Ecosystem Assessment (pdf)

Elizabeth Siddon Auke Bay Laboratories, Alaska Fisheries Science Center, NOAA Fisheries
Contact: elizabeth.siddon@noaa.gov

Last updated: November 2023

Introduction

The eastern Bering Sea (EBS) transitioned from an ecosystem governed by interannual variability (1982– 2000) into one that has experienced multi-year stanzas of warm (2000–2005, 2014–2021) and cold (2007–2013) periods (Baker et al., 2020) of varying durations. At present, the North Pacific is also un- dergoing a transition from three consecutive years of La Niña conditions to predicted El Niño conditions by early 2024. Since 2021, oceanographic metrics (i.e., sea ice extent, sea surface and bottom temper- atures) have cooled to near average based on respective time series. However, biological metrics like zooplankton and fish dynamics have lagged in their expected response to cooler conditions. Therefore, ecologically, the EBS remains in a transitional state in 2023.

Seasonal sea ice and the resulting cold pool extent are defining features over the Bering Sea shelf. The recent warm stanza (2014–2021) included unprecedented low sea ice extents in winters 2017/2018 and 2018/2019 with near nonexistent cold pool extents in summers 2018 and 2019. The lack of thermal barrier resulted in the northward distributional shifts of groundfish stocks (e.g., Thorson et al., 2019; DeFilippo et al., 2023) that potentially impacted the food web dynamics and carrying capacity of the northern Bering Sea (NBS) ecoregion.

The delineation between the southeastern and northern Bering Sea is often considered at 60°N latitude on the basis of the physical and biological distinctions between these ecological systems, existing research and analyses in these areas, and available data and survey designs. This delineation is supported by broad-scale analyses of the physical oceanography and hydrography (Stabeno et al., 2012; Baker et al., 2020) and zoogeography of the region (Sigler et al., 2017). For an in-depth review of distinguishing characteristics between these ecoregions, see Baker (2023).

An assessment of the ecological trends during the recent transitional state (since 2021) and current 2023 status in the eastern and northern Bering Sea ecoregions is provided.

Southeastern Bering Sea

The recent warm stanza (2014–2021) in the EBS was unprecedented in terms of magnitude and duration (Figure 13), and included the near-absence of sea ice in the winters of 2017/2018 and 2018/2019 and subsequent absence of cold pool the following summers. Groundfish and crab stocks shifted their distribution in response to changes in sea ice and cold pool extent (Thorson et al., 2019; DeFilippo et al., 2023). However, since that time stocks have experienced variable responses in their population trends. Stocks that experienced increased reproductive success and recruitment during the warm stanza include the 2018 year class of pollock, the 2014–2019 year classes of sablefish (with juvenile sablefish increasing in the EBS; Goethel et al. (2022)), the 2017 year class of Togiak herring (see p. 118), and 8 years of Bristol Bay sockeye salmon returns (year classes precede returns by 3-5 years; see p. 128). Conversely, stocks that experienced reduced survival and stock declines include several crab stocks (notably snow crab and Bristol Bay red king crab; Figure 97) and multiple Western Alaska Chinook, chum, and coho salmon runs whose marine residency time may include the southeastern Bering Sea (SEBS) ecosystem.

Since 2021, oceanographic metrics of the thermal state of the SEBS shelf, including sea surface tem- perature (SST; see Figures 16, 18, and 25), bottom temperature (BT; see Figure 25), wintertime sea ice extent and thickness (see Figures 30 and 34), and cold pool extent (Figure 2) have largely returned to near their respective historical baselines through August 2023. While the summer 2023 cold pool pool was of moderate extent and among the largest of the past several years, it was significantly below the large cold pool extents that were common prior to the recent warm stanza.

