Current research from the Intergovernmental Panel on Climate Change (IPCC 2007) predicts a worldwide increase of 0.2° C (0.36° F) per decade for the next few decades. However, the polar regions have already been warming twice as fast as the rest of the earth. This trend is expected to continue.

These temperature changes will have profound effects on Alaska's marine ecosystems. Commercial fisheries and traditional subsistence ways of life will be changing in uncertain ways as Alaska's climate changes.

Melting Sea Ice in the Bering Sea

One of the most significant consequences of climate change on Alaska’s marine ecosystems is thinning of the ice pack and retreat of the winter extent of sea ice in the Bering Sea. The seasonal expansion and melt of sea ice in the Bering Sea is a defining feature of the highly productive ecosystem. The timing of the phytoplankton bloom, which supplies energy to the entire ecosystem, is regulated by the timing of the ice retreat.  As temperatures increase, less sea ice forms and it melts earlier in the spring, resulting in delayed spring phytoplankton bloom.

What difference does the timing of ice retreat make?

Algae and tiny animals inhabit sea ice, living in and on the under surface. In the spring when sunlight is returning, ice in the Bering Sea melts discharging those plants and animals into the water column where they stimulate a massive phytoplankton bloom. There is more plankton present than can be consumed by the zooplankton and so most of the nutrients fall to the seafloor feeding benthic animals. The bottom of the northern Bering Sea is a rich living seafloor providing abundant food for diving predators including walrus, gray whales and spectacled eiders.

Warmer temperatures causes the melt to happen earlier than usual. Under this scenario, there has been less growth of ice algae and it is discharged before sufficient sunlight is present to cause the phytoplankton bloom. The bloom is then delayed until sunlight is available but without the added fuel from the ice algae. Less phytoplankton is produced and it is consumed by zooplankton before it reaches the seafloor. This scenario is considered more favorable to fish in the pelagic zone feeding on zooplankton.

The change in timing of the phytoplankton bloom affects which predators consume the phytoplankton and the effect is carried all the way up the food chain. Colder temperatures and more sea ice normally support benthic (bottom-dwelling) communities like crustaceans and in turn the marine mammals and diving sea ducks that prey on them. In contrast, warmer temperatures and reductions in sea ice result in more food available for fish in the pelagic zone (water column). Scientists are concerned that a loss of spring phytoplankton production may in turn reduce the overall productivity of the Bering Sea ecosystem.

photo: J. Wasley

photo: USFWS/B. Larned

The entire world population of spectacled eiders winters in open water leads within the northern Bering Sea pack ice. Here they dive for clams and rest on the ice. Since they lose more heat while in the water, eiders need the ice to stay warm. Scientists are studying spectacled eider ecology to understand what the consequences might be for them as Bering Sea ice diminishes. Spectacled eiders are a threatened species under the Endangered Species List

Sea ice loss is  profoundly affecting marine mammals like seals, walrus and polar bears. They use the sturdy pack ice to haul-out and rest after swimming and feeding. As the ice retreats, this safe resting ice is farther away from favorable continental shelf foraging areas, and they have to swim farther and expend too much energy to get the food they need. Thinner ice and earlier melting also cause the snow dens of some ice seals to collapse. These "lairs" are needed to protect seal pups from frigid air temperatures and predators.

Northward Shifts in Distribution and Migration

Temperature is one of the primary factors controlling fish distribution. The marine ecosystem is naturally very dynamic with cold and warm cycles causing pendulum changes in dominant species. However, scientists are concerned that the projected continuous warming trend will soon overwhelm natural variation. Sea ice creates a “cold pool” that persists through the summer on the Bering Sea shelf, supporting a specialized benthic ecosystem. With reduced winter ice, the cold pool is shrinks, changing the species composition in northern Bering Sea waters.

As the ocean warms and the seasonal extent of sea ice is reduced, species that are adapted to ice and arctic conditions retreat to higher latitudes. Sub-arctic species will also shift northward, since they are better suited to take advantage of the warmer, reduced ice environments. Supporting evidence for these northward shifts are already being seen in annual survey data.  From 1982-2006 45 fish species shifted the center of their distribution north.

Average summer location of the Bering Sea cold pool (temperature < 2 degrees C) during 1982-86 and a recent five-year period (2002-06).

Source: M. Litzow, AFSC

Mismatches Between Prey Availability and Predator Needs

Another factor controlling fish distribution after water temperature is prey availability. Changes in the ecosystem can separate predators from their prey both in space and time. A mismatch occurs, for example, if  warmer water temperatures cause fish to spawn before the spring plankton bloom. At vulnerable larval or juvenile life stages, fish starve without sufficient plankton available to eat.

Changing Rates of Metabolism

Fish are ectothermic (cold-blooded) so their body temperature and their metabolism are controlled by the temperature of the surrounding water. Living in cold water slows their metabolism while warmer conditions increase their metabolism. Fish in warmer water will need to consume more prey to sustain increased metabolism and will face increased risk of starvation if sufficient prey is not available.

Starvation is a big concern especially for young fish during their first winter. As little fish with small energy reserves, they must forage enough in the summer to survive the winter when there is little food available. In a warmer ocean, their metabolic rate will increase and their energy reserves will be depleted faster. As a result, fish populations could be very vulnerable to warming temperatures: the warmer it is, the faster they use up their resources. Increased metabolism will exacerbate the problems from other impacts of warming temperatures like a food resource mismatch or distribution changes. 

