Aleutian Island Assessment - 2023

Ecosystem Assessment (pdf)

Ivonne Ortiz1 and Stephani Zador2

1Cooperative Institute for Climate, Ocean and Ecosystem Studies, University of Washington
2Resource Ecology and Fisheries Management Division, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA


Last updated: October 2023

The Aleutian Islands ecoregions

The Aleutian Islands ecosystem assessment and Report Card are presented by three ecoregions. The ecoregions were defined based upon evidence of significant ecosystem distinction from the adjacent ecoregions by a team of ecosystem experts in 2011. The team also concluded that developing an assessment of the ecosystem at this regional level would emphasize the variability inherent in this large area, which stretches 1900 km from the Alaska Peninsula in the east to the Commander Islands in the west. For the purposes of this assessment, however, the western boundary is considered the U.S.–Russia maritime boundary at 170° E.


Figure 5: The three Aleutian Islands assessment ecoregions.

The three Aleutian Islands ecoregions are defined from west to east as follows (Figure 5). The western Aleutian Islands ecoregion spans 170° to 177° E. These are the same boundaries as the North Pacific Fishery Council fishery management area 543. This ecoregion was considered to be distinct from the neighboring region to the east by primarily northward flow of the Alaska Stream through wide and deep passes (Ladd, pers. comm.), with fewer islands relative to the other ecoregions.

The central Aleutian Islands ecoregion spans 177° E to 170° W. This area encompasses the North Pacific Fishery Council fishery management areas 542 and 541. There was consensus among the team that the eastern boundary of this ecoregion occurs at Samalga Pass, which is at 169.5° W, but for easier translation to fishery management area, it was agreed that 170° W was a close approximation. The geometry of the passes between islands differs to the east and west of Samalga Pass (at least until Amchitka Pass). In the central ecoregion the passes are wide, deep and short. The Alaska Stream, a shelf-break current, is the predominant source of water (Figure 6). There is more vertical mixing as well as bidirectional flow in the passes. This delineation also aligns with studies suggesting there is a biological boundary at this point based on differences in chlorophyll, zooplankton, fish, seabirds, and marine mammals (Hunt and Stabeno, 2005).


Figure 6: Ocean water circulation in the Aleutians. Currents are indicated with black lines. Currents are indicated by grey arrows. Selected passes are indicated by straight light grey lines

The eastern Aleutian Islands ecoregion spans 170° W to False Pass at 164° W. The passes in this ecoregion are characteristically narrow, shallow and long, with lateral mixing of water and northward flow. The prominent source is from the Alaska Coastal Current, with a strong freshwater component. This area encompasses the NPFMC fishery management areas 518, 517 (EBS) and the western half of 610 (GOA).

Aleutian Islands Ecosystem Assessment

As the Aleutian Islands are situated between the Bering Sea and the Pacific Ocean, they are influenced by different ocean currents, eddies, and geographic constraints. Given these challenges, there are large gaps in knowledge about the local physical processes. The use of technology to address these challenges can be limited. For example, persistent cloudiness can preclude obtaining comprehensive satellite-derived data, and strong currents preclude the use of various unmanned underwater vehicles. The long distances involved in surveying the island chain make comparing west–east trends in indicators difficult due to time lags during oceanographic surveys across the region. The archipelago is also influenced by different processes in the eastern than in the western Aleutian Islands. Differ- ences in survey timing and longitudinal gradients may also affect detection of biological patterns, as gradients are seldom monotonic in any direction. Integrative biological indicators such as fish or marine mammal abundances may be responding to physical indicators such as temperature, but are less sensitive to timing of when they are surveyed compared with direct measurements of temperature. Also, the extensive nearshore component of the ecosystem is a long, narrow shelf relative to the entire ecosystem, and strong oceanographic inputs mean that some metrics commonly used as ecosystem indicators in other systems may not be as informative in the Aleutian Islands. Therefore, our synthesis of ecosystem indicators by necessity includes speculation

The Alaska Fisheries Science Center last completed its standard bottom trawl survey in 2022. While the bottom trawl survey is typically done biennially in even-numbered years, there was no survey in 2020 due to COVID-19, so the 2022 survey was the first in four years.

