The physical oceanographic conditions that have occurred on the Chukchi Sea shelf during June through October of the years 1979–2014 were investigated using the ORAS4 ocean reanalysis product. Time series of vertically integrated temperatures (especially during September and October) indicate greater warming in the first half of the 36-year record. This change in the long-term temperature trend may be in part due to trends in the mean currents, which in Herald Canyon and off the coast of Northwest Alaska tended to be slightly less poleward after 2000. A k-means cluster analysis of monthly mean sea surface height anomaly distributions was used to describe five distinct patterns of flow. Two of the five patterns (clusters 2 and 4) relate to the strength of the Alaskan Coastal Current (ACC). Another pair of patterns (clusters 3 and 5) had their strongest expressions in the northwest Chukchi Sea and relate to periods of weak southeastward versus strong northwestward flow in this region associated with the presence or absence of the Siberian Coastal Current. The fifth pattern (cluster 1) was defined by weak southeastward flow anomalies in the western Chukchi and a slightly suppressed ACC relative to the mean in the eastern Chukchi Sea. The composite sea surface height anomaly patterns of the five cluster types correspond closely with the mean sea level pressure anomaly distributions during the months constituting each cluster type. Our findings for the Chukchi Sea region provide a long-term context for previous field observations and may be useful for interpretation of past ecosystem variations.

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.

The timing and magnitude of phytoplankton blooms in subarctic ecosystems often strongly influence the amount of energy that is transferred through subsequent trophic pathways. In the eastern Bering Sea, spring bloom timing has been linked to ice retreat timing and production of zooplankton and fish. A large part of the eastern Bering Sea shelf (~500km wide) is ice-covered during winter and spring. Four oceanographic moorings have been deployed along the 70-m depth contour of the eastern Bering Sea shelf with the southern location occupied annually since 1995, the two northern locations since 2004 and the remaining location since 2001. Chlorophyll a fluorescence data from the four moorings provide 37 realizations of a spring bloom and 33 realizations of a fall bloom. We found that in the eastern Bering Sea: if ice was present after mid-March, spring bloom timing was related to ice retreat timing (p<0.001, df=1, 24); if ice was absent or retreated before mid-March, a spring bloom usually occurred in May or early June (average day 148, SE=3.5, n=11). A fall bloom also commonly occurred, usually in late September (average day 274, SE=4.2, n=33), and its timing was not significantly related to the timing of storms (p=0.88, df=1, 27) or fall water column overturn (p=0.49, df=1, 27). The magnitudes of the spring and fall blooms were correlated (p=0.011, df=28). The interval between the spring and fall blooms varied between four to six months depending on year and location. We present a hypothesis to explain how the large crustacean zooplankton taxa Calanus spp. likely respond to variation in the interval between blooms (spring to fall and fall to spring).

Alaska's Bering Sea ecosystem is changing rapidly, and the people and animals living in this area must quickly adapt. The US National Oceanic and Atmospheric Administration's Ecosystems and Fisheries-Oceanography Coordinated Investigations program has been monitoring the Bering Sea ecosystem for more than 20 years with a multidisciplinary toolbox of biophysical moorings, ship-based operations, and satellite-tracked drifters. Physical and biological time-series data collected from a series of three-to-seven-year programs have supported foundational ecosystem science and provided great insight into how climate can influence fisheries recruitment. In this article, we highlight the major discoveries made during nearly two decades of observations in the Bering Sea.

