S. Zador1, K. Holsman1, J. Ianelli1, E. Siddon2, A. Whitehouse1
1Resource Ecology and Fisheries Management Division, Alaska Fisheries Science Center, National Marine Fisheries Service,
National Oceanic and Atmospheric Administration, 7600 Sand Point Way NE, Seattle, WA 98115, USA
2Auke Bay Laboratories, Alaska Fisheries Science Center, National Marine Fisheries Service,
National Oceanic and Atmospheric Administration, 17109 Point Lena Loop Rd, Juneau, AK 99801, USA
- The eastern Bering Sea supports one of the largest fisheries in the world: walleye pollock (Gadus chalcogrammus).
- The annual cycle of sea ice cover in the eastern Bering Sea influences ecosystem productivity via the formation of a cold pool of bottom water that varies in extent each year.
- Years with extensive cold pools are typified by energy-rich foodwebs, which favor walleye pollock and their preferred prey. However, multiple studies predict overall declines in the intensity and extent of cold pools under future warming conditions.
- Better scientific understanding of how ecosystem components interact enables ecosystem science to inform fisheries management practices that are robust to future warming.
The Eastern Bering Sea (EBS) is a highly productive ecosystem characterized by a broad (>500 km) continental shelf (defined as <200 m) and narrow slope to a deep-sea basin (Fig. 1, Stabeno et al., 1999). Commercial fisheries in the EBS represent over 40% of fish landed annually in US waters, and the system has long supported one of the largest fisheries in the world: walleye pollock (Gadus chalcogrammus; hereafter pollock; FAO, 2017; Fissel et al., 2016). In addition to pollock, key commercial fisheries include King, snow and Tanner crab, Pacific cod, yellowfin sole, Pacific halibut, other flatfish, salmon and Pacific herring. The US federal government is responsible for sustainably managing the groundfish species, which include the bottom or near-bottom dwelling pollock, Pacific cod, rockfish and a variety of flatfish. While there is currently no commercial fishing allowed in US Arctic waters north of the EBS, there is awareness that warming oceans may extend the distributions of some fish species northward, increasing the potential for commercial fishing and associated vessel traffic (Cheung et al., 2015; Pinsky et al., 2013; Haynie and Pfeiffer, 2013).
Fig. 1. The Eastern Bering Sea.
The annual cycle of sea ice coverage in the EBS directly influences ecosystem productivity. Winter winds that bring cold arctic air and ice from the north into the Bering Sea affect the extent of the annual sea ice formation in the EBS and the timing of sea ice breakup in the spring (Hunt et al., 2002, 2011; Stabeno et al., 2012). The extent of the winter sea ice influences the formation and structure of the summer "cold pool", a body of dense cold water (<2° C) that remains on the shallow shelf bottom after the ice retreats in the spring (Fig. 2). In "cold" years with extensive winter sea ice, the cold pool extends south to the Alaska Peninsula; in "warm" years it does not extend far beyond St Matthew Island, a difference of about 800 km. The cold pool influences spring and summer oceanographic conditions (e.g., stratification) as well as the distribution of species that may have different tolerances or preferences for the cold bottom water. Thus, food-web interactions can vary each year as predators and their prey respond differently to the extent and location of the cold pool (Mueter et al., 2011; Stabeno et al., 2012; Sigler et al., 2016).
Fig. 2. The annual extent of the cold pool of bottom water on the Eastern Bering Sea shelf from 2001-2017. Maps represent interpolated hydrographic observations and were developed by Carol Ladd (Siddon and Zador, 2017).
The EBS experiences shifts between consecutive years of predominantly warm or cold conditions (i.e., years typified by small or large cold pools, respectively), which provide contrast to evaluate the drivers of biological productivity (Fig. 2; Stabeno et al., 2012). While phytoplankton blooms occur regularly in the spring, the duration and timing of blooms, as well as the sizes of phytoplankton, vary substantially between warm and cold years. These divergent bloom characteristics, in turn, influence the size and energy content of zooplankton prey (Hunt et al., 2011). In general, cold years are typified by large phytoplankton, which support higher abundances of krill and larger species of copepods, which themselves become energy-rich prey for their fish predators, including pollock. In contrast, during warm years there are more small-sized species of copepods, which are not as energy-rich. Feeding on energy-rich prey can improve fish condition and increase their overwinter survival rates. For example, when young pollock at the end of their first summer have high average energy content (a combination of energy density and fish weight), their overwinter survival is also higher (Siddon et al., 2017a; Heintz et al., 2013).
