Ecosystem Assessment (pdf)
Stephani Zador1 and Ellen Yasumiishi2
1Resource Ecology and Fisheries Management Division, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA
2Auke Bay Laboratories Division, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA
Last updated: November 2019
The primary intent of this assessment is to summarize and synthesize climate, biological, and fishing effects on the shelf and slope regions of the Gulf of Alaska (GOA) from an ecosystem perspective and to provide where possible an assessment of the possible future effects of climate and fishing on ecosystem structure and function. This serves the larger goal of the Ecosystem Status Reports (ESRs) to provide ecosystem context for tactical fisheries management decisions. This assessment ties together the myriad indicator data into a narrative of the current and likely future ecosystem state, including information based on new or unexpected observations that may have implications for groundfish management. Report cards are presented at the front of this ESR to provide a succinct summary of the state of the ecosystem based on a short list of indicators.
This assessment reflects the recognition that the western and eastern GOA ecosystems have substantial differences. The GOA is characterized by topographical complexity, including: islands; deep sea mounts; continental shelf interrupted by large gullies; and varied and massive coastline features such as the Cook Inlet, Prince William Sound, Copper River, and Cross Sound, which bring both freshwater and nutrients into the GOA. The topographical 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 report card indicators were selected to best reflect the complexity of the GOA. Although there are many more people living in both large and small communities throughout the GOA relative to the Aleutian Islands or eastern Bering Sea, the complexity of the system requires a high-degree of local understanding to disentangle broad-scale patterns from local processes. We consider the GOA to be ecosystem data-moderate relative to the Aleutian Islands (data-poor) and eastern Bering Sea (data-rich). However, the division of the GOA into separate ecoregions highlights data gaps. For example, several zooplankton indicators are available for the western GOA, while the only comparable indicators in the eastern GOA sample inside waters (Icy Strait). The report card indicators are described at the end of the Ecosystem Indicators section of this ESR. We will continue to revise and update these indicators in future editions of this report.
Complete Recap of the 2018 Ecosystem State
Some ecosystem indicators are updated to the current year (2019), while others can only be updated to the previous year (or earlier) due to the nature of the data collected, sample processing, or modelling efforts. Therefore, some of the 'new ' updates in each Ecosystem Considerations Report reflect information from the previous year. This year we include a complete summary of the ecosystem status of the GOA during 2018 that includes information from both previous and current indicators. The next section (Current conditions: 2019) provides separate summaries of the 2019 ecosystem state for the western and eastern GOA based on indicators updated this year. We plan to continue developing the ecosystem assessments with this ecoregional focus in future editions.
2018 marked the last year of a 2-year stretch of generally more 'typical ' conditions in the GOA following the 2014-2016 marine heatwave and before warm conditions returned in 2019 (see next section). Here we review our set of indicators for 2018 and attempt to distinguish which ecosystem components continued to show signs of lagged effects of the heatwave versus those that were more responsive to the generally cooler conditions.
The North Pacific atmosphere-ocean system in 2018 was similar to that of 2017, as seen in the continuation of largely average conditions in the western GOA following the end of the 2014-2016 marine heatwave. The Pacific Decadal Oscillation index shifted to a neutral state, reflecting a broad scale pattern of warmer-than-average sea surface temperatures across the North Pacific. Across the GOA, 2018 sea surface temperatures were largely average with some warming during the spring and summer. At a local scale, 2018 sea temperatures within the top 100m along the Seward Line showed average to slightly cooler than average temperatures during May. However, in Auke Creek near Juneau, freshwater temperatures during summer 2018 were the warmest since 1980. Surface currents during winter 2017/2018 were not strongly directional, indicating weak flow into the Alaska Current. Eddy kinetic energy continued to be low, indicating less cross-shelf exchange of nutrients than in years with strong eddy activity. By late September 2018, temperatures within the western GOA shelf area crossed a threshold to be considered a heatwave. Climate models had forecast anomalously warm sea surface temperatures throughout winter 2017/2018 in the GOA; while the signs of the anomalies proved to be correct, actual temperatures exceeded the forecast.
