Aleutian Islands Ecosystem Assessment (pdf)
Ivonne Ortiz 1 and Stephani Zador 2
Last updated: October 2020
The primary intent of this assessment is to summarize and synthesize historical climate and fishing effects on the shelf and slope regions of the Aleutian Islands (AI) from an ecosystem perspective, and to provide an assessment of the possible future effects of changes in the climate and fishing on ecosystem structure and function. The Ecosystem Considerations section of the Groundfish Stock Assessment and Fishery Evaluation (SAFE) report provides the historical perspective of status and trends of ecosystem components and ecosystem-level attributes using an indicator approach. For the purposes of management, this information must be synthesized to provide a coherent view of ecosystems effects in order to clearly recommend precau- tionary thresholds, if any, required to protect ecosystem integrity. The eventual goal of the synthesis is to provide succinct indicators of current ecosystem conditions. In order to perform this synthesis, a blend of data analysis and modeling is required annually to assess current ecosystem states within the context of its history, as well as past and future climate.
The Aleutian Islands ecosystem assessment area
The Aleutian Islands ecosystem assessment and Report Card are presented by three ecoregions. The ecore- gions 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 border at 170 °E.
The three Aleutian Islands ecoregions are defined from west to east as follows (Figure 8). 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 oW, but for easier translation to fishery management area, it was agreed that 170 oW 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 7). 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).
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).
Figure 6: The three Aleutian Islands assessment ecoregions.
Figure 7: Ocean water circulation in the Aleutians. Currents are indicated with black lines. Passes are indicated with white lines. Image from Carol Ladd.
This year, due to the COVID-19 pandemic, most surveys and fieldwork were cancelled, so there are no biological indicators updated for 2020. The new information in this assessment is largely from remote- sensing, updated analysis of 2019 data, and local observations. Whenever possible we included data for 2019 as an update from the previous 2018 Aleutian Islands Ecosystem Status Report. Cancelled surveys and data streams include:
Most of what we can say about the Aleutians Islands ecosystem is based upon biological trends. There are large gaps in knowledge about the local physical processes and, as a result, their impact on biological processes. These gaps are largely due to geographic reality. For example, persistent cloudiness and strong currents preclude obtaining comprehensive satellite-derived data and the use of various unmanned underwater vehicles. In addition to the sheer distances involved in surveying the island chain that make comparing west- east trends in indicators such as bottom temperature difficult due to difference in timing of oceanographic surveys across the region, the archipelago is also influenced by different processes in the eastern than in the western Aleutians. Differences 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 sea lion abundances may be responding to physical indicators such as bottom 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, narrow shelf relative to the entire ecosystem, and strong oceanographic input mean that some metrics commonly used as ecosystem indicators in other systems may not be as informative in the Aleutians. Therefore, our synthesis of ecosystem indicators by necessity includes speculation.
During 2019-2020, the state of the North Pacific atmosphere-ocean system featured the continuance of warm sea surface temperature anomalies in the Gulf of Alaska with an almost year-long marine heat wave in 2019 that decreased significantly towards the west, with subsurface warmer temperatures throughout the chain that reached the western Aleutians. Bottom trawl survey temperatures from 2019 support model results from the Global Ocean Data Assimilation System that show the persistence of subsurface warmer temperatures in the 100-250 m deep layer that have stayed statistically above the long-term mean. The warm temperatures can be attributed in part to slower at-depth processes. In 2020, the surface temperatures cooled, and climate indices were near average, potentially offering more favorable environmental conditions for biota relative to recent years.
Newly estimated indices show eddies have a distinctly different signature across the island chain, with discrete, strong events characterizing the east and multiple or multi-year but less intense events towards the west. The role of these eddies and how they are processed within the system are yet to be understood, as stocks and overall populations are subject to the dynamics in the east and the west throughout their life cycle. Eddy kinetic energy has remained low since 2013 in the east, and this coincides with the North Pacific Gyre Oscillation more than with the North Pacific Index, which is typically the more characteristic index of the region. Model results suggest moderate increases in the strength of the Alaskan Stream Current increases flow through the eastern passes such as Amukta, while stronger flows carry the current westward, decreasing flows through the eastern passes and increasing them through the wider and deeper passes prevalent in the central and western Aleutians.
With average or close to average climate conditions throughout, 2020 is expected to be a return to more favorable conditions for the biological components of the Aleutian Islands ecosystem.
Biological summary through 2019
In general, warmer temperatures increase bioenergetic costs for ectothermic fish, and all else being equal, prey consumption must increase to maintain fish condition. These increased bioenergetic costs and consumption demands may partly explain why the observed body condition of several commercial groundfish (adult pollock, Pacific cod, northern rockfish and Pacific ocean perch) has been lower than the survey mean since 2012, as last measured by length-weight residuals during the biennial summer bottom trawl survey during 2018. We note however, that for Pacific Ocean perch and northern rockfish, intraspecific competition might be a contributing factor, as their abundance increased and appears to have now stabilized at high biomasses (e.g. Pacific Ocean perch) that now surpass that of Atka mackerel and pollock combined. While Pacific Ocean perch condition has also been lower than the long term mean, it has decreased less than that of the rockfish. The poorer condition of fish, particularly of species such as Atka mackerel and pollock that when small serve as prey for piscivorous seabirds and apex fish predators like Pacific cod and arrowtooth flounder, also means that that their quality as prey has decreased, with potential cascading effects on their predators.