Alaska's Bering Sea ecosystem is changing rapidly, and the people and animals living in this area must quickly adapt. The US National Oceanic and Atmospheric Administration's Ecosystems and Fisheries-Oceanography Coordinated Investigations program has been monitoring the Bering Sea ecosystem for more than 20 years with a multidisciplinary toolbox of biophysical moorings, ship-based operations, and satellite-tracked drifters. Physical and biological time-series data collected from a series of three-to-seven-year programs have supported foundational ecosystem science and provided great insight into how climate can influence fisheries recruitment. In this article, we highlight the major discoveries made during nearly two decades of observations in the Bering Sea

Adding acoustic systems onto ocean moorings and observatories provides additional data to more fully document ecosystem responses to environmental perturbations. A passive acoustic recorder and three-frequency echosounder system were integrated into a biophysical mooring on the central eastern Bering Sea continental shelf. An unexpected, transient, mid-winter retreat of the seasonal sea ice was observed over the mooring for a 2-week period in March 2009. Interpretation of the passive acoustic data provided information about sea ice conditions and included the detection and identification of vocalizing marine mammals, while the acoustic backscatter provided information on relative zooplankton and fish abundance before, during, and after the retreat. Hydrographic data confirmed the acoustic signal was associated with changing surface ice conditions, and the combined information from the biophysical mooring sensors revealed changes in winter trophic level dynamics during the retreat, which would have otherwise been undetected by traditional ship-based observations. Changes in the acoustic environment, zooplankton dynamics, and acoustic detection of marine mammals were observed amidst a physically stable and uniform water column with no indication of a phytoplankton bloom. These data demonstrate the value of acoustic technologies to monitor changing ecosystems dynamics in remote and hazardous locations. (C) 2012 Elsevier Ltd. All rights reserved.

Adding acoustic systems onto ocean moorings and observatories provides additional data to more fully document ecosystem responses to environmental perturbations. A passive acoustic recorder and threefrequency echosounder system were integrated into a biophysical mooring on the central eastern Bering Sea continental shelf. An unexpected, transient, mid-winter retreat of the seasonal sea ice was observed over the mooring for a 2-week period in March 2009. Interpretation of the passive acoustic data provided information about sea ice conditions and included the detection and identification of vocalizing marine mammals, while the acoustic backscatter provided information on relative zooplankton and fish abundance before, during, and after the retreat. Hydrographic data confirmed the acoustic signal was associated with changing surface ice conditions, and the combined information from the biophysical mooring sensors revealed changes in winter trophic level dynamics during the retreat, which would have otherwise been undetected by traditional ship-based observations. Changes in the acoustic environment, zooplankton dynamics, and acoustic detection of marine mammals were observed amidst a physically stable and uniform water column with no indication of a phytoplankton bloom. These data demonstrate the value of acoustic technologies to monitor changing ecosystems dynamics in remote and hazardous

Fishery managers require an understanding of how climate influences recruitment if they are to separate the effects of fishing and climate on production. The southeastern Bering Sea offers opportunities to understand climate effects on recruitment because inter-annual oscillations in ice coverage set up warm or cold conditions for juvenile fish production. Depth-averaged temperature anomalies in the Bering Sea indicate the past nine years have included three warm (2003–2005), an average (2006), and five cold (2007–2011) years. We examined how these climatic states influenced the diet quality and condition (size, energy density and total energy) of young-of-the-year (YOY) pollock (Theragra chalcogramma) in fall. The implications of fall condition were further examined by relating condition prior to winter to the number of age-1 recruits-per-spawner the following summer (R/S). The percentage of lipid in pollock diets was threefold higher in cold years compared with warm years, but stomach fullness did not vary. Consequently, fish energy densities were 33% higher in cold years (P<0.001) than in warm years. In contrast, neither fish size (P=0.666), nor total energy (P=0.197) varied with climatic condition. However, total energy was significantly (P=0.007) and positively correlated with R/S (R2=0.736). We conclude that recruitment to age-1 in the southeastern Bering Sea is improved under environmental conditions that produce large, energy dense YOY pollock in fall.

