Climate Change and Carrying Capacity

## Exploring Climate Change and Carrying Capacity in the North Pacific

### by Anne Hollowed

U.S. GLOBEC recently convened a workshop to discuss a future research program on Climate Change and the Carrying Capacity (CCCC) of the North Pacific. The workshop was held at the Battelle Conference Center in Seattle, Washington, April, 19-20, 1995, and over 75 scientists attended. The need for the workshop stemmed from the development, in October 1994, of a Science Plan for coordinated research on Climate Change and the Carrying Capacity by the North Pacific Marine Science Organization (PICES). In response to the PICES Science Plan, the U.S. GLOBEC Scientific Steering Committee agreed to support a community-wide workshop to explore U.S. GLOBEC-relevant issues of the oceanic and coastal domains of the subarctic Pacific Ocean, and the Bering Sea. PICES is an intergovernmental organization established in 1992 to promote and coordinate marine scientific research in the temperate and subarctic region of the North Pacific and its adjacent seas. PICES' member countries are Canada, China, Japan, Korea, Russia, and the United States. The PICES Second Annual Meeting (1993) authorized the preparation of a draft Science Plan for what was called the PICES GLOBEC-International Program on Climate Change and Carrying Capacity (CCCC). The Plan was then discussed at a workshop and approved at the PICES Third Annual Meeting (1994) where it was agreed to establish a Scientific Steering Committee (now called Implementation Panel) to initiate development of an implementation plan. An Executive Committee met in May 1995 to prepare a draft for review and revision during the summer. A preliminary draft of this implementation plan was available to the participants of the Seattle workshop.

#### Central Questions

The PICES/GLOBEC CCCC Science Plan emphasizes activities at two spatial scales:

- Basin-scale studies to determine how plankton productivity and the carrying capacity for higher trophic level, pelagic carnivores in the North Pacific change in response to climate variations.

- Regional scale ecosystem studies comparing how variations in ocean climate affect species dominance and fish populations in the coastal margins of the Pacific Rim.

The Key Scientific Questions postulated in the Science Plan have since been consolidated into the following set of so-called Central Scientific Issues:

- Physical forcing: What are the characteristics of climate variability, can interdecadal patterns be identified, how and when do they arise?

- Lower trophic level response: How do primary and secondary producers respond in productivity, and in species and size composition, to climate variability in different ecosystems of the subarctic Pacific?

- Higher trophic level response: How do life history patterns, distributions, vital rates, and population dynamics of higher trophic level species respond directly and indirectly to climate variability?

- Ecosystem interactions: How are subarctic Pacific ecosystems structured? Do higher trophic levels respond to climate variability solely as a consequence of bottom-up forcing? Are there significant intra-trophic level and top-down effects on lower trophic level production and on energy transfer efficiencies?

Key research activities related to these issues will include retrospective analyses, development of models, process studies, development of observational systems, and data management. The next steps in developing the CCCC implementation plan on the regional scale are expected to include efforts to design the proposed comparison of ecosystem properties and responses to climate variability in cooperation with national GLOBEC programs. On the basin scale, a more comprehensive effort to develop an international cooperative program will be required.

#### Program Rationale

The North Pacific is an attractive site for a GLOBEC program for many reasons. Many commercial industries in the Pacific Northwest and Alaska are heavily dependent on natural resources. For example, approximately half of the total U.S. fisheries catch is removed from waters off the coast of Alaska. Studies have shown a strong connection between climatic variables and indices of fish abundance and distribution in the North Pacific (see collection of papers Beamish 1995, and Beamish and McFarlane 1989). These strong responses to climatic change translate into direct impacts on the efficiency and sustainability of the region's fishing industry. Elucidation of long term influences of climate change on these natural resources could have important benefits to the nation by improving our knowledge of functional relationships between climatic conditions and biological production that would allow for the development of long range plans for resource conservation and management.