Seasonally, winter conditions contribute to determining summer oceanographic conditions over the EBS shelf and impact vertical (through water column stratification) and horizontal (through cold pool extent) thermal barriers important for predator/prey dynamics (p. 116). The overall moderate winter conditions in 2022/2023 led to summer conditions that, while variable relative to long-term means, did not show any extreme characteristics. Winter 2022/2023 was on the warmer side while summer 2023 was cooler (Figure 19). Sea-ice dynamics are driven by both temperature and winds. Winter winds (Nov 2022-Mar 2023) were more southerly (Figure 8), which brings warm and moist air from the south and inhibits sea ice advance (Figure 31). This resulted in the sea ice phenology (winter ice advance and spring meltout) to be shifted ∼ 1 month later than 2021/2022. Sea ice did eventually reach as far south as St. Paul Island in 2023, providing a source of freshwater as ice melted at the ice edge, and reversing a trend of increasing salinity during the recent warm stanza (Figure 23).

Overall, water temperature patterns in 2023 moderated relative to recent years. Winter through spring SSTs (Dec 2022-May 2023) were above the long-term average for the outer and middle domains. However, bottom temperatures (from the ROMS model) were below average over the outer domain all year (Figure 21), resulting in a potentially strongly stratified system. Stratification can impact phytoplankton blooms and the vertical distribution of prey and predators. By summer, both observed (from the NOAA bottom trawl survey) and modeled (from the ROMS) bottom temperatures noted a cold ‘tongue’ of bottom water along the 50m isobath that contributed to the coldest inner domain temperatures since 2013 (Figure 26).

Wind patterns did not show a consistent directional signal. Winds can impact transport and surface (upper ∼ 30–40m) drift of early life stages of groundfish (e.g., pollock) and crab species (e.g., snow crab). For example, recruitment of winter-spawning flatfish (e.g., northern rock sole, arrowtooth flounder) is higher when spring winds result in in-shore advection to favorable nursery grounds (Wilderbuer et al., 2002, 2013). In 2023, spring (Mar-May) winds generally favored off- shelf surface transport while summer (Jun-Aug) 2023 winds favored on-shelf surface transport (Figure 12). These patterns suggest reduced recruitment for winter-spawning flatfish.

Metrics of benthic habitat condition also show mixed signs following the recent warm stanza. There are continued declines in the estimated percentage of habitat disturbance by fishing gear (see p. 202), which is assumed to be beneficial to structural epifauna. The biomass of some structural epifauna (e.g., anemones and sea whips) increased in 2022 and 2023, however sponges have had a steady decline since 2015 (Figure 38). Meanwhile, the catch of structural epifauna in groundfish fisheries had been declining since 2013 but increased slightly in 2021 and 2022 (Figure 114, middle).

Indirect measures of benthic productivity include trends in motile epifauna (e.g., sea stars, brittle stars, crabs), benthic foragers (such as small-mouthed flatfish), and some benthic-associated species. Motile epifauna have declined since the peak in 2017 but remain above the long term mean in 2023 (1982–2023; Figure 2). Some of this decline can be attributed to the rapid increase immediate prior to the 2017/2018– 2018/2019 marine heatwaves, then subsequent collapse of the snow crab stock. Echinoderms, and specifically brittle stars, continue to dominate this group that, while below their peak abundance, still reflect decades of steady increase in abundance and biomass in the ecosystem. Several crab stocks showed biomass declines in 2023 (Figure 97), including Bristol Bay red king crab males (female biomass increased from 2022, but remains below 1SD below the time series average; 1982–2023), St. Matthew Island blue king crab males, snow crab, and Pribilof Islands blue king crab. Tanner crab males and females remain below the time series average, though female biomass increased from 2022 (see p. 169). Trends in benthic foragers indirectly indicate availability of infauna, which are their prey. Trends of several component flatfishes (e.g., yellowfin sole, flathead sole) decreased, and the guild remains below 1 standard deviation (SD) below the time series mean (1982–2023; Figure 2), suggesting that some stocks could be experiencing prey limitations. These coincident trends among small-mouthed flatfish may warrant further attention to identify potential common stressors. Eelpouts have been increasing since 2019 and are just above the time series average while poachers decreased to just below the time series average (1982–2023; Figure 95).

