Gulf of Alaska Assessment - 2023

Ecosystem Assessment (pdf)

Bridget Ferriss, Resource Ecology and Fisheries Management Division, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA

Last updated: November 2023

This assessment reflects the recognition that the western and eastern GOA ecosystems (divided at 147°W) have substantial differences (Waite and Mueter, 2013; Mueter et al., 2016). The GOA is char acterized by topographical complexity, including islands, deep sea mounts, a continental shelf interrupted by large gullies, and varied and massive coastline features such as Cook Inlet, Prince William Sound, Copper River, and Cross Sound, which bring both freshwater and nutrients into the GOA. The topo- graphical complexity leads to ecological complexity, such that species richness and diversity differ from the western to eastern GOA. Thus, local effects of ecosystem drivers may swamp basin-wide signals. With this in mind, we present report cards and assessments of current ecosystem state for the western and eastern GOA ecoregions separately to highlight inherent differences.

The Status of the Gulf of Alaska 2023

The Gulf of Alaska shelf marine ecosystem had an average year of productivity in 2023, with some declining trends from the highly productive previous year. Some highlights for 2023 include an increase in Pacific cod although still very low population levels) and capelin populations (both had not shown signs of recovery since declines related to the 2014–2016 marine heatwave), and a transition from three consecutive years of La Niña to El Niño conditions. Despite the generally productive year, some concerns persist around a decline in the zooplankton prey base. Total zooplankton biomass in 2023 was variable, but overall declined to below average, as indicated by multiple zooplankton surveys, low biomass of age-0 pollock and cod (WGOA), and low energy density of juvenile pink and sockeye salmon (eastern GOA; predators of zooplankton). Given the current El Niño status and the associated warming surface waters predicted in winter/spring of 2024, the reduction in zooplankton availability and quality may persist into the coming year. The last El Niño event occurred in 2016, with warming effects augmented by the ongoing 2014–2016 marine heatwave. If we do not experience another separate marine heatwave event, the upcoming El Niño is predicted to be of a strength similar to that in 1997/1998 (Bond in this report, p.27). Vulnerable groundfish in 2024 (due to warm surface waters and reduced zooplankton quality) potentially include the larval and age-0 juveniles of Pacific cod, walleye pollock, and northern rock sole. Warm surface waters can be favorable for larval rockfish and sablefish. Adult zooplanktivorous groundfish may have reduced prey availability (walleye pollock, Pacific Ocean perch, dusky and northern rockfish) but the deeper adult habitat is not predicted to warm unless El Niño-related warming persists long enough to be mixed to depth.

Gulf of Alaska Shelf 2023

Ocean temperatures were approximately average to cooler than average in the winter and spring (surface and depth) extending to above average in the summer, ranging from 5.8°C (WGOA Bottom Trawl Survey, O’Leary, in this report, p.40) to 10.5°C (Icy Strait, SEAK, Fergusson in this report, p.40). The cool early spring surface temperatures were favorable for walleye pollock, Pacific cod, northern rock sole egg and larval survival. The warm late spring/early summer surface temperatures may have been favorable for rockfish larval feeding and survival. Winter across- and along-shelf transport was reduced but variable, characterized by anomalous winter winds from the west that resulted in relaxed downwelling conditions. Variable eddy kinetic energy (strength of eddies on the shelf edge), and strong spring gap winds around Kodiak may have reduced the ability for groundfish larvae to be retained in favorable nearshore habitat, such as Shelikof Strait for juvenile pollock. Reduced cross-shelf transport is less conducive to the movement of larval arrowtooth flounder, Pacific halibut, and rex sole (slope spawners) to more favorable shelf habitat.

The spring chlorophyll-a concentration (an indicator of primary production) continued a multiyear below average trend, and peak bloom timing was considerably late (western GOA) to average (eastern GOA) across the regions (Gann in this report, p.74). While late peak spring blooms can be driven by colder springs, this event may also be explained by a deeper mixed layer in the winter/spring. Weaker stratifi- cation of the water column and a deeper mixed layer depth can reduce the opportunity for wind mixing to bring plankton and nutrients to the surface to promote spring blooms. Stratification strengthened in early May, one of the factors contributing to the spring bloom along the Seward Line (Danielson in this report, p.79).