Current Conditions 2023

The past year in the NE Pacific, as a whole, was characterized by continuing moderate La Niña conditions and a continued negative PDO phase that followed the marine heat wave years of 2014–16. El Niño conditions devel- oped along the equator over the summer, and El Niño is predicted to continue through the Northern Hemisphere spring (with an 62% chance during April–June 2024)3. Conditions in the Aleutian Islands were relatively stormy during the winter of 2022–23 and summer 2023. The year started with the warmest winter on record since 1900 based on long-term sea surface temperatures. Satellite sea surface temperatures across the Aleutian Islands show waters were in or near moderate marine heat wave status last winter. Temperatures cooled but remained above average during spring and early summer, periodically crossing the threshold to marine heatwave status. Persistent heatwave conditions returned in late summer. Both summer and winter temperature trends in the Aleutian Islands have been increasing since 2013–2014, concurring with a 2013–2014 annual mean sea surface temperature regime shift in the North Pacific recently identified by Xiao and Ren (2023). Mid-depth temperatures were above average in 2023, as they have been since 2014. Winds suppressed northward flow through Unimak Pass, and eddy kinetic energy was near or below its long-term average (1993–2022) across the chain. Together these indicators suggest that there were decreased fluxes of heat and nutrients from deeper water and through the Aleutian Island passes. The cooler conditions during late spring early summer of 2023 were accompanied by greater upper mixed layer depths than during 2022, which may have impacted the vertical distribution and availability of prey throughout the water column.

Seabirds at Buldir Island in the western Aleutians had mixed reproductive success, in contrast to the exceptionally- successful previous year. This suggests that there was not enough available prey to support successful reproduction for some seabird species in 2023. Tufted puffins that dive to capture their fish and squid prey had above average reproductive success. In contrast, fork-tailed storm petrels and kittiwakes that forage at the surface for fish and invertebrates, and thick billed murres and parakeet auklets that dive for and zooplankton respectively, had below average or complete reproductive failure. This contrasting pattern suggests that there was less available prey throughout the water column in general but perhaps more available prey below the surface and at depth. This summer continues a successful trend at Aiktak, where reproductive success has been average or above av- erage since 2019 (there were no surveys in 2020) for most seabirds breeding at Aiktak. This indicates uniformly high prey availability for both nearshore and offshore foragers, including surface feeders and divers and across a broad spectrum of zooplankton to forage fish prey (Figure 34). However, the report of a die-off in August of over 150 shearwaters on Akutan indicates the presence of a potential stressor(s) in the region (more details below).

Eastern Kamchatka pink salmon abundance in 2023 was the third highest on record. Exceptionally high numbers of pink salmon suggest that there could be top-down pressure on their zooplankton prey and competitive impacts on species that forage on the same prey. Several papers report that the strong pink salmon biennial pattern of abundance seems to be causing trophic cascades, showing impacts on fish growth (Atka mackerel, Matta et al. 2020 ) and food availability for seabirds (Zador et al., 2013; Batten et al., 2018; Springer et al., 2018). High pink salmon abundance in 2023 poses a particular concern for Pacific ocean perch, Atka mackerel, and tufted puffins (Springer and van Vliet, 2014; Batten et al., 2018; Springer et al., 2018), for which some aspect of their biology has shown to be negatively correlated to pink salmon abundance. The notable steep increases in Eastern Kamchatka pink salmon abundance trends in 2009, for the more abundant odd-year classes, and in 2014 and 2016, for the less abundant even-year classes, seem to have made biennial signals potentially driven by pink salmon abundance more noticeable across ecosystem components.

Lastly, paralytic shellfish toxins increased significantly this year compared to 2022, posing a continued seasonal concern for human health and food webs in the region. In 2023, maximum concentrations of 3,793 µg per 100 g were recorded in blue mussels. This is 47x (vs 3.4x in 2022) above the regulatory limit (80 µg per 100 g, FDA4). Concentrations above 1000 µg per 100 g are considered potentially fatal for humans. The level of toxins observed this year is still substantially lower than the toxins observed at 75x the limit in Unalaska during 2021. More details on this year’s trends are in the regional highlights section below.