In 2012, the EBS experienced the coldest average bottom temperature (Fig. 3) and largest cold pool since 1999, which provided an extreme case to test the hypothesis that cold conditions favor young pollock (Fig. 2). During the cold summer, the spatial overlap between young pollock and their preferred zooplankton prey appeared high (Siddon et al., 2017b). However, the pollock were much smaller than average at the end of their first summer. While their energy density was high, as is typical in cold years, their small size contributed to a low average energy content per fish, which led to the prediction of poor overwinter survival (Heintz et al., 2013b). However, current estimates of the pollock stock suggest that this 2012 year class of pollock may in fact be one of the largest in the past three decades (Fig. 4). The 2012 year class now dominates the estimates of total pollock spawning biomass, which is currently at an overall high level and supports a large fishery quota (Fig. 5; Ianelli et al., 2016).
Fig. 3. Average summer surface (green dots) and bottom (blue triangles) water temperatures (°C) of the eastern Bering Sea shelf collected during the NOAA Fisheries summer bottom-trawl surveys. Dotted lines represent the time-series mean for 1982-2016. From Zador and Siddon, 2016.
Fig. 4. Recruitment estimates (the number of age-1s) for Eastern Bering Sea pollock from the 2016 stock assessment for all years since 1964 (1963-2015 year classes). Error bars reflect 90% credible intervals based on model estimates of uncertainty. From Ianelli et al., 2016.
The explanation for the surprisingly strong 2012 year class remains uncertain and is an active area of study. Better scientific understanding of how ecosystem components interact enables ecosystem science to inform fisheries management (Zador et al., 2017). For example, despite the current high EBS pollock stock size, the fisheries managers selected a more conservative catch limit than the maximum permissible in 2016 (Federal Register, 2017). The justification for this reduction included the recent warm environmental conditions; low abundances of age-1 and age-10 and older pollock; the anticipated poor survival of age-0 pollock; the continued decline in acoustic survey estimates of krill, an important prey; and the continued declines in northern fur seals, which prey upon pollock.
The current status of groundfish stocks in the EBS appear favorable overall. However, the longer-term effects of the recent, and record, warm years in the EBS will continue to be closely watched. Long-term climate projections for the EBS indicate significant warming of both surface and bottom summer temperatures, especially in the southern EBS shelf, as well as declines in sea ice extent in the winter and the extent and intensity of the cold pool. Concomitant shifts in productivity are anticipated; these shifts include declines in large zooplankton biomass, which may impact survival of groundfish species including pollock (Ortiz et al., 2016; Hermann et al., 2016). Multiple studies predict overall declines in pollock biomass in the EBS under future warming conditions (Ianelli et al., 2016b; Seung and Ianelli, 2016; Spencer et al., 2016).
Fig. 5. Estimated Eastern Bering Sea female pollock spawning biomass. Biomass units are in thousands of tons. Data from Ianelli et al., 2016.
Projected effects in adjacent large marine ecosystems will likely differ from those in the EBS, based on inherent differences in geography, oceanography, and food webs. For example, the adjacent Aleutian Islands ecosystem encompasses a long chain of islands with extensive nearshore habitat, a narrow shelf, and abyssal waters to the north and the south. Local ecological communities typified by rockfish (Sebastes spp.) and sea otters (Enhydra lutris) will not be able to simply shift their distributions northward due to the absence of nearshore habitat. North of the EBS, continuing winter sea ice cover, in spite of less spring sea ice (due to earlier breakup), remains a barrier to ecosystem expansions. This can influence the scope for fisheries and fishing communities to adapt to climate change. For example, the ability of pollock fisheries to adapt to reduced productivity in the south EBS and northward shifting groundfish distributions may be limited by continued winter sea ice and existing fishery closure areas that limit northward expansion of the fishery. Thus, understanding ecological and socioeconomic implications of climate-driven changes requires ecosystem-specific understanding of mechanistic processes and species physiological threshold (Pörtner and Farrell, 2008) to project trends in species and environmental conditions.
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