Prey availability for zooplankton-eating adult groundfish (e.g., walleye pollock, Pacific Ocean perch, dusky and northern rockfish), and larval/juvenile groundfish, was below average to average across the GOA shelf. Total zooplankton biomass progressed from below average in the spring to indications of greater abundance in the summer, with variable copepod biomass but relatively high euphausiid biomass across the GOA (Shelikof St., Kimmel in this report, p.84, Seward Line, Hopcroft in this report, p.92, Icy Strait, Fergusson in this report, p.94). Larval pollock biomass was low in 2023 (as surveyed in the spring and summer, Rogers in this report, p.105), another common prey item when abundant. Planktivorous seabird reproductive success, an indicator of zooplankton availability and nutritional quality, was approximately average in the western (Chowiet Island) and the central GOA (Middleton Island) (Drummond in this report, p.167, Whelan in this report, p.167).

The reduced total zooplankton biomass could be explained by lower production, potentially connected to the late and reduced spring phytoplankton bloom, or by increased top down grazing pressure. Predators of zooplankton increased in 2023, relative to 2022, driven by large returns of pink salmon (Whitehouse in this report, p.130, Vulstek in this report, p.140), relatively large and increasing populations of Pacific Ocean perch (Hulson et al., 2023) and walleye pollock (Monnahan et al., 2023), and continued produc- tion of large year classes of juvenile sablefish in recent years (Goethel et al., 2023). Regardless of the mechanism, there appears to have been adequate, but not abundant, zooplankton available to support predators in 2023. Signs of a restricted prey base include a decline from above average to average zooplanktivorous seabird reproductive success, lower body condition (weight at length) of adult pollock, below average energy density of juvenile salmon, and juvenile pink salmon diet dominated by gelatinous prey (less nutritious alternative to zooplankton) (Icy Strait, SE Alaska, Fergusson in this report, p.94). Predictions for 2024 returns of pink salmon seem less favorable based on juvenile CPUE, length, and energy density in 2023, bolstering the indication of a reduced zooplankton prey base in 2023 (Yasumiishi in this report, p.129).

Prey availability for fish-eating groundfish (e.g., Pacific cod, sablefish, arrowtooth flounder, yelloweye rockfish) was approximately average with signs of reduced abundance in 2023. Capelin populations are rebounding for the first year since their decline during the 2014–2016 marine heatwave (McGowan in this report, p.113, Whelan in this report, p.167). Herring population biomass remains elevated, but is decreasing due to a declining 2016 strong year class (as assessed in EGOA but assumed GOA-wide trends; Hebert in this report, p.119, Pegau in this report, p.209). Age-0 pollock, a common prey in western GOA, had very low abundance (Rogers in this report, p.105). The reproductive success of piscivorous, diving seabirds (e.g., common murres and tufted puffins), decreased from 2022 to below average to average across the GOA (Drummond in this report, p.167 Whelan in this report, p.167), indicating less than sufficient/adequate prey to meet reproductive needs. In particular, black-legged kittiwakes failed to produce chicks on Chowiet Isl. (AK Peninsula), potentially due to lack of available age-0 pollock and Pacific sandlance in that area.

The predominant GOA groundfish species, by biomass, continue to be characterized by reduced popula- tions of Pacific cod, Pacific halibut, and arrowtooth flounder, and increased sablefish and Pacific ocean perch populations. While the ecological implications of the Pacific ocean perch population expansion (biomass and spatially) are not well understood, the biomass has grown large enough for the signal of this longer-lived, zooplankton-eating species to influence trends in various GOA groundfish community metrics (e.g., groundfish community stability and average groundfish lifespan, Whitehouse in this report, p.188 and p.192).

GOA Shelf/Upper Slope 2023

The GOA shelf edge (200 – 300m) and upper slope demersal/benthic habitat is habitat for numerous managed groundfish species, including sablefish, Pacific ocean perch, rockfish (thornyhead rockfish, rougheye/blackspotted rockfish, shortraker rockfish, and the slope subgroup of the Other Rockfish complex), and the deepwater flatfish complex (e.g., Dover sole). A number of these species migrate onto the shelf to spawn, and others are capable of changing depths in response to environmental conditions (Yang et al., 2019), increasing their ability to mitigate unfavorable habitat and forage conditions.

