Ecosystem Assessment (pdf)
Auke Bay Laboratories, Alaska Fisheries Science Center, NOAA Fisheries Contact: email@example.com
Last updated: November 2022
During 2022, operational impacts due to COVID-19 had a negligible effect on information used in this report, due in large-part to effective mitigation strategies put in place to protect the health and safety of field research personnel and communities. The Alaska Fisheries Science Center's Ecosystem Status Reports are informed by the continuation of survey- and lab-based data streams, as well as information contributed through new and existing partnerships.
The Recent Warm Stanza
Beginning in approximately 2014, the eastern Bering Sea (EBS) entered a warm phase of unprecedented duration (e.g., Figures 15 and 20). The impact of this sustained warming can be seen in a variety of ecosystem indicators, as described below. Ecosystem response can be immediate (i.e., occurs in the same year as the perturbation), can be lagged (i.e., seen in a subsequent year from the perturbation), or can be cumulative (i.e., carry-over impacts that have positive or negative feedback loops). The past year (fall 2021 through summer 2022) has seen a relaxation to more average thermal conditions. With "reasonably normal conditions" (p. 64) forecast into 2023, and a potential bookend to the recent prolonged warm phase, the ecosystem response and impacts to managed groundfish and crab stocks are assessed below.
Physical environment responses to the recent warm stanza:
Immediate ecosystem responses to warming can be seen in surface and bottom temperatures from the NOAA- AFSC bottom trawl survey (Figure 27), which were above their long-term means beginning in 2014 and largely remained above average through 2022 (bottom temperatures were at the long-term mean in 2017 and 2022). The spatial extent of the cold pool (<2 °C bottom water; Figures 35 and 36) is a direct re ection of sea-ice extent over the eastern Bering Sea shelf the preceding winter. The cold pool extent dropped below the time series average beginning in 2014; years 2018, 2019, and 2021 (no survey in 2020) were the lowest cold pool extents in the time series (see the 2022 Report Card, Figure 2).
Cumulative ecosystem responses are best exemplified through sea-ice dynamics. Throughout the recent warm stanza, residual warmth in the system resulted in delayed sea-ice formation (Figure 29). Delayed freeze-up led to shortened ice seasons that in turn had impacts on ice thickness (Figure 33), ice algae, and thermal modulation of the ecosystem. Thinner sea ice resulted in earlier ice retreat, as it was more susceptible to being eroded by storms, further truncating the ice season (Figure 31) and perpetuating the residual warmth into the following year (i.e., carry-over impacts). The additive effects of residual warmth in the system and loss of sea ice resulted in an increased rate of warming in the northern Bering Sea (see p. 27, Figure 5).
The loss of sea ice over time may also have contributed to an observed increase in salinity at the Pribilof Islands. Community-led monitoring of temperature and salinity on St. Paul Island shows an increasing trend in salinity that corresponds to the recent warm phase (Figure 24). In the Bering Sea, ice growth occurs in the north, which extrudes salts and results in localized increases in salinity. The sea ice is advected south, largely due to winds, and melting occurs at the ice edge, resulting in decreased salinity (i.e., sea ice "conveyor belt"; Pease (1980)). Changes in the salinity structure of the water column can impact the vertical stratification and, ultimately, vertical mixing of primary and secondary productivity. If production is mixed deeper in the water column, for example, a vertical mismatch of prey for surface-foraging seabirds or forage fish may occur. It is worth noting that the salinity observations indicate decreased salinity in 2022, potentially due to sea-ice extent reaching the Pribilof Islands during winter 2021/2022.
Biological responses to the recent warm stanza:
Structural epifauna, such as sea anemones and sponges, provide habitat to benthic-associated organisms and fishes, including rockfishes. Declines in structural epifauna have been observed, both in the NOAA AFSC bottom trawl survey (see p. 66) and as non-target catch reported from groundfish fisheries in the Eastern Bering Sea (EBS (see p. 177), since approximately 2013. While the causes of these declines are not fully understood at this time, the resulting trends may indicate that there are system-wide changes in benthic versus pelagic energy flow (Grebmeier et al., 2006). Satellite measures of chlorophyll-a, an estimate of phytoplankton biomass in the surface level and an indicator of primary production available to the food web, along the shelf break were low in 2022, continuing a decreasing trend since 2014 (see off-shelf region in Figure 40) suggesting potential limitations at the base of the food web.