The North Pacific is the location of one of the major storm tracks in the northern hemisphere. Models suggest that the southern side of the Arctic front will be the region of greatest alteration due to global climate change. The storm track responds to two global teleconnections patterns, the West Pacific oscillation that influences the location of storm generation and the Pacific-North American pattern that influences the track of storms across the subarctic Pacific. The Pacific-North American pattern is often considered the major mode of planetary variability of the atmosphere. We can hypothesize the shift in storm frequency and track due to climate change and its potential impact on the physical environment. At present, considerable natural variability exists on time scales from seasonal to decadal. This variability has a profound impact on circulation, mixed layer depths and the extent of ice coverage, all of which influence the rich biological resources of the subarctic Pacific.

U.S. GLOBEC and PICES are now poised to take advantage of newly developed tools that will enable us to examine the carrying capacity of the subarctic Pacific. These include measurement technologies and complex computer models. The vast time-space scope of the environmental questions requires application of technologies such as remote sensing via aircraft and satellite, shipboard data acquisition systems such as those employing acoustic sampling of currents and biota, and moored platforms to collect time series of biological and physical observations. Advances in computer technology now permit using large-scale models that assimilate field observations and integrate biological and physical processes.

A U.S. GLOBEC program in the North Pacific would benefit from parallel development of complementary research programs of other nations through the PICES-GLOBEC CCCC program. International cooperation on a common research program will inevitably enhance our national research efforts. In the case of coastal programs, Japanese and Russian studies in the Bering Sea, and Canadian research off British Columbia will augment U.S. investigations of ecosystem responses to climate variability.

U.S. GLOBEC research in the North Pacific would complement proposed research on the influence of climate variability on marine ecosystems in the California Current (U.S. GLOBEC Report 11). Coordination with the California Current program is highly desirable because large scale forcing for both regions could be modeled simultaneously, and because of earlier suggestions that the physical and biological systems of the two regions-California Current and Alaskan GyreÑoperate oppositely in phase (Chelton and Davis, 1982).

#### Linkages to Other Field Programs

There are opportunities for U.S. GLOBEC research in the North Pacific to coordinate with other existing process oriented research programs: Fisheries Oceanography Coordinated Investigations (FOCI), Bering Sea FOCI, Exxon Valdez Oil Spill Trustees, and NMFS Ocean Carrying Capacity studies (OCC). FOCI, and Bering FOCI are NOAA programs focussing on the biological and physical processes that influence survival of walleye pollock (Theragra chalcogramma). FOCI is comprised of scientists at the Pacific Marine Environmental Laboratory, the Alaska Fisheries Science Center, and several other institutions. The biotic and abiotic environment, including processes within larval patches, have been examined during the past decade through integrated field, laboratory and modeling studies. The original focus of FOCI was recruitment to the pollock population spawning in Shelikof Strait. Bering Sea FOCI, a component of NOAA's Coastal Ocean Program, has been studying production of walleye pollock in the Bering Sea since 1991. The Bering Sea FOCI program is investigating stock structure of pollock in the Bering Sea, and recruitment of walleye pollock in the southeastern Bering sea, where significant spawning takes place. The Exxon Valdez Oil Spill Trustees support research leading to the development of an integrated science plan for restoration of species potentially injured by oil spills in Prince William Sound, Gulf of Alaska. Currently, the trustees are sponsoring the Apex Predator Ecosystem-Experiment (APEX) and the Sound Ecosystem Assessment (SEA) programs. SEA is an interdisciplinary, multi-component program designed to understand factors constraining pink salmon and herring production in Prince William Sound, Alaska. The NMFS Auke Bay laboratory initiated the OCC study on Pacific salmon in the Gulf of Alaska in 1995. The OCC study is focused around cooperative Canada-U.S. research surveys on the marine phase of the life history of Pacific salmonids and will include studies of: age-at-maturity, modeling and diet studies, and retrospective studies of salmon growth. These process oriented research programs will provide: a) estimates of many of the critical biological parameters required to develop a coupled bio-physical model, and b) spatially explicit physical models for the region.