Multi-year patterns

Overall, there seem to be three major drivers of the multi-year patterns observed across the Aleutian Islands: persistent warm conditions, increasing pink salmon abundance, and increasing Pacific Ocean perch abundance. Jointly, these factors might indicate a transition of the ecosystem to a new state. The likelihood and detection of such a transition may depend on how long the current conditions prevail. As one indication of an ecosystem transition, preliminary analysis of Pacific cod stomachs in 2023 shows their diet has shifted from a majority of fish to diets dominated by invertebrates, with decreased average amount of prey consumed (as percent of predator weight). The change in diets is observed across medium (30 cm) to large (85 cm) Pacific cod (see Noteworthy, page 24). Further analysis of the food web to see if predator/prey relationships have changed over time would help to determine if these changes reflect a broader transition in the ecosystem. This would require both more diet sample collections (from fish, birds, and mammals) and analysis to inform underlying changes in the structure of the ecosystem. The data-poor nature of Aleutian Islands relative to the eastern Bering Sea or Gulf of Alaska, limits the ability to identify the extent of cascading or cumulative effects of these drivers. Additionally, as the bottom trawl surveys, from which stomach content and other data are collected, occur only in even-numbered years, potential ecosystem impacts of superabundant pink salmon in odd-numbered years is not observed in data collected during those surveys.

Persistent warm conditions since 2013 : As mentioned above, Xiao and Ren (2023) identified a 2013-2014 annual mean sea surface temperature regime shift across the North Pacific. This agrees with temperature data observed in the Aleutian Islands, where surface to mid-depth and bottom waters have remained above the long term mean since 2013/14. The warm temperatures can be attributed in part to slower processes such as weaker wind/mixing, warmer air temperature, and advection of warm water from the North Pacific Ocean. However, the relative im- portance of each mechanism on maintaining warm temperatures is hard to assess without a detailed heat budget analysis. Other aspects of the physical environment that influence nutrient availability continue to show variability over the same time period. For example, eddy kinetic energy (EKE) averaged in regions along the Aleutian Islands with historically high long-term mean EKE was at or below average in 2023, with EKE in the eastern box reaching the historical minima (Figure 26). This suggests that there were decreased fluxes of heat and nutrients across the island chain, which has been the case in recent years except for 2021-2022. Phytoplankton biomass, as represented by satellite-derived chl-a concentration, was below average in the AI for much of spring 2023, with overall mean concentrations comparable to the previous two lowest years in the observed time series, 2016 and 2018. At present (9/12/23) there is evidence for a negative trend in spring AI chl-a across the GlobColour time series. These trends are being driven by the Alaska Stream and southeastern Aleutian Basin areas, while areas to the west of Bowers

Ridge, or south of ˜ 50° N, do not show trends (Figure 28). This coincides with the decrease in satellite-derived chl-a observed in the off-shelf region of the eastern Bering Sea (Hennon et al, EBS ESR 2023), suggesting a widespread area with little primary production. Negative zooplankton biomass anomalies and the smaller mean size of the copepod community also suggest low secondary (zooplankton) production (Figure 31). Cumulatively, these conditions suggest that there has been lower productivity across the ecosystem, concomitant with increased bioenergetic needs for fish, faster growth rates for zooplankton and larvae, and shorter incubation periods for eggs due to the warm conditions. These changes in bioenergetics and development rates can potentially lead to mismatches between egg hatching/larvae release and prey availability, which can negatively affect recruitment.

Also of note is that the beginning of the period of lower fish condition as reported in the 2022 ESR and lower satellite-derived chl-a estimates seem to coincide with the notable increases of Eastern Kamchatka pink salmon in 2009 and 2014/16 for odd and even year-classes, respectively.