Zooplankton form the prey base for pelagic stages of groundfish and crab, including important forage fish and age-0 pollock. In spring, small copepods form the prey base for earlier life stages of pollock; by late-summer, age-0 pollock overwinter survival and recruitment success is correlated with the abundance of large, lipid-rich copepods (Eisner et al., 2020) and/or euphausiids (Andrews III et al., 2019). During the recent warm stanza, spring zooplankton surveys documented a distinct increase in small copepods alongside a decrease in large copepods (Figure 48). Late-summer zooplankton surveys over the southern shelf noted no long-term trend in the abundance of small copepods, but a shift to markedly lower abundances of large copepods during the recent warm stanza (Figure 51). These observations suggest that prey conditions in spring have been favorable for early life stages of pollock, but that the availability of large, lipid-rich copepods in fall was low and may have limited age-0 overwinter survival (Heintz et al., 2013).
Concomitantly, increases in forage fish have been observed in biennial surface trawl surveys during this warm stanza (e.g., 2014, 2016, 2018) compared to lower abundances during the preceding cooler stanza (∼2009-2013) (Figures 60, 61, and 66). The combined forage fish index includes age-0 pollock, age-0 Pacific cod, capelin, herring, and juvenile chum, Chinook, coho, pink, and sockeye salmon biomass. The trends in this index are driven by age-0 pollock, particularly during warm years, as age-0 pollock occur closer to the surface during warmer years (Spear and Andrews III, 2021). This trend suggests that the summer foraging conditions were more robust during the recent warm stanza, especially for surface-feeding organisms like piscivorous seabirds.
Bristol Bay adult sockeye salmon returns showed a large increase during the recent warm stanza, with inshore run sizes in 2015-2022 that all exceeded 50 million salmon. The 2022 Bristol Bay sockeye salmon inshore run estimate is the largest on record since 1963 (see p. 109; Figure 70). These large run sizes indicate favorable ocean conditions for juveniles at entry since the summer of 2012-2013 and winters 2012/2013 and 2013/2014. In contrast, declines in NBS juvenile Chinook salmon have been observed since ∼2013 (Figure 67) and adult salmon runs (e.g., Chinook, chum, and coho) throughout the Arctic-Yukon-Kuskokwim region have experienced unprecedented failures in recent years (see Liller (2021) for 2021 Noteworthy and p. 24 for 2022 Noteworthy). These contrasting trends highlight different responses to changing ocean conditions; the dynamic life histories within salmon species are impacted by a myriad of freshwater and marine habitat conditions.
Adult groundfish condition provides indication of prey availability, growth, general health, and habitat condition (Blackwell et al., 2000; Froese, 2006). Below-average condition has been observed in adult pollock since 2015 (except 2019) while juvenile pollock (100-250mm) have experienced above-average condition since 2014 (except 2015) (Figure 75). The 2018 year class of pollock in the EBS is well above-average (Ianelli et al., 2022), likely due to a combination of factors. For example, several indicators of bottom-up drivers of recruitment success support the hypothesis of increased overwinter survival to age-1 in 2019. Specifically, age-0 pollock experienced relatively cool summer sea surface temperatures in 2018 that were followed by warmer spring conditions for age-1 fish in 2019 (see p. 133). Diet composition of age-0 pollock in 2018 revealed a large proportion of euphausiids (Andrews III et al., 2019), supporting the hypothesis that increased euphausiid abundances during warm years may compensate for lower large copepod abundances (Duffy-Anderson et al., 2017). Additionally, the CEATTLE model (see p. 128) has shown continued declines in predation mortality on age-1 pollock due to declines in total predator biomass (i.e., reduced predation and mortality 2019-2021). A reduction in predator biomass is combined with a likely reduction in the spatial overlap between juvenile and adult pollock (Mueter and Litzow, 2008). The reduction in cold pool extent, and subsequent expansion of the adult pollock distribution into the NBS, further released predation pressure on the 2018 year class.