This deeper habitat is often buffered from variable environmental conditions occurring in the upper water column (e.g., the predicted surface warming of El Niño in 2024). However, this habitat can be exposed to warmer temperatures mixed from shallower depths over time, such as during the 2014– 2016 marine heatwave. Decreased dissolved oxygen, pH, and aragonite saturation can also occur from deep water intrusions from the central GOA gyre (Hauri et al., In Press). Bottom temperatures in 2023 along the shelf edge (250m) cooled to average after being consistently above average since 2016 (Temperature Synthesis, p.40). In fall 2022 and winter 2023, the Gulf of Alaska experienced weaker downwelling conditions on the shelf, favoring intrusion of deeper, saltier, and more acidic water from the central GOA gyre onto the upper slope and shelf (Bond in this report, p.26, Pages in this report, p.64). Modeled and observed time series along the Seward Line show statistically significant long-term decreasing trends of bottom water pH, aragonite saturation, and dissolved oxygen, an indication of steady degradation of the habitat. Observed bottom pH at GAK 9 (an outer shelf station of the Seward Line) was particularly low in spring of 2023, reaching values potentially detrimental to Tanner crab (pH = 7.56; Pages in this report, p.64). However, these environmental characteristics are not currently within the known range of detrimental effects of groundfish species in the shelf edge/slope region.

Structural epifauna (primarily sponges), which are measured poorly and indirectly from various surveys, continue to show signs of a multi-year decline in the WGOA (bottom trawl survey CPUE Laman in this report, p.61, non-target catch, Whithouse in this report, p.217). These slow growing structures are important habitat for rockfish, but any mechanistic link to rockfish population survival and productivity is currently unknown, and worthy of further investigations.

Looking Ahead to 2024 (El Niño)

Surface temperatures are predicted to warm in late winter/early spring of 2024, in alignment with the current El Niño (Bond in this report, 35). The most recent El Niño events occurred in 2015/2016, 2002/2003, and 1997/1998. The warming impacts of the 2016 event were compounded by the ongoing multi-year marine heatwave. The mass of warm water (termed “the Blob’) that moved from the central north Pacific onto the Alaskan shelf to initiate the 2014–2016 marine heatwave has persisted offshore since then. To-date it does not show signs of moving back onto the shelf (as of Oct 2023). The trajectory of that warm mass would determine if Alaska experiences strong, but more typical, El Niño warming conditions (perhaps similar to 1997/1998) or a more persistent and intense separate marine heatwave/El Niño combination (similar to 2016). Past El Niño’s have been associated with a stronger Aleutian Low, driving stronger southerly winds, and an increase in eddy strength (Crawford et al., 2002; Whitney and Robert, 2002) (potentially resulting in increased cross-shelf transport of slope-spawned larvae ATF, halibut, rex sole, rockfish and sablefish (Bailey and Susan J. Picquelle, 2002). These climate-ecosystem relationships can be tenuous and their potential impacts remain to be seen for 2024 (Litzow et al., 2020).

Warm winter/spring surface temperatures in the GOA drive early spring phytoplankton bloom (Gann in this report, p.74), early hatch times of cod eggs (up to 19 days earlier) and potentially larger age-0 cod (Laurel et al., 2023). There is potential for the surface temperatures to exceed the optimal temperatures for larvae survival and feeding of the early spring shelf spawners (Pacific cod, walleye pollock, northern rock sole), negatively impacting the 2024 year classes of these species. Conversely, warm surface waters in spring and summer can be favorable for rockfish larval survival. As it takes time for warm surface waters to mix to depth, the extent of warming that might occur in the deeper habitat of adult groundfish is dependent on the intensity and persistence of the surface warming, but would be a delayed effect of the winter El Niño warming event.

Most groundfish populations have one or more recent strong year classes that could help the population persist through a challenging year (e.g., potentially 2024 for some stocks). The warm period driven by 2014–2016 and 2019 marine heatwaves followed by a multi-year cooling during the 3 consecutive La Niña events (2020/2021–2022/2023) have jointly produced some strong year classes for numerous groundfish species across the spectrum of temperature affiliations. The warmer late spring/summers favored rockfish larvae (Sebastes spp., including Pacific Ocean perch). Sablefish have had multiple strong year classes since 2014, although fewer in recent years. The cooler and productive winter/springs favored larval walleye pollock (2017, 2018, 2020) and Pacific cod (2020, 2022). The Pacific cod population is still at very low abundance, though, so would be most vulnerable to any population fluctuations. Some important forage species have also benefited from strong year classes, including herring (2016, 2020), capelin (2023) and Tanner crab (2019). In summary, these stocks will be entering a year of greater environmental uncertainty and variability (2024) from a year of average productivity (2023) (with some signs of reduced lower level productivity. Some of the better surveyed stocks have a few known strong year classes to buffer potential population fluctuations. While Pacific cod appears to have a good recruitment year class entering the fishery in 2023, their low population would be most vulnerable to potential declines.