Eastern Kamchatka pink salmon abundance in odd years continues to increase: The biennial pattern of high pink salmon abundance in odd years and low abundance in even years continues. As mentioned above, this year was the third-highest abundance on record since 1952 (Figure 32). However, since 2009, high abundances of odd year-classes have doubled and even tripled (315 million adult fish) compared to prior levels of around 100 million fish. Low abundance [even] years reached the 100 million fish mark in 2016 and 2018 (perhaps related to higher temperatures mentioned above) but also had a notable increase in 2014 from previous years (48 million compared 24 million in 2012 and 14 million and 10 million in 2010 and 2008 respectively). In 2020, pink salmon abundance decreased to pre-2014 levels, perhaps due to low availability of prey that was suggested by anomalously low numbers of large meso-zooplankton sampled in the Continuous Plankton Recorder. The following indicators, tracked in past ESRs, also show a biennial pattern: satellite chl-a (lower in even years, Figure 28), hatch timing of tufted puffins (earlier in odd years, Figure 35), catch estimates of age-2 Atka mackerel (Lowe et al., 2022), number of age 3+ Pacific Ocean perch (Spencer et al., 2022), and the bycatch of all seabirds combined (increases in years of high pink salmon abundance and decreases during low pink salmon abundance, not available this year). That the timing of some of these biennial patterns coincides with the step changes in Eastern Kamchatka pink salmon abundance suggests that a threshold has been reached where potential ecosystem impacts increase. Interestingly, the biennial pattern seen in the age-2 Atka mackerel catch has not been observed in recruitment estimates or surveys. The biennial pattern in tufted puffin hatch timing indicates they find more favorable conditions in winter/spring when Eastern Kamchatka pink salmon abundance is low. However the abundance of pink salmon does not correlate with their reproductive success during summer. Further evaluation is needed to fully assess the influence of Eastern Kamchatka pink salmon in the system.

Rockfish have replaced Atka mackerel and pollock as the main pelagic foragers: The increase of rockfish across the Aleutian Islands has slowly changed the ratio of Atka mackerel and pollock to northern rockfish and Pacific Ocean perch, with rockfish now contributing a higher percent of the local biomass across the archipelago to pelagic foragers. Stock assessment estimates support rockfish remaining dominant, although their biomass is decreasing. An ecosystem state where sustained high biomass of Pacific Ocean perch and northern rockfish is outcompeting or displacing pollock and Atka mackerel would signal a return to conditions that existed before Pacific Ocean perch was heavily fished by the foreign fleets. The effect of rockfish dominance on the ecosystem is best captured by the mean lifespan of the groundfish community, a proxy for the mean turnover rate of species (AI ESR 2022). Mean lifespan has increased from 35 years in the 1980s to 60 years in 2018-22. Longer-lived species help to dampen the effects of environmental variability, and in ecological terms, increase the stability of the ecosystem (AI ESR 2022). The persistent poor groundfish condition suggests that Pacific Ocean perch and northern rockfish could potentially be experiencing density dependence or some other mechanism that leads to less optimal foraging conditions. Also, rockfish prefer habitats with vertical structure (Rooper 2019), particularly deep coral and sponges, and may be exerting spatial pressure on other fish in this habitat. This in turn might lower the availability of Atka mackerel and pollock to other predators such as Pacific cod, whose diet shows a general decreasing trend in the amounts of Atka mackerel consumed in NMFS areas 543 and 542. It is unclear whether this change in pelagic foragers (Figure 48) has contributed to the observed decline of harbor seals (AI ESR 2021) and/or Steller sea lions. Together with eastern Kamchatka pink salmon, Pacific ocean perch and northern rockfish consume a large portion of zooplankton previously consumed by Atka mackerel, walleye pollock and other forage fish.

Western Aleutians

Sustained high sea surface temperatures during winter resulted in a moderate marine heat wave in the western Aleutians (Figure 19). Temperatures cooled in later spring and early summer, but remained above the long-term (Dec 1985-Nov 2015) average. Heatwave conditions returned in late summer and have prevailed through early November5. The heatwave could have potential negative impacts on Atka mackerel as they move to shallower areas during the spawning season, when surface temperatures were close to 11–11.5° C, which is the upper limit of the observed temperatures during Atka mackerel spawning. These higher temperatures may shorten egg development time and lead to faster growth rates in larvae. Atka mackerel nests are typically found between 32—144 m depth (Lauth et al., 2008) potentially making the shallowest nests more vulnerable to the heatwave. Bottom temperatures averaged 4.4° C in 2022, which is well below the lethal temperature of 15° C for Atka mackerel eggs (Gorbunova, 1962). Eddy kinetic energy was below average, suggesting low fluxes of nutrients, heat and salt through the passes in the Western Aleutians (Figure 26). Satellite-derived chl-a concentration was below average throughout spring, and improved somewhat in fall (Figures 28, 29).