Trends in primary productivity over the SEBS shelf have not tracked synchronously with thermal con- ditions. With the moderation from the recent heatwave conditions, the expectation is that primary productivity indicators would increase. In contrast, chlorophyll-a concentrations have generally been decreasing, with 2023 being among the lowest across sub-regions (see Figures 45 and 24). Also, coc- colithophore blooms have been more prominent over the shelf since 2017 with the 2023 index being the highest observed in the time series (1997–2023; p. 86). Coccolithophore blooms are not considered a positive sign of ecosystem productivity as their large blooms are thought to negatively impact foraging success of visual foragers such as pollock and seabirds. The size structure of the phytoplankton commu- nity reflects the quantity and quality of primary production as smaller phytoplankton assemblages lead to longer food webs and a less efficient energy transfer. While the observed size structure of phytoplankton was variable during the recent warm stanza, it was average in 2022, which is the most recent available data (Figure 40). Additionally, measures of large diatoms from the continuous plankton recorder (CPR) show a declining trend 2020–2022 (Figure 50).

The Rapid Zooplankton Assessment in the SEBS in spring 2023 noted a moderate abundance of small copepods, but low abundance and low lipid content of large copepods and euphausiids. In spring, sea ice and cooler temperatures over the shelf reduced zooplankton abundances and limited the development of the zooplankton community. In fall, the moderate abundance of small copepods continued, and while the abundance of large copepods and euphausiids remained low, abundances increased from south to north (see p. 93). This spatial pattern reflects more favorable conditions in the north; cold pool extent is one of the most important factors that correlates to the presence of Calanus spp. (Eisner et al., 2018; Kimmel et al., 2018). As large, lipid-rich copepods are linked to better overwinter survival of age-0 pollock, the low availability of large copepod prey in 2022 (Figure 50) may result in reduced overwinter survival and recruitment of pollock to age-3 in 2025 (see p. 162). The abundance of jellyfish over the SEBS shelf was average in 2023 (Figure 61, top), representing no significant change in competitive pressure for planktivorous predators.

Trends in forage fish abundance were also mixed in 2023. For example, capelin and eulachon were at near all-time lows while Pacific herring has been above average for the last several years. As the numerically dominant forage fish in the EBS, age-0 pollock are an important component of available forage over the SEBS shelf to piscivorous predators such as Pacific cod, pollock, seabirds, and marine mammals. In 2023, age-0 pollock showed continued declines in several metrics of condition (i.e., length/weight residuals, energy density residuals, and %lipid) (see p. 112), indicating that predators may have experienced some prey limitations. The extent of any negative impact may be influenced by whether predators could access alternate prey.

Some species, such as herring and salmon, experienced strong year classes during the recent warm stanza that have buoyed their adult biomasses. Togiak herring biomass has been increasing as a result of strong 2016 and 2017 year classes (see p. 118). Bristol Bay sockeye salmon remained abundant in 2023, continuing a 9-year pattern of high returns (see p. 128). These consistent high returns represent extended foraging pressure on their prey, such as euphausiids. It is not currently known whether this has created competitive pressure on other species that feed on similar prey. Interestingly, juvenile salmon condition in 2022, measured by energy density anomalies, was negative for pink, chum, sockeye, and Chinook salmon in the SEBS, consistent with lower energy stores and a reduced capacity for overwinter survival (Figure 70). This represents a departure from recent trends and may indicate lower future returns for sockeye salmon.

Groundfish condition, as measured by length-weight residuals of fish collected during the SEBS bottom trawl survey, was poor for pollock and arrowtooth flounder in 2023, indicating these species may have experienced some prey limitations. Condition of several species (e.g., Pacific cod, northern rock sole, yellowfin sole, flathead sole, Alaska plaice) showed slight increases in condition from 2021 to 2022 followed by declines from 2022 to 2023. Juvenile (100–250mm) and adult (>250mm) pollock condition has declined since 2021 and 2019, respectively, with adult pollock condition this year being second- lowest in the time series (1999–2023) (Figure 78). Additionally, pollock condition as inferred from the mean biomass anomaly in the commercial fishery was the lowest that has been recorded. Through 2022, continued favorable top-down conditions existed for juvenile groundfish survival due to predator release as a result of the declining biomass of groundfish (see p. 153).