Complex Community Re-Organization

Research published in 2006 indicates that the current level of warming and associated loss of sea ice has already resulted in a reorganization of the northern Bering Sea from an arctic to sub–arctic system. The Bering Sea cold pool contracted as a result of warmer air temperatures and lower winter ice cover. Observations over the past 12 years show increased bottom temperatures and onset of ice melt occurring three weeks earlier. Without the reduced temperatures in the cold pool, species composition has clearly shifted. The abundance of benthic species, such as bivalves, has declined in the northern Bering Sea. As a result, it appears gray whales who prey on bivalves have shifted farther northward. It is also cause for concern for bottom feeding walrus and spectacled eiders.

This sentinel study correlates with observations by Alaska Natives in Bering Sea coastal communities. Similarly, the offshore fishing fleet is experiencing a northward concentration of some species beyond previous core fishing grounds. To preempt the expansion of bottom trawl fishing fleets to the north, AMCC has been working with communities in the Bering Sea to establish a northern bottom trawl boundary.

The greatest concern in terms of climate change impacts on marine ecosystems is that even as scientists are beginning to understand individual aspects of the effects of climate change, they cannot predict the cumulative or synergistic effects on the whole ecosystem. We will not simply see a wholesale northward shift in marine ecosystems as temperatures increase. Instead, as each species responds differently to changing environmental factors, they will interact and affect each other in different and complex ways. These inherent uncertainties in the impacts of climate change on marine ecosystems are of great concern to scientists, resource managers, fishermen, and coastal Alaskans.

Not All Years Are the Same

Even though summer sea ice in the Arctic was the lowest on record in 2007, sea ice in the Bering Sea the following winter extended very far to the south. How can the Bering Sea have lots of ice some years if Alaska is warming up? Global climate change is not linear—especially in the oceans, where conditions are driven by complex oceanographic and atmospheric forces and cycles on various time scales. Alaskans should expect the Bering Sea to be cold some years and especially warm during others. Scientists at the Pacific Marine Environmental Lab project a warming trend characterized by strong interannual variability. It is also ecologically important to track sea ice thickness and timing — not just location of the ice edge. For example, the Cupik word for November means “when the ice comes in.” In November 2007 there was no ice on lakes or around the coast of Nunivak Island. However when the ice came in late, it extended far into the southern Bering Sea compared to most years on record. The trajectory of warming oceans may overwhelm decadal shifts between warm and cold cycles. This is a variable that fishery managers have not encountered and have not addressed in management strategies.

The Management Imperative for the Bering Sea

The dramatic consequences of climate change demand a multi-faceted response including new approaches to ocean management that foster ecological resilience. With the loss of sea ice, fisheries management in the Bering Sea will confront stark challenges as ecosystem-wide changes affect the abundance and distribution of fish species throughout the region.

Fishery managers face unprecedented circumstances brought about by climate change, the loss of sea ice and a cascade of associated ecological consequences.

Primary challenges include:

• How to detect ecosystem shifts as they occur, and
• How to factor in a new degree of environmental variability in accounting for the impacts of fisheries on the ecosystem.

Effective fisheries management will require new approaches that foster ecological resilience including:

• Stretching the horizon time for management decisions from annual to a 5, 10 or 15–year timeframe. The likely result will be to throttle back on exploitation rates — 

• Some assumptions that fish managers have relied on to date are not likely to apply in the future. Today’s single species management strategy is based on the principle that maximum sustainable yield is achieved when fish populations are reduced to 40% of their unfished biomass. Maximum sustainable yield is already a dubious strategy for maintaining ecosystem health but clearly it will not be an appropriate approach in a dramatically altered, unpredictable and highly stressed Bering Sea ecosystem.

• In ecological terms, increased uncertainty manifests as increased population variability. The best insurance against potential extinction is population size, because large populations during periods of abundance provide a size buffer during periods of declining productivity, and also help the population adapt because natural selection can operate on a larger number of individuals. The correct management response is thus to maintain a higher minimum viable population size for species of concern. This means that fishing is not only reduced as population size declines (the current practice); it is also reduced on average as population size becomes more variable and uncertainty increases.
• Establish control areas to track trends
• Identify and protect ecologically important places under stress from climate change or to pre–empt adverse effects of industrial fisheries now following northward moving fish populations into previously unexploited waters of the Bering Sea.

Criteria for selecting candidate areas to protect or monitor include sites with:

• High abundance of vulnerable species
• High species diversity
• Large seasonal concentrations of seabirds or marine mammals
• Habitat sensitivity to fishing effects
• Sensitive ecosystem functions especially stressed by climate change
• Importance for local subsistence use

Sources:

Arctic Climate Impact Assessment (ACIA). 2004. Chapter 9, Marine Systems, H. Loeng, lead author. International Arctic Research Center, University of Alaska Fairbanks, Secretariat. Prepared for the Arctic Council. http://www.acia.uaf.edu/pages/scientific.html

Grebmeier, J., Overland J., Moore S., Farley E., Carmack E., Cooper L., Frey K., Helle J., McLaughlin F., McNutt L. 2006. A major ecosystem shift in the northern Bering Sea. Science, vol. 311, Mar. 10, 2006.www.sciencemag.org

Hunt, G. L., Stabeno, P., Walters, G., Sinclair, E., Brodeur, R. D., Napp, J. M., Bond, N. A. 2002. Climate change and control of the southeast Bearing Sea ecosystem. Deep-Sea Research II 49:5821-5853.

Litzow, M. 2007. Warming climate reorganizes Bering Sea biogeography.  Alaska Fisheries Science Center, Jan-Feb-Mar Quarterly Report.

McNutt, L. 2006. How does ice cover vary in the Bering Sea from year to year?

WWF and AMCC. 2006. Notes from Roundtable Discussion on Climate Change and Fisheries Management. June 27, 2006. Anchorage, AK. www.beringclimate.noaa.gov/climate-roundtable