During 2017-2018, phytoplankton blooms appeared to be earlier and with higher concentrations overall relative to the previous warm years. Satellite-derived chlorphyll estimates indicated that the spring phytoplankton bloom occurred early, in April, during 2017 and 2018, particularly in the western GOA. This differed from 2012-2014, when the peak occurred later, in May. Chlorophyll concentrations were high in 2017 and exceptionally high in 2018. The continuous plankton recorded showed higher diatom abundances overall in 2017, and to a lesser extent in 2018, and confirmed an earlier bloom in 2018. These patterns suggest increased availability of food for phytoplankton consumers such as zooplankton and larval fish.
Indicators of zooplankton abundance for 2018 showed mixed signals. The biomass of large copepodsduring May 2018 along the Seward line was above average and similar to that during 2015-2017. While this was the fourth spring of abundant copepods, it was the first for euphausiids since 2014, potentially indicating an increase in higher quality zooplankton prey for predators during spring. However, higher abundance of large copepods during spring can also reflect faster growth during warmer temperatures, not necessarily higher abundance for predators. In contrast, the decline in zooplankton biomass seen in the CPR in 2018, after the higher biomasses observed from 2014- 2017, was mostly due to a drop in the numbers of small copepods, which had been numerous during the warm years. Zooplankton sampled in Icy Strait in 2018 showed an increase in total densities consisting primarily of small copepods and a higher lipid content of most zooplankton taxa. Although zooplankton densities were above average, the community consisted primarily of small calanoid copepods. Densities of larger zooplankton, which are prey items for small fish, were below average in 2018.
Another indicator of zooplankton availability to predators in 2018 is the reproductive success of planktivorous seabirds. Parakeet auklets had poor reproductive success in the Semidi Islands south of the Alaska Peninsula, where they feed mostly on euphausiids. This is notable because these auklets maintained average-to-high reproductive success through the heatwave years and last had poor reproductive success in 2011. In the Barren Islands at the mouth of the Cook Inlet, fork-tailed storm petrels had average reproductive success, suggesting zooplankton availability was moderate in that region. In early fall 2018, euphausiid biomass was among the highest in the Seward Line time series during September sampling, likely due to the cooler spring temperatures and indicating that euphausiid prey continued to be abundant over summer. If this represented a true record abundance of euphausiids in late summer, the 2018 year-class of pollock may have benefited from this high-quality forage going into winter.
With respect to forage fish, seabird-derived indicators suggested increases in abundance relative to the heatwave years. In 2018, piscivorous seabirds had above average reproductive success at the Semidi Islands, indicating that there were sufficient forage fish prey (possibly including age-0 gadids) to raise chicks. Kittiwakes and rhinoceros auklet chick diets at Middleton Island in 2018 showed notable increases in sand lance, an important forage fish, but few capelin, which disappeared from chick diets during the heatwave. In contrast, based on reproductive success, prey appeared to be limiting for the surface-foraging kittiwakes in 2018, but sufficient for average production for diving rhinoceros auklets. Kittiwakes also had below average reproductive success at the Barren Islands. Taken together, these observations suggest that forage fish were abundant and available around the Semidi Islands, but less so, at least to surface foragers, to the northeast. Given that the 2018 year-class of pollock appears to be strong as age-1s, we may assume that there were numerous age-0 pollock to support the seabird reproductive trends observed, particularly in the western GOA. Also, one may speculate that the reduced stock sizes of large piscivorous groundfish (e.g, Pacific cod and arrowtooth flounder) and cooler temperatures, which modulated their energetic demands relative to the heat wave years, may have decreased natural mortality on forage fish, leaving more for the seabirds.