The impacts of marine heatwave induced increases in thermal experience and metabolic demand, and declines in growth potential, are borne out in bioenergetic-based metrics that integrate thermal expe- rience, prey quantity and quality, and metabolic demands (see p. 146). Available metrics for age-0, juvenile, and adult pollock allow finer-scale tracking of the impacts of temperature and prey conditions across life stages. The %lipid and energy density of age-0 pollock underwent a step-change to lower values in the recent warm stanza beginning in 2014 and have remained low through 2023 (Figures 63 and 64), indicating potential declines in prey availability or a switch to less energetically valuable species. For adult pollock, prey limitation is indicated by multiple consumption indices (see p. 146). The cu- mulative effects of the multi-year warm stanza, in terms of thermal experience, metabolic demand, and prey availability, are evident in the 2023 adult pollock fish condition (Figure 78).

Seabirds are indicators of secondary productivity and shifts in prey availability that may similarly affect commercial fish populations. Trends in seabird reproductive success were mixed on the Pribilof Islands in 2023, with higher reproductive success for both fish-eating and plankton-eating species on St. George Island than on St. Paul Island (Figures 98 and 99). Species that experienced recent population losses (least auklets and common murres) do not appear to be rebounding to historic numbers. On St. Paul Island common and thick-billed murres had very low egg abundance early in the season, therefore no subsistence harvest took place in 2023. Community observations throughout the summer reported eventually seeing “a lot” of murre eggs, though murres seemed to experience nest failure later in the summer. Overall, reproductive success was mixed across species, but generally higher for species on St. George Island. This may indicate differences in local availability of zooplankton and small schooling forage fish in feeding areas utilized by seabirds of each island. No major seabird die-off events were observed in 2023 (Figure 101).

Metrics of stability in the fish community (for species regularly caught in the SEBS bottom trawl survey) indicate overall stability and resilience, although there are anomalous peaks in individual species (e.g., capelin, sablefish). Trends in mean lifespan (Figure 102) show little year-to-year variability and give no indication of shifts between short-lived and longer-lived species. The mean length of the fish community remained above the time series mean in 2023 (1982–2023; Figure 103) while the biomass index for pollock was below average, corroborating declines in fish condition (Figure 78). The stability of the groundfish community also remained above the time series mean in 2023 (Figure 104).

We track emerging stressors like ocean acidification (OA) and strive to better understand their role and potential impacts to the ecosystem. Metrics of OA (pH and Ω_arag) continued a multi-decadal decline, indicating more corrosive bottom-water conditions for marine calcifiers, though values improved slightly from 2022 to 2023. At this time, there is no evidence that OA can be linked to recent declines in crab populations. It is worth noting that Ω_arag is approaching the threshold value (<1.0) for pteropod shell dissolution that could have subsequent biological significance through the food web (Figure 107).

Northern Bering Sea

Ecosystem-wide impacts have been observed in the northern Bering Sea (NBS) since the two winters (2017/2018 and 2018/2019) of little sea ice and two summers (2018 and 2019) of reduced cold pool extent. Northward shifts in the distribution of groundfish species and concerns about the food web dynamics and carrying capacity have existed since 2018, highlighted by the gray whale Unusual Mortality Event and short-tailed shearwater mass mortality event (Siddon, 2020). Both species feed in the NBS during summer before embarking on long migrations south for breeding.

In 2021, multiple ecosystem ‘red flags’ occurred in the NBS: (1) the NOAA bottom trawl survey demonstrated a substantial drop in total CPUE between 2019 and 2021 that reflected large decreases in dominant species, including crab and pollock, (2) salmon run failures in the Arctic-Yukon-Kuskokwim region, and (3) seabird die-offs combined with low colony attendance and poor reproductive success (Siddon, 2021). Although the events are coincident, the multi-year warm phase resulted in cumulative impacts of increased thermal exposure and metabolic demands.