The persistent decline in fish condition as represented by weight/length residuals from fish sampled during the bottom trawl survey may be linked to the change in prey, as suggested by a preliminary analysis of cod stomachs from the region (p. 24). Atka mackerel were almost absent from cod stomachs collected in recent years, whereas the proportion of squid and shrimp in the stomach increased over the same time period. The transition to a higher proportion of invertebrates in their diet seems to coincide with the temperature regime shift/ higher temperatures in 2013–2014, indicating that there may have been a change of the structure of the ecosystem in this region. The decline in fish condition may also be indicative of several interacting factors, including poor prey quality, low availability of prey, density dependence, and increased metabolic rate (Holsman et al., in press). Based on biomass estimates from the 2022 bottom trawl survey compared to 2018 estimates, apex predator abundance increased 3% overall. This increase was driven by Pacific cod (20%), rougheye/ blackspotted rockfish (84%) and large sculpins (2%), while all large flatfish decreased. The below average fish condition of Pacific cod and arrowtooth flounder suggests that they experienced either poor prey quality and/or low availability of prey. In contrast, the overall biomass estimate of pelagic foragers increased 35%. This increase was driven by Pacific ocean perch and Atka mackerel (33% and 58%), while pollock decreased 63%. The fish condition for all three was below average, and while this would suggest low quality and/or availability of prey, in the case of Pacific ocean perch it may also be due to density dependence, given its increasing biomass trend.

Long-term average hatch dates for fish-eating seabirds are between mid-June to late July (Dragoo et al., 2019), along with average hatching periods of 30 to 42 days. In general, hatch dates in the western Aleutians in 2023 were average or later than the 1993–2022 average. This suggests that there were poor foraging conditions during winter and early spring, which may also reflect potentially poor foraging conditions for commercial groundfish. Reproductive success was also mixed in the region, with puffins having average or above average success while all other species saw a decline in their reproductive success. Fork-tailed storm petrels and kittiwakes (both surface feeders on invertebrates and fish) had below average success or complete reproductive failure (red-legged kittiwakes). Tufted puffin chick diets at Buldir were mainly composed of squids 63% (up from 35% in 2022) and 18% Pacific saury, while horned puffin chick diets there were primarily composed of Atka mackerel (43%). The dominance of species in puffin chick diets concurs with stable or increasing biomass of these species based on bottom trawl survey data. The increase of rockfish in seabird diets observed in 2021 was not observed in 2022 or this year, as Sebastes spp. was only 2% of the chicks’ diets. It will be interesting to see if the increase in age-0 rockfish in chick diets in 2021 lines up with future estimates of 2021 rockfish age-classes. We use reproductive success of zooplankton-eating seabirds (auklets) as indicators of zooplankton production. Their reproductive success was above average from 2019–2022, but this year declined to average. These species feed their chicks mainly euphausiids and copepods. Their average hatch dates and reproductive success this year suggest that there were sufficient foraging conditions from early spring through summer.


Central Aleutians

Similar to the Western Aleutians, the central Aleutians experienced a moderate marine heatwave in winter, warm temperatures during spring, and a return to heatwave conditions in late summer that have prevailed through early November. Mid-depth temperatures in this region have not been as warm as those in the eastern Aleutians (see 2022 ESR). Eddy kinetic energy in this region is usually lower in magnitude compared to those in the western and eastern Aleutians. Eddy events in this area are characterized either by multiyear or continuous eddies of low intensity. In 2023, eddy kinetic energy was generally below the 1993–2022 average, indicating potentially below-average flux of nutrients and heat across the passes. Phytoplankton biomass, as represented by chl-a concentration, was also generally below average except for an increase primarily in August of 2022 (Figures 28 and 29).

Pacific cod stomachs sampled in this region (NMFS areas 541 and 542) show a strong decrease in the proportion of fish in their diet over time and an increase in shrimp, squid, and other invertebrates. In some years, Atka mackerel are more common in diets (p. 24). Further analysis is needed to see if these periodic increases coincide with local Atka mackerel recruitment trends. Fish prey types also change along a west to east gradient in this region with Atka mackerel more common in the west and a higher proportion of shelf demersal fish to the east.

We report on school enrollment as an indication of trends in coastal, rural community populations. There are currently 2 active public schools in the Central region. School enrollment bottomed out at the state level in Alaska during 2020–2021 and decreased even further during the 2022–2023 school year. Barring renewed activity by the now-closed fish processing plant in Adak and the lost potential to be a hub for clean energy (fuel) along the great circle route, the future stability of the Adak community and school is uncertain.