Canadian scientists also have a long history of fisheries oceanographic research in the Pacific. The Canadian La Perouse program provides a continuous time series of biological and physical oceanographic conditions off the outer coast of Vancouver Island since 1985.

The FOCI and the Canadian La Perouse programs are among the most mature fisheries oceanography programs in the world. Very few fisheries oceanography programs have been able to maintain continuous coordinated research for more than a decade. The results from these two programs provide many of the critical parameters for the development of the larger scale ecosystem models necessary to study climate change and carrying capacity. For example, the FOCI program has enumerated abundance trends at various life stages of early development; examined processes affecting life stages; mapped horizontal, vertical, and temporal distributions; described the oceanic and atmospheric environment; developed coupled bio-physical models of the Gulf of Alaska, and developed techniques to examine recruitment-process hypotheses.

#### Regional Boundaries

For the purposes of the workshop, the Bering Sea included all regions north of the Aleutian Islands (Figure 1). The geographic boundary between the coastal regions of the Gulf of Alaska and the open sub-Arctic was not defined by the PICES/CCCC working group. The following working definition is offered by U.S. GLOBEC:

The open sub-Arctic region will include Pacific waters north of the position of the isohaline of 34.0 psu in the upper mixed layer, with the exception of the coastal regions over the continental shelf and slope (to depths of 1000 m).

Coastal regions of the subarctic Pacific will include all waters over the continental shelf and slope to depths of 1000 m. This region will include areas south of the Aleutian Islands to the western boundary of U.S. waters at 173 E.

Some species, such as salmon, undertake seasonal migrations that cross both the coastal Gulf of Alaska and the open subarctic. In such situations, it may be necessary to include processes from adjacent regions (such as the coastal Gulf of Alaska or Bering Sea), if they significantly affect the physics, chemistry or biology of the subarctic gyre.

#### Workshop Structure and Breakout Session Summaries

The workshop began with background briefings on U.S. GLOBEC planning and research activities, and an introduction to the PICES Climate Change and Carrying Capacity Science Plan. Participants were then divided into six multi-disciplinary breakout groups. These six breakout groups covered issues that were relevant to the development of a research plan designed to address the impact of climate variability on biological systems: climate change, regime shifts, carrying capacity, modeling, technology, spatial and temporal scales. The following day, participants were divided into groups to discuss specific recommendations for future research in three geographic regions: oceanic subarctic, Bering Sea and coastal Gulf of Alaska. The following summaries provide a synopsis of the discussions and recommendations made in each of the breakout sessions.

Breakout Session 1. Climate Change: What are the likely scenarios for climate change in the North Pacific and how would they influence the ecosystem?

This group discussed the potential impact of climate change caused by increased CO2 and other greenhouse gases from anthropogenic sources. Climate change would influence North Pacific ecosystems primarily through four physical factors: mixed layer depth (MLD), volume and location of marine habitat, sea ice, and river flows. Time variation in late spring/summer MLD is the physical oceanographic measurement which may correlate most highly with primary and secondary productivity in the coastal Gulf of Alaska and Bering Sea shelf. Changes in marine habitat, thus the zoogeographic distribution of marine species, are expected to accompany ocean warming, with particular impacts on species at the edge of their ranges. Sea ice is foreseen to decrease both in space and seasonal duration, with effects on the Bering Sea's primary productivity and distribution of many marine mammals. The overall magnitude and seasonal cycle of river flows may change significantly, with implications for coastal currents and freshwater habitats for salmon.

Breakout Session 2. Regime shifts: Can they be detected, what is their impact, are they predictable?