Figure 8: Compared indicators before and after 2012, from top to bottom left side: NPGO, summer SST by AI region, CPR, EKE (top WAI, middle CAI, bottom, EAI), right side, Fish Condition.
Warmer temperatures may also impact ontogenesis of Atka mackerel eggs (Lauth et al., 2007). Surface tem- perature was found to be the most important determinant of egg and larval stage distribution of commercial fish in Alaska based on the distribution models used to define EFH. For many of the commercial groundfish for which the youngest age in the stock assessment is 4 years old or older, effects of this sustained warmer temperature on recruitment will not be immediately apparent.
These generally unfavorable conditions seem to be improving, as seabirds - both plankton and fish-eating species - had earlier to average hatch dates and average to above-average reproductive success in 2019. This seems particularly true for surface-feeding seabirds which have been shown to respond more consistently with changes in their phenology as warmer temperatures bring earlier spring blooms. This flexibilty and higher response to fluctuations in the environment is also coherent with the lower response to variable environmental conditions that is observed in fish and seabirds used to generally more stable processes at depth throughout their lifespan.
In addition to physical drivers, Kamchatka pink salmon (a new indicator this year), with a marked biennial signal in their abundance that peaks in odd years, has been shown to be correlated with copepod abundance, otolith growth in Atka mackerel, planktivorous seabird reproductive success (Batten et al., 2018; Matta et al., 2020; Springer and van Vliet, 2014), and potentially, Pacific ocean perch young of the year. With record abundance in 2019 and an increasing trend over the past decade, their potential for competitive impacts on prey availability for other groundfish and cascading ecosystem effects warrants consideration. This competitive impacts may differ for fish feeding in shallow versus deeper waters as other biological processes may confound physical forcing driven by surface temperatures or may have a lagged effect in deeper waters. While, in general, Kamchatka pink salmon abundance correlates with a lower copepod abundance in off years, 2019 was an exception, as shown by the CPR timeseries which shows an increase in the mean size of the copepod community and its abundance - as supported by the decreased biomass of large diatoms which signals a potential increased predation pressure from copepods. With a potential cascading effect on plankton feeding species and young-of-year fish, this may partly explain the success of fish feeding seabirds in 2019. Understanding the interplay of vertical and horizontal spatial variability in food-web and oceanographic dynamics is particularly relevant given the higher reliance on plankton in the western Aleutians versus more piscivorous and invertivore feeding habits of fish and seabirds towards the eastern Aleutians.
The largest total biomass of both fish apex predators and pelagic foragers is located in the central Aleutians, the ecoregion with the largest shelf area under 500m. The lowest apex predator biomass is located in the western Aleutians whereas that of pelagic foragers is found in the eastern Aleutians. This pattern has been consistent since 1991, though individual species group fluctuations do not necessarily follow the same behavior. Finally, the increase of Pacific Ocean perch biomass and its stable high population, might be driving some spatial dynamics, where it may be encroaching onto other species' habitats, as seen by the estimated increase in the area occupied shown in the Pacific Ocean perch stock assessment. This increase in abundance and area occupied may be the cause of the increased bycatch of Pacific Ocean perch.
Western Ecoregion In the western ecoregion, the reproductive success of planktivorous auklets, serving as indicators of zooplankton production, was above average during 2019. Both least and crested auklets hatched chicks earlier than the long term average. These species feed their chicks mainly euphausiids and copepods, respectively. Parakeet, whiskered, and crested auklets all had high reproductive success in 2019, while that of least auklets was average. While the overall timing of breeding for fish-eating seabirds was average in 2019, their reproductive success varied. Glaucous-winged gulls and horned puffins had high reproductive success, tufted puffins and thick billed murres had average reproductive success, and common murres failed. There was an increase in the variety of fish brought back to feed tufted puffin chicks. Increased diversity in chick diets may indicate that more favored prey were less available. There was a slight increase in the proportion of gadids fed but lower proportions of hexagrammids (likely age-0) and Ammodytes. It is still unknown whether the high number of hexagrammids seen in 2013 and 2014 possibly indicated high recruitment in Atka mackerel, as their overall abundance has been in decline since 2006. Steller sea lion non- pup counts continue to decline with the lowest estimated numbers yet in 2019. The diet of Steller sea lions consists primarily of commercially fished species, many of which seem to have had poorer body condition in recent years. The declining Steller sea lion trends in both numbers and birth rates are topics of active research, and prey quality may play a role in their lack of recovery.