Species guilds derived from samples collected during the standard (southern Bering Sea) bottom trawl survey are grouped by functional roles within the ecosystem, and trends inform dynamics across these roles (e.g., predation pressure, prey availability) (Report Card, Figure 2). While functional guilds provide ecologically relevant information, species-specific trends within a guild may be "masked", such as in the motile epifauna guild. Trends in motile epifauna indicate benthic productivity, and this guild has increased since 2014 and remains above the long-term mean. However, within the guild, the biomass of brittle stars, sea stars, and other echinoderms are well above their long-term means while the biomass for king crabs, tanner crab, and snow crab are all below their long-term means (see also p. 140). Comparing species-specific trends within the broader guild trend can inform niche partitioning within the ecosystem. The pelagic foragers guild, predominantly driven by pollock, decreased from above the long-term mean to below the long-term mean between 2015 and 2021, reflecting the decline in pollock biomass through 2021. This trend reversed in 2022 due to the strong recruitment of the 2018 year class of pollock. The apex predator guild declined from above the long-term mean to within ±1 standard deviation of the mean between 2014 and 2022. Trends in this guild are largely driven by Pacific cod and Arrowtooth flounder; however, trends should be interpreted with caution as individual stock dynamics continue to shift and fluctuate between the southern and northern shelves (e.g., EBS Pacific cod; Barbeaux et al. (2022)).
The recent warm stanza in the eastern Bering Sea has resulted in protracted ecosystem conditions as well as pulse perturbations. The warm stanza was unprecedented in terms of magnitude and duration (Figure 15), but also contained a pulse event of near-absence of sea ice (and subsequent absence of cold pool over the southern Bering Sea shelf) in the winters of 2017/2018 and 2018/2019. The ecosystem indicators contained in this Ecosystem Status Report provide evidence of various ecosystem responses, both immediate and cumulative, that have direct and indirect implications on groundfish and crab stocks in the eastern Bering Sea. Overall, shifts in the distribution of groundfish and crab stocks in response to changes in sea ice and cold pool extent have been documented (see p. 155, Thorson et al. (2019)). There are several examples of stocks that are "winners" and "losers" (Stabeno et al., 2012) in the EBS ecosystem, although the exact mechanisms may not be fully understood at this time. Stocks experiencing increased survival for recent year classes 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. 102), and the last 8 years of Bristol Bay sockeye salmon returns (year classes precede returns by 3-5 years; see p. 109). Conversely, stocks experiencing reduced survival and stock declines include several crab stocks (notably snow crab and Bristol Bay red king crab; Figure 92) and multiple Western Alaska Chinook, chum, and coho salmon runs. With cooler conditions predicted into 2023 (i.e., sea-ice extent and bottom temperatures near historical averages, see p. 64)(Figure 37), the ecological responses to this recent prolonged warm phase will continue to come into sharper focus.
Current Conditions: 2022
Observations over the last year (September 2021-August 2022) indicate that the extended warm phase experienced by the EBS has ended, with a variety of metrics showing the relaxation to average thermal conditions. The combined states of three climate indices (positive North Pacific Index and Arctic Oscillation, and continued La Nina; see p. 33 and Figure 7) meant a return to more average sea surface temperature conditions for the EBS shelf. During this past year, marine heatwaves have been infrequent and brief compared to recent years (Figure 19). Rapid sea-ice growth in November 2021 resulted in above-average early ice extent (highest since 2012; Figure 29) that was followed by dramatic ice loss in April due to thin ice (Figure 33) that retreated quickly (Figure 31) with relatively warm air temperatures. The area of the 2022 cold pool (<2° C bottom water) expanded and was near the time series average, representing a major change from the three prior survey years (2018, 2019, 2021) (Figure 2).
Ocean acidification (OA) research shows an expansion of bottom water conditions (Ωarag and pH; Figure 108) that have been experimentally shown to negatively impact pteropods and red king crab. In 2022, relatively lower pH was predicted for most of the outer and middle shelves and near Bering Strait. However, at this time, there is no evidence that OA can be linked to recent declines in surveyed snow crab and red king crab populations (see p. 164).
Lastly, a localized pulse disturbance event heavily impacted the western Alaska region when Typhoon Merbok hit on September 17, 2022. The storm's timing (i.e., early in the fall for a storm of this strength) and intensity were fueled by warm ocean waters from the north-central Pacific to the northern Bering Sea (developing warmth can be seen in Figure 16d). Immediate impacts of the storm included damage to infrastructure (e.g., seawalls) and disruption of the fall subsistence harvest season3. Longer-term impacts due to storm surges and coastal and river flooding are not yet known, but may include disturbance of HAB cyst beds or salmon eggs. We anticipate these longer-term impacts to be identified over time.