Indicators suggest poor conditions for salmon survival in the Gulf of Alaska during 2017 and 2018. Juvenile chum, coho and pink salmon captured during the summer surface trawl surveys in Icy Strait were in low numbers and had small body sizes. Abundances in 2018 increased for juvenile Chinook salmon (2nd highest) and sockeye salmon (but still low) since 1997 for the 22-year time series, although sockeye body sizes remained low. Juvenile salmon lengths and weights were below average in 2018, indicating poor feeding conditions or a delayed migration. Salmon monitoring at Auke Creek weir in northern southeast Alaska showed low adult returns and poor freshwater and marine survival of pink and coho salmon. Age-0 coho salmon that outmigrated as smolts in 2018 had record low marine survival when they returned later in 2018. Age-1 coho salmon that migrated from freshwater to saltwater in 2017 and returned to Auke Creek as adults in 2018 had the second lowest marine survival for the weir since 1980. The 2018 outmigrating smolts experienced warm creek temperatures and low water depths due to lack of snowfall and snowmelt. Pink salmon had low marine survival, and adult returns were the lowest in 2018 since 1980.
Indications of groundfish biomass trends in 2018, an 'off-year ' for the GOA-wide bottom trawl surveys, are based on ADF&G surveys off Kodiak Island over Barnabus Gully and in two inshore bays. Catch rates were below the long-term mean for arrowtooth flounder, Pacific halibut, Pacific cod, skates, and flathead sole. Catch rates in 2018 were above the long-term mean for pollock offshore, but below at the inshore bays. While there has been a generally decreasing trend in total catch rates for 10-15 years, there was a slight increase in 2018. In this offshore region, the increase from 2017 was primarily due to increases in the pollock and arrowtooth flounder catch relative to 2017. Overall, and including the observations and assessments based on the 2017 bottom trawl surveys, large piscivorous groundfish such as Pacific cod and arrowtooth flounder (but not sablefish which are longer-lived and inhabit deeper depths) remain at low stock sizes relative to before the heatwave.
Marine birds and mammals appear to continue to show signs of negative impacts from the marine heatwave. The numbers of piscivorous murres breeding in the Semedis in 2018 were low-although those that were there did well reproductively-possibly reflecting a population impact of the im- mense die-off seen during the heatwave. Similarly, encounter rates of humpback whales in PWS fall surveys were very low in 2018, similar to what was observed in 2017. The highest encounter rates were noted just before and at the beginning of the heatwave. The murre die-off (2015-2016), large whale Unusual Mortality Event (2015-2016), and decline in sea lion counts during and immediately after the heatwave signaled the extreme impact of the heatwave and lagged responses to adverse conditions that would be expected in long-lived, k-selected species such as these. Humpback whale presence in southeast Alaska waters continued to remain low. In Glacier Bay, the number of calves and juvenile return rates of humpback whales have declined substantially since 2015. In Glacier Bay, crude birth rates-number of calves per adult whale sighted-remained anomalously low from 2016-2018. During whale surveys of northern southeast Alaska, crude birth rates of humpback whales continue to drop. Only two calves were seen in southeast Alaska waters during the summer of 2018, one of these is believed to have died, and no calves were seen during the survey window (June-August). Some whales had poor body condition but to a lesser extent than was observed in 2016 and 2017. These changes in calving and juvenile return rates may be related to changes in whale prey availability and/or quality, which may be negatively affecting maternal body condition and therefore reproductive success and/or overall juvenile survival.
Current Environmental State-Western Gulf of Alaska
The dominant physical signature of 2019 was the return to warm conditions following the largely average temperatures in 2017-2018. The western GOA shelf area sea surface temperatures remained largely above the 90% threshold over consecutive 5-day periods to qualify as a heatwave (see p.27). This was particularly the case during the summer, when the cumulative intensity (oC days) exceeded that of summer 2015. Despite that, the cumulative intensity for the entire year was very similar between 2015 and 2019. The winter of 2018/2019 was not as warm as those of 2014/2015 and 2015/2016, which may have modulated the negative impacts on the ecosystem relative to those previous heatwave winters, which had extensive impacts on populations of piscivorous predators such as Pacific cod and murres.