Long term variations in ocean conditions appear to occur at two different time scales and the biological responses appear to differ in magnitude. The temporal periods most commonly mentioned are: 1) decadal and bi-decadal scale shifts, including 6-12 year warm and cool eras and the 18.6 year cyclic phenomenon (Trenberth and Hurrell 1994; Hollowed and Wooster 1992; Royer 1993), and 2) regime shifts that are 30-60 year cycles and appear to generate measurable ecosystem responses (Francis and Hare 1994; Baumgartner et al. 1992; Kawasaki 1992). There is compelling evidence of interdecadal changes in the physical environment of the North Pacific and Bering Sea. The most recent regime shift occurred in the late 1970s. The changes appear to be linked to large scale shifts in atmospheric processes. Marine organisms seem to respond to these decadal scale changes in the physical environment. The group acknowledged that research is required to improve our understanding of the mechanisms underlying the response of marine organisms to shifts in physical conditions. North Pacific basin modeling shows promise in simulating and explaining decadal fluctuations of the ocean over coarse scales. Regional and mesoscale oceanographic models exist for the Gulf of Alaska and need to be developed for other regions. Several physical and biological variables were identified that could be used as diagnostic indicators of regime shifts.

Breakout Session 3: What is carrying capacity?

This group discussed the concept of carrying capacity and methods of measuring it. The group adopted the following definition of carrying capacity: "Carrying capacity is a measure of the biomass of a population that can be supported by the ecosystem. The carrying capacity changes over time with the abundance of predators and resources (food and habitat). Resources are a function of the productivity of the prey populations and competition. Changes in the biotic environment affect the distributions and productivity of all populations involved." Rather than measuring carrying capacity as an absolute value, or providing a rigorous definition, the group discussed indices of carrying capacity that could be used to assess relative changes in the status of a population. The group noted that size spectrum theory, which relates rates of productivity to the size class of organisms in the ecosystem, is a potentially valuable conceptual framework for examining carrying capacity questions.

Breakout Session 4: What is required to model the impact of climate change on the carrying capacity of the region?

Participants discussed a variety of modeling approaches and suggested that different types of models be nested spatially, temporally and trophically. Physical models of the North Pacific and Bering Sea already exist and could be utilized in the U.S. GLOBEC program. While the formulation of governing equations and choice of parameters for biophysical models is difficult, reasonable choices can be made. Encouraging results have been obtained from the application of coupled biophysical models in other areas of the world (such as the North Atlantic).

Breakout Session 5: What are the technological impediments to measuring the effects of climate change on the carrying capacity?

Climate change by definition is a large scale, long term process (decadal) and will require ample measurements collected over a large geographical area for a long duration. A successful program will require careful selection of study sites at key or pulse points where the variance is minimized and the effects of climate change on carrying capacity are indicative of large scale change. A variety of technological issues were discussed and the disadvantages and advantages of each were identified. The group encouraged efforts to measure sea-surface salinity from satellites to map the large-scale distribution of this variable which can be dynamically more important than temperature in the Gulf of Alaska and Bering Sea. They also noted that deep ocean currents could be monitored using electro-magnetic observations from submarine telephone cables and identified the Kamchatka Current and Alaskan Stream as possible pulse points. Finally they noted the need for research on non-commercial species such as jellyfish or forage fish. These species may play a critical role in determining the carrying capacity of oceanic systems, and at least in the case of the "jellies" require specialized sampling.

Breakout Session 6: What are the spatial and temporal scales required to resolve questions concerning climate change and the carrying capacity?

This group concluded that the spatial scale of climate forcing is large-basin scale at least. The group noted that while considerable attention has been devoted to interannual variations, decadal and longer time scales may be more important for resolving issues of climate forcing and its impact on marine ecosystems. Participants acknowledged that the response time to climate change differs among species, which complicates the interpretation of biological/ecological systems to climatically driven physical changes. Criteria for selecting specific time and space scales for a future U.S. GLOBEC study must include: 1) important sources of variability must be concentrated, 2) relationship to plausible mechanisms of interaction, and 3) related to applied problems.