Sufficient oceanographic measurements have been made in recent years to describe the latitudinal variation in the physics of the eastern Bering Sea shelf and the potential impact of climate change on the species assemblages in the two ecosystems (north and south). Many of the predicted ecosystem changes will result from alterations in the timing and extent of sea ice. It is predicted that the sea ice in the northern Bering Sea will be less common in May, but will continue to be extensive through April. In contrast, the southern shelf will have, on average, much less sea ice than currently observed, but with large interannual and multiyear variability until at least 2050. Thus, even under current climate warming scenarios, bottom temperatures on the northern shelf will remain cold. Based on biophysical measurements, the southern and northern ecosystems were divided by a North–South Transition at ∼60°N. The northern middle shelf was characterized by a freshwater lens at the surface, cold bottom temperatures, and a thicker pycnocline than found on the southern shelf. Subsurface phytoplankton blooms were common. In contrast, the southern shelf stratification was largely determined by temperature alone; the pycnocline was thin (often<3m) and subsurface blooms were uncommon. Biological responses to climate warming could include greater north–south differences in zooplankton community structure, the transport of large Outer Shelf Domain crustacean zooplankton to the middle shelf, and the disappearance of two principal prey taxa (Calanus spp. and Thysanoessa spp.) of planktivorous fish, seabirds and whales. The response of commercially and ecologically important fish species is predicted to vary. Some species of fish (e.g., juvenile sockeye salmon, Oncorhynchus nerka) may expand their summer range into the northern Bering Sea; some (e.g., pink salmon, O. gorbuscha) may increase in abundance while still other species (e.g., walleye pollock and arrowtooth flounder; Theragra chalcogramma and Atheresthes stomias, respectively) are unlikely to become common in the north. The projected warming of the southern shelf will limit the distribution of arctic species (e.g., snow crab, Chionoecetes opilio) to the northern shelf and will likely permit expansion of subarctic species into the southern Bering Sea. The distribution and abundance of baleen whales will respond to shifts in prey availability; for instance, if prey are advected northward from the southeastern Bering Sea, an extension of range and an increase in seasonally migratory baleen whale numbers is anticipated. Thus, alteration of this ecosystem in response to climate change is expected to result in something other than a simple northward shift in the distribution of all species.

We examined the influence of environmental conditions on walleye pollock (Theragra chalcogramma) early life history in discrete stages at two ecological scales using a 17-year time series from the southeastern Bering Sea. Generalized additive models (GAMs) were used to quantify relationships between walleye pollock stages (eggs, yolksac larvae, preflexion larvae, late larvae, and juveniles), the fine-resolution environment (temperature, wind speed, salinity, and copepod concentration), and the broad-resolution environment (annual spawning stock biomass, temperature, zooplankton biomass, and wind mixing). Early stages (eggs, yolksac larvae, and preflexion larvae) were associated with high spawning stock biomass, while late stages (late larvae and juveniles) were not associated with spawning stock biomass. The influence of temperature increased with ontogeny: high egg abundance was associated with temperatures from −2 to 7°C and negative annual temperature anomalies and high juvenile abundance was associated with temperatures from 4 to 12°C and positive temperature anomalies. Winds enhanced the transport of early stages from spawning locations to shallower sampling depths, but did not affect feeding stages (preflexion larvae, late larvae, and juveniles) in a manner consistent with the encounter-turbulence hypothesis. Feeding stages were positively associated with localized copepod concentrations but not zooplankton biomass anomaly, suggesting that the localized measurements of potential prey is a better indicator compared to broad-scale conditions measured in areas where these stages do not necessarily occur. Broad-resolution covariates, however, explained a greater portion of the overall variation than did fine-resolution models. Of the environmental conditions examined, temperature explained more variation in abundance of walleye pollock early life stages than any other covariate. Temperature is likely a major driving force structuring variability in populations of walleye pollock in their first year of life, acting directly upon them and indirectly upon their physical habitat and prey community.