Several surveys confirmed that the warm waters at the surface during summer 2019 extended to depth. The thermal profile measured during the biennial bottom trawl survey suggests that water temperatures in 2019 may have been as warm or warmer than those observed in 2015 and 2017, particularly near the surface in the western Gulf of Alaska. Bottom temperatures observed during the biennial larval fish surveys were higher in summer 2019 relative to 2015. It is important to note that neither of these surveys took place during summer 2016, which we believe was the warmest of the 2014-2016 heatwave. The annual temperature profile of the top 100 m from the Seward Line indicates that temperatures during spring (May) 2019, while warmer than average, were not as warm as those from 2015 or 2016; summer temperature profiles were not available.
Chlorophyll concentrations during 2019 in the GOA indicated: (1) a late phytoplankton bloom that did not show up until June, and (2) low early season biomass, which could possibly affect phytoplankton consumers such as zooplankton and some larval fish populations, depending on their timing and location. Indeed, zooplantkon indicators suggested that only moderate to low abun- dances were available to predators. During May 2019, both large calanoid copepod and euphausiid biomasses were below average on the Seward Line. During the summer acoustic survey, euphausiid biomass was estimated to be slightly below average in this 7-year time series. Planktivorous para- keet auklets nesting at the Semedi Islands had above average reproductive success, suggesting that there were sufficient euphausiid or larval fish prey available to successfully rear chicks. However, planktivorous fork-tailed storm petrels nesting at the Barren Islands at the mouth of the Cook Inlet did not fare well reproductively, so zooplankton may not have been as available to them in that area. Also, record abundances of jellyfish were caught during the bottom trawl survey, across all regions. As jellyfish such as Chrysaora melanaster feed on zooplankton and small fish, high numbers could represent significant additional predation pressure on their prey.
Larval fish surveys during spring found few to no age-0 gadids, and their near-absence was confirmed during surveys at the end of summer. No age-0 pollock were found to the south and east of Kodiak Island, or near the Shumagin Islands in the southwest. The majority of fish found were in Shelikof Strait and near the Semedi Islands. Interestingly, black-legged kittiwakes, which have a mixed fish/invertebrate diet, failed reproductively at the Semedi Islands during the chick-rearing stage. The timing of failure suggests that they arrived in good condition in early summer but failed to find sufficient prey for their chicks. Kittiwakes are known to feed on age-1 pollock through late winter and early spring, when they become too big for kittiwakes to eat. It's reasonable to assume that kittiwakes had sufficient prey available to them at the surface prior to the breeding season, including age-1 pollock, but then suffered from the lack of age-0 pollock during the chick-rearing stage, when they had to abandon chicks. Surveys confirm that there appear to be abundant age-1 pollock (the 2018 year class). While the diving, piscivorous seabirds at the Semedi Islands did well reproductively, anecdotal data suggest that these birds were bringing in more diverse prey than the typical sand lance and age-0 gadids (pers. comm H. Renner). On Middleton Island, the occurrence of herring and other coastal species in seabird diets possibly reflect more use of nearshore/inner shelf habitat because of reduced availability of their offshore primary prey such as capelin. In fact, GPS-tracking of foraging seabirds conducted during chick-rearing revealed that birds from Middleton Island were commuting a considerable distance (˜80 km one-way) and foraging principally in nearshore waters. Overall, these patterns suggest that forage fish were not abundant in 2019.
Groundfish condition, as measured by length-weight residuals, was once again below average for all groundfish in the bottom trawl survey except for Pacific cod. This was the same overall pattern that was seen during the last survey in 2017, and indicates that foraging conditions were not sufficient for optimal growth for most species of groundfish. However, when fish condition is analyzed at finer spatial scales, a few patterns are discernible. There appeared to be an east-to-west trend in condition with heavy age-2+ pollock and southern rock sole per length in the eastern areas of the GOA relative to the western areas. While the survey is prosecuted in a west-to-east direction, and pollock typically put on weight during the summer, this spatial pattern in condition is not persistent across years. The pattern in 2019 is not likely to be due solely to sampling error, but may reflect regional differences in foraging landscapes, with pollock in the western GOA experiencing more limitations in prey than those in the east. Further supporting evidence of below average foraging conditions is the negative anomalies in condition of Pacific Ocean perch, which have similarly planktivorous diets to pollock. Pacific cod biomass is currently at an extremely low level. Little density-dependent competitive effects in combination with their large growth potential may have enabled cod to build up their weight.