#### Coastal Gulf of Alaska Breakout Session

Participants were asked to define research questions that should be investigated to improve our understanding of the impact of climate change on: physical forcing, lower trophic level species, and higher trophic level species. Forcing questions focused on four forcing factors: atmospheric forcing, interactions between the Alaska Stream and Alaska Coastal Currents, the influence of bottom topography on coastal circulation, and tidal influence on nutrient flux. These four large scale factors influence important physical processes such as: mixed layer depth, mixed layer temperature, retention times (eddies), turbidity, and cross shelf transport. Research on the functional relationship between large scale forcing and local conditions will be required. Lower trophic level questions focused on five research topics: the effect of ocean transport on the composition and production of plankton communities, the role of grazing and predation on the structure of plankton communities, trophic phasing, climate change effects on over wintering plankton communities, and freshwater influences on plankton communities. Potential research topics relevant to higher trophic levels focussed on identifying climate change effects upon: a) the spatial distribution of predators, b) prey abundance, c) species composition of fish communities, and d) seasonality of resources to apex consumers.

In the Gulf of Alaska, bio-physical models have been developed for British Columbia, Prince William Sound and Shelikof Strait. Efforts to nest regional models into a large-scale biophysical model of the Gulf of Alaska were recommended. A broad-scale biological model of the Gulf might include the following: phytoplankton, protozoa, euphausiids, copepods, jellyfish salmon, herring, and pollock.

#### Oceanic Subarctic Breakout Session

Three subgroups formed to discuss projects relevant to 1) physical forcing/lower trophic level response, 2) higher trophic level response, and 3) ecosystem interactions. The first group identified three projects for future study: a program to document changes in standing stocks of plankton, a project to distinguish the effects of iron, Ekman pumping, cloud variation, and other factors on primary production, and a test of the Chelton hypothesis on the split of the west wind drift as it nears North America. Five questions were identified for future research of higher trophic level responses. These questions focused on mechanisms responsible for sustained high biomass of higher trophic level species since 1976-77, identifying historical biomass levels, studies to examine the coherence between the eastern and western gyres, bio-physical interactions, and regulatory factors controlling the carrying capacity of salmon. The ecosystem subgroup identified four research topics: effects of Kuroshio/Oyashio currents on coastal ecosystems of Asia and the deflection of these currents into the eastern subarctic, effects of subarctic currents and ENSO events on the subarctic coastal ecosystem, effects of the transition zone on the subarctic ecosystem and the effect of deep water species on near surface ecosystems. Retrospective, monitoring studies, process oriented and modeling projects were identified to address each of the research topics.

#### Bering Sea Breakout Session

The Bering Sea is possibly the most productive of the northern high latitude seas. The group noted that a first order understanding of the Bering Sea has been obtained and that a U.S. GLOBEC program should focus on studies aimed at elucidating the mechanisms linking environmental change to responses of the system. Four specific research topics were identified by the group: 1) what is the relation of the range of storm activity to the annual production budget and food web dynamics of the mixed layer?; 2) What is the relation of the sea ice melt-back bloom to total annual production?; 3) How does the nature (esp. timing) of the spring bloom determine the partition of energy between the pelagic and benthic ecosystem components?; and 4) Will climate change alter habitat/domain volumes and how will this influence recruitment? Retrospective, modeling, process oriented studies, and monitoring activities designed to answer these four questions were identified.

#### Products

Contributions of a U.S. GLOBEC CCCC program might include:

- The development and/or refinement of coupled bio-physical models that could be used to examine hypotheses regarding potential impacts of climate variability on marine ecosystems.

- Improved knowledge of the impact of climate variability on marine ecosystems of the North Pacific. Specifically, the program could elucidate mechanisms controlling marine populations (including commercially important fish species) and provide quantitative information that would improve the assessment, conservation and management of our nations valuable marine resources.

- Data sets will be assembled during the program that will provide