Eastern Aleutians

This region encompasses the islands east of Samalga Pass to Unimak Pass. As in 2022, sea surface temperatures in the eastern Aleutians during 2023 were not as high during winter as they were in the western and central Aleutians. The marine heat wave periods were also shorter, with those in spring and early summer largely reduced, and with almost no heatwaves as of early November. However, temperatures were still above the 1985–2015 baseline in the region. Mid-water temperature profiles for 2023 show an increased warm band of water between 150–300 m with cooler temperatures above and below (Figure 24). The predominant wind pattern in 2023 suppressed flows through Unimak Pass. Eddy kinetic energy, which is typically driven by a strong pulse eddy in this area, was significantly lower this year similar to the generally low value that has largely been observed since 2012 (Figure 26) wiht the exception of 2021-2022. The spatial structure of chl-a anomalies suggest almost uniformly low chlorophyll across the region during April-June 2023, with the exception of some small positive anomalies in the Alaska Stream east of 187° E, and in some areas further south (Figure 29).

Fish-eating seabirds, such as murres, puffins and gulls, all had above average reproductive success as in the past couple years. No auklets (primarily zooplankton-eaters) were surveyed in the region. Storm-petrels, which feed on a mix of invertebrates and zooplankton, have had average above average reproductive success since 2022. Fork-tailed storm-petrels had average hatch dates but above average reproductive success. Leach’s storm-petrels had both average hatch dates and average reproductive success (Figures 36, 34). These indicators suggest good availability of forage fish to rear chicks and potentially for fish-eating groundfish. Storm petrels and murrelets, which feed on fish, invertebrates and zooplankton, had average or above average reproductive success. While it is unclear whether the conditions were as favorable for obligate zooplankton-eating seabirds as for fish-eating seabirds, the overall reproductive success suggests there was sufficient prey. Tufted puffins chick diets were fed primarily capelin (87%) followed by pollock (6%), indicating that high-quality forage fish were available to foraging seabirds.

In late August an opportunistic report of a die-off of over 150 shearwaters was reported at Akutan Island. This is particularly notable as die-offs have been rarely reported in the Aleutian Islands, likely in part due to the remote and unobserved nature of most of the coastline. Die-offs such as this one have been linked to poor food supply and/or disease. In recent years, the global spread of the Highly-Pathogenic Avian Influenza (HPIA) has been of great concern for seabirds, particularly those that nest in dense colonies where an introduction of HPIA could spread quickly due among densely-aggregated seabirds. In the past year, HPIA has been increasingly detected in seabird colonies worldwide. Six carcasses from this die-off were sent to the Alaska Department of Environmental Conservation where samples tested negative for HPAI; some carcasses are also planned to be tested for HABS toxins. It is reasonable to assume that HPIA will eventually be detected in seabirds in the Aleutian Islands, particularly as samples from the southeastern Bering Sea tested positive in August this year. Human infections with bird flu viruses are rare and most often occur after close or lengthy unprotected contact, but global efforts continue to monitor HPAI.

Pacific cod diets have been alternately dominated by fish and invertebrates in this region (the South Bering Sea bottom trawl area) (Figure 12). There does not appear to be a temporal transition from fish to invertebrates in this area as there are in the western and central Aleutians. The most common fish prey is pollock, but it is not dominant in all years. Shrimp are the most common invertebrate seen, but cod diets as a whole have a more diverse array of invertebrates relative to cod diets in the other regions.

Shellfish samples from several locations including Little Priest Rock in Summer Bay, Unalaska are collected weekly and analyzed for harmful algal blooms. Monitoring indicated that peak toxin levels occurred during June this year. Blue mussels had toxins of 47x (vs 3.4x in 2022) above the regulatory limit of 80 µg per 100 g, (FDA6, Figure 41), which is still below the maximum of 75x documented in 2020. Public awareness efforts continue in the area to minimize impacts on human health.

Lastly, school enrollment in this region declined in 2020–21 and has not recovered. The decrease in the eastern Aleutians enrollment was driven by a large decline at Unalaska Elementary, but enrollment has also been decreasing for high school grades. All other schools (Akutan, False Pass) had similar enrollment as last year.