Overall productivity within the EBS ecosystem shows immediate (e.g., primary production) and potentially lagged (e.g., higher trophic level) responses to the return to more average thermal conditions in 2022. Primary production, as measured by chlorophyll-a concentration, varied spatially over the shelf in 2022 and estimates of the spring bloom peak timing suggest that 2022 was similar to the long-term average (see p. 68). The direct mechanisms linking primary production to groundfish and crab stock dynamics are not fully understood, though continued research into the relationship between eddies, chlorophyll-a blooms, and spatial hotspots is on-going and future contributions to this Ecosystem Status Report are anticipated. The 2022 coccolithophore bloom index for both the south inner and middle shelf was among the highest ever observed (p. 73). The milky aquamarine color of the water during a bloom can reduce the foraging success for visual predators, such as seabirds, though monitored seabird species at the Pribilof Islands had an exceptional year in terms of reproductive success (except thick-billed murres) (Figures 93 and 94) suggesting that their foraging conditions were not limited by the coccolithophore bloom.
Zooplankton community composition was observed on three surveys in the Bering Sea during 2022: (1) spring along the 70-m isobath, (2) late summer over the southern shelf, and (3) late summer over the northeastern shelf (see p. 79). During spring 2022, the zooplankton composition appeared similar to previous warm years (i.e., relatively lower abundance of large copepods, higher abundance of small copepods). By late summer over the southern shelf, both large and small copepods were in low abundance; this decrease in available prey base over the southern shelf may have been mitigated by an increased abundance of euphausiids sampled during the same survey. Euphausiids were also sampled in higher abundances over the northeastern shelf, suggesting widespread abundance over the Bering Sea shelf.
Forage fish represent a critical trophic linkage in the ecosystem and are prey for larger fish, seabirds, and marine mammals. An aggregate forage fish index derived from surface trawl surveys in the southern and northern Bering Sea indicates lower availability of forage in surface waters, especially in the northern Bering Sea during 2022 (Figure 60). Patterns in seabird reproductive success track patterns in prey availability, with planktivorous seabirds doing well at the Pribilof Islands and on St. Lawrence Island. While piscivorous seabirds did well at the Pribilof Islands, reproductive failures were observed for piscivorous seabirds on St. Lawrence Island (see p. 142), in line with the low forage fish availability suggested by the aggregate index.
Adult fish condition reflects prey availability and growth potential, both impacted by climate-driven changes in metabolic demand (higher in warmer conditions) and trophic interactions (changes in prey quality and quantity). Through 2021, bioenergetic work indicates declining conditions for groundfish during the recent warm stanza (see p. 123) that is reflected in groundfish condition (Figure 75). The cooler sea surface temperatures and bottom temperatures that began in 2021 (though still above average), and the relaxation to average thermal conditions in 2022, would be expected to coincide with better groundfish condition. In fact, groundfish condition improved from 2021 to 2022 for all monitored species over the southern shelf, except adult pollock that remained comparable to 2021 (Figure 75), while groundfish condition trends were more variable for monitored species over the northern shelf (Figure 78).
The groundfish community shifted northward during the recent warm stanza and remained near its northern maximum through 2019 before shifting south again in 2021 as conditions cooled (see p. 155). The mean latitude did not change between 2021 and 2022, but the groundfish community on average shifted into slightly deeper waters (Figure 101). The bottom trawl survey catch-per-unit-effort (CPUE; kg/ha) between the southern and northern shelves (Figure 98) also demonstrates a northward shift in the groundfish community. The drop in CPUE in the northern Bering Sea between 2019 and 2021 reflected large decreases in all of the dominant species (Figure 99) that had moved northward with the lack of cold pool in 2018/2019. The drop in CPUE in the NBS may indicate migration out of the survey area or that the carrying capacity of the system was exceeded during the exceptionally warm years.
The salmon run failures in the Arctic-Yukon-Kuskokwim Region (see Liller (2021)) continued in 2022, neg- atively impacting the human communities of the region. In response to the Scientific and Statistical Committee's request (see p. 217), and in collaboration with tribal, state, federal, and NGO partners, the factors affecting the 2022 Western Alaska Chinook salmon runs and subsistence harvest are explored in a Noteworthy topic (see p. 24).