The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice extent, temperature, and the distribution and abundance of species at multiple trophic levels over the past four decades. From 1972–2000, there was high interannual variability of areal extent of sea ice during spring (March–April). In 2000, this shifted to a 5-year (2001–2005) period of low ice extent during spring, which transitioned to a 4-year (2007–2010) period of extensive sea ice. High (low) areal extent of sea ice in spring was associated with cold (warm) water column temperatures for the following 6–7 months. The ocean currents also differed between warm and cold years. During cold years, the monthly-mean currents over the shelf were largely westward, while in warm years the direction of currents was more variable, with northward flow during December–February and relatively weak flow during the remainder of the year. The types and abundance of zooplankton differed sharply between warm and cold years. This was especially true during the prolonged warm period (2001–2005) and cold period (2007–2010), and was less evident during the years of high interannual variability. During the warm period, there was a lack of large copepods and euphausiids over the shelf; however, their populations rebounded during cold period. Small crustacean zooplankton taxa did not appear to vary between and warm and cold years. For both walleye pollock and Pacific cod, year-class strength (recruitment) was low during the prolonged warm period, but improved during the following cold period. Year-class strength did not appear to vary as a function of warm and cold years during the period of high year-to-year variability. Also, recruitment of arrowtooth flounder (a predator of pollock and cod) did not appear influenced by the warm or cold years. Finally, the distribution and relative abundance of fin whales appeared to differ in warm and cold years, with fewer whales on the southeastern, middle shelf during warm years.

ESR Endangered Species Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials ESR 13:99-109 (2011) - DOI: https://doi.org/10.3354/esr00324 Rare detections of North Pacific right whales in the Gulf of Alaska, with observations of their potential prey P. R. Wade1,*, A. De Robertis1, K. R. Hough1, R. Booth2, A. Kennedy1, R. G. LeDuc3, L. Munger4, J. Napp1, K. E. W. Shelden1, S. Rankin3, O. Vasquez1, C. Wilson1 1Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Point Way NE, Seattle, Washington 98115, USA 2Center for Conservation Biology, Department of Biology, University of Washington, Seattle, Washington 98195, USA 3Southwest Fisheries Science Center, National Marine Fisheries Service, 3333 N. Torrey Pines Ct, La Jolla, California 92037, USA 4Scripps Institution of Oceanography, La Jolla, California 92037, USA *Email: paul.wade@noaa.gov ABSTRACT: The North Pacific right whale Eubalaena japonica was heavily exploited throughout the Gulf of Alaska by both historical whaling and 1960s illegal Soviet catches. It is now extremely rare in this region (2 sightings between 1966 and 2003 and passive acoustic detections on 6 days out of 80 months of recordings at 7 locations). From 2004 to 2006, 4 sightings of right whales occurred in the Barnabus Trough region on Albatross Bank, south of Kodiak Island, Alaska, USA. Sightings of right whales occurred at locations within the trough with the highest density of zooplankton, as measured by active acoustic backscatter. Net trawls through a high-density demersal layer (~150 to 175 m) revealed large numbers of euphausiids and oil-rich C5-stage copepods. Photo-identification and genotyping of 2 whales failed to reveal a match to Bering Sea right whales. Fecal hormone metabolite analysis from 1 whale estimated levels consistent with an immature male, indicating either recent reproduction in the Gulf of Alaska or movements between the Bering Sea and the Gulf of Alaska. Large numbers of historic catches of right whales occurred in pelagic waters of the Gulf of Alaska, but there have been few recent detections in deep water. Given that there is no other location in the Gulf of Alaska where right whales have been repeatedly seen post-exploitation, the Barnabus Trough/Albatross Bank area represents important habitat for the relict population of North Pacific right whales in the Gulf of Alaska, and a portion of this area was designated as critical habitat under the US Endangered Species Act in 2006. KEY WORDS: North Pacific right whale · Eubalaena japonica · Prey · Gulf of Alaska · Kodiak Island · Whaling Full text in pdf format PreviousNextCite this article as: Wade PR, De Robertis A, Hough KR, Booth R and others (2011) Rare detections of North Pacific right whales in the Gulf of Alaska, with observations of their potential prey. Endang Species Res 13:99-109. https://doi.org/10.3354/esr00324 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in ESR Vol. 13, No. 2. Online publication date: January 27, 2011 Print ISSN: 1863-5407; Online ISSN: 1613-4796 Copyright © 2011 Inter-Research.