Seabirds and marine mammals continue to show lagged effects of the 2014-2016 heatwave. While the generally high reproductive success of murres indicated favorable foraging conditions in 2019, the numbers of birds on breeding cliffs at the Semedi Islands was still only a little more than half the number that was counted, on average, in years before the 2014-2016 heatwave, and specifically before the die-off in 2015/2016, which was estimated to comprise approximately 1 million individuals (Piatt et al., in review). While murres may defer breeding in some years, likely due to poor foraging conditions, the continuation of low numbers at colonies may indicate population-level impacts of the 2015/2016 murre die-off. More humpback whales were observed during September 2019 in Prince William Sound than had been observed in 2017 and 2018; however, they remain well below 2008-2014 numbers. One positive note is that unlike in 2017 and 2018, no whales in poor body condition were observed. There was a large unusual mortality event that included 48 dead grey whales found within Alaska waters, 19 of which were in the western GOA. Given that benthic prey (primarily ampelecid amphipods) in the Bering, Chukchi, and Beaufort Seas are considered the mainstay of gray whale foraging, it is reasonable to assume that the mortalities located in the GOA are linked to the extreme changes in their foraging grounds to the north (see EBS ESR).
Current Environmental State-Eastern Gulf of Alaska
Here we highlight those trends that are specific to the eastern GOA. However, separating an assessment of the current state of the eastern GOA ecosystem highlights the limited data relative to the western GOA. Data-limitations are highlighted further when we distinguish current indicators for inside waters from offshore waters.
Sea surface temperatures during summer 2019 in the eastern GOA, while not as warm as in the western GOA, were also elevated and similar to those during summer 2016. Warm temperatures were also recorded in inside waters in southeast Alaska. Inside waters, while not as warm as the outside waters, were similar to those in 2015 and 2016 overall. This pattern was confirmed by direct sampling in Icy Strait, where surface temperatures were warmer than those in 2015 and very similar to those in 2016.
In contrast to the western GOA, phytoplankton bloom timing was only slightly late (May), and chlorophyll concentrations were nearly average. Zooplankton density in Icy Strait dropped in 2019 relative to 2018, and lipid content was also low. However, planktivorous storm-petrels nesting on St. Lazaria Island had high reproductive success, suggesting that zooplankton prey were abundant in offshore waters. This is further supported by the positive anomalies in pollock condition in this region. The contrast among zooplankton indicators may reflect differences in zooplankton abundance in inside waters compared with outside waters. However, it is difficult to draw conclusions from such few indicators.
The lack of forage fish indicators for the eastern GOA limit assessments about their condition or abundance. Gulls and murres nesting at St Lazaria Island had very successful reproduction in 2019; as both are primarily forage fish eaters, this suggests that there were sufficient prey to support chick-rearing in offshore waters. In contrast, juvenile salmon catch rates in Icy Strait were among the lowest since the survey began in 1997, although their average length increased relative to 2017-2018. Adult pink salmon returns and marine survival rates at Auke Creek were very low, despite being an odd-year brood.Humpback whale calving and juvenile return rates in Glacier Bay remained low in 2019, and mothers appeared to be in suboptimal condition, suggesting poor foraging conditions and continuation of negative impacts of the 2014-2016 heatwave. While only 3 of the 48 dead gray whales found throughout the state were found in the eastern GOA, this is not likely an indication of better foraging conditions relative to other Alaskan areas, but simply the lagged negative impacts of their foraging landscape in the Northern Bering Chukchi seas in 2018.