Since 2021, like the SEBS, the NBS ecosystem has been transitioning to more average conditions. There have been no prolonged marine heatwaves in the NBS since January 2021 (Figure 17) and sea ice thickness in 2023 was above the time series average (2011–2023) for the Bering Strait region, Norton Sound, and the area between St. Lawrence Island and St. Matthew Island (Figure 33). SSTs have been within 1SD, therefore organisms have experienced reduced cumulative thermal exposure and metabolic stress (Figure 18). Also similar to the SEBS, the fall and winter 2022/2023 were on the warmer side (Figure 19) with SSTs in the outer domain above average from December 2022 through May 2023 (Figure 21). By summer 2023, bottom temperatures in Norton Sound were below average (Figure 26).

Fewer metrics of benthic habitat condition are currently available for the NBS, but trends in anemones show low biomass in 2023. Sponges are more variable and biomass was moderate in 2023 (Figure 39). Indirect measures of benthic productivity show continued low biomass of eelpouts in 2023 and continued declining trend in poachers since 2017 (Figure 96).

Overall bottom-up productivity indicators showed mixed signals this year, although none were extreme. In the NBS, sea ice retreat regulates the timing of the spring phytoplankton bloom. In general, earlier ice retreat results in an earlier bloom (Waga et al., 2021), except in the years 2018–2019 where ice retreated so early that open water blooms formed in large areas (Nielsen et al. review). In 2023, chlorophyll-a biomass was among the lowest across sub-regions (Figure 45). In fall, the Rapid Zooplankton Assessment noted that small copepods were ubiquitous across the survey area and increased in abundance from south to north. Hot spots of large copepods and euphausiids were observed around St. Lawrence Island (see p. 93). The abundance of jellyfish, potential competitors for zooplankton prey, also increased over the NBS shelf (Figures 60 and 61, bottom). Taken together these suggest it is reasonable to assume that there were moderate amounts of prey for planktivorous predators.

Yukon and Kuskokwim River salmon runs have experienced precipitous declines in recent years, largely attributed to ecosystem conditions experienced in both the freshwater and marine residency phases (see p. 131), though indicators suggest recent improvement. In 2022, juvenile salmon condition, measured by energy density anomalies, was positive for all species in the NBS (Figure 71). Positive energy stores may contribute to higher overwinter survival (when food is limited) and higher adult returns (see p. 125). In fact, slight increases were observed in juvenile Chinook and chum salmon indices in 2023 (see p. 122).

Groundfish condition, as measured by length-weight residuals collected during the NBS bottom trawl survey, showed mixed trends precluding an overall assessment of groundfish foraging conditions. Adult pollock (>250mm) condition has increased since 2021 to the highest value in the time series (2010, 2017, 2019 and 2021–2023), while juvenile pollock (100–250mm) condition has decreased since 2021. The condition of Pacific cod increased from a time series low in 2021 to average condition in 2023. Yellowfin sole condition has decreased since 2019 to a time series low in 2023 (Figure 80).

On St. Lawrence Island, qualitative observations indicated that seabirds did well in 2023. Auklet numbers, especially crested auklets, were very high and colonies that had been essentially empty the last few years were at levels comparable to 2016. Together, these observations suggest favorable conditions for seabirds in the NBS (see p. 172).

The prevalence of harmful algal blooms (HABs) in marine food webs of the NBS are important indicators of ecosystem health and of potential threats to wildlife and human health. Recent oceanographic changes have made conditions more favorable for HAB species, particularly Alexandrium catenella and diatoms in the genus Pseudo-nitzschia. Dedicated research has documented a consistent trend of higher prevalence of saxitoxin than domoic acid in Arctic food webs as HABs continue to be observed in all regions, including the Bering Strait (see p. 192). This concerning trend will continue to be monitored.