US marine fisheries management in Alaska has been cited worldwide as an example to highlight successful management that incorporates ecosystem considerations. A review of historical federal fisheries management and ecosystem advice in Alaska is provided that shows some of the more significant ecosystem-based management actions that have been taken. These include conservative exploitation rates, limits on bycatch and discards, habitat protection measures, endangered and protected species considerations, and including humans as part of the ecosystem. An important aspect of the US approach in Alaska is to assess not only fishing impacts on ecosystem components, but also the impacts of other pressures of human or natural origin on the ecosystem. In addition to human-induced ecosystem impacts, primarily through fishing, Alaska marine ecosystems are strongly influenced by climate variability. Understanding and quantifying the contributions of fishing and climate influences to ecosystem change have been important aspects of the research effort in this region. A variety of ecosystem indicators have been derived to evaluate and assess how various stressors may affect the ecosystem-based objectives of maintaining predator–prey relationships and energy flow, maintaining diversity, maintaining habitat, and incorporating/monitoring the effects of climate change. Uncertainty regarding the mechanisms influencing ecosystem structure and function is influencing research efforts. A combination of field research and predictive modeling is leading to the design of management systems that are robust to a wide range of predictive uncertainty. Integrating social science and economic considerations into the ecosystem-based approach in Alaska is also a key consideration.

Walleye pollock (Theragra chalcogramma) is an important component of the eastern Bering Sea ecosystem and subject to major fisheries. The Oscillating Control Hypothesis (OCH) predicted that recruitment of pollock year classes should be greatest in years with early ice retreat and late blooms in warm water, because more energy would flow into the pelagic (vs. benthic) community. The OCH further predicted that, with pollock population growth, there should be a shift from bottom-up to top-down regulation. New data support the predictions that in those years with early ice retreat, more primary production accrues to the pelagic compartment and that large numbers of age-0 pollock survive to summer. However, in these years, production of large crustacean zooplankton is reduced, depriving age-0 pollock of lipid-rich prey in summer and autumn. Consequently, age-0 pollock energy reserves (depot lipids) are low and predation on them is increased as fish switch to age-0 pollock from zooplankton. The result is weak recruitment of age-1 recruits the following year. A revised OCH indicates bottom-up constraints on pollock recruitment in very warm periods. Prolonged warm periods with decreased ice cover will likely cause diminished pollock recruitment and catches relative to recent values.

In spring and fall 2005, cross- and along-shelf transects were sampled to evaluate the influence of physical forcing, including sea ice, tides, and winds, on the lower trophic levels of the Bering Sea ecosystem. The hydrography, nutrients, chlorophyll, and zooplankton abundance and species composition were all affected by the presence or absence of sea ice on a north–south transect along the 70-m isobath. In May, shelf waters between ∼59°N and 62°N were cold and relatively fresh, and benthic invertebrate larvae and chaetognaths were a significant fraction of the zooplankton community, while to the south the water was warmer, saltier, and the zooplankton community was dominated by copepods. The position of the transition between ice-affected and ice-free portions of the shelf was consistent among temperature, salinity, nutrients, and oxygen. This transition in the hydrographic variables persisted through the summer, but it shifted ∼150km northward as the season progressed. While a transition also occurred in zooplankton species composition, it was farther north than the physical/chemical transition and did not persist through the summer. Mooring data demonstrated that the change in the position of the transition in physical and chemical properties was due to northward or eastward advection of water onto and across the shelf. From south to north along the 70-m isobath, tidal energy decreased, resulting in a less sharply stratified water column on the northern portion of the middle shelf, as opposed to a well-defined, two-layered system in the southern portion. This more gradual stratification in the north permitted a greater response to mixing from winds, which were homogeneous from north to south. Thus the physical and biological structure at any one location over the middle shelf is dynamic over the course of a year, and results from a combination of in situ processes and climate-mediated regional forcing which is dominated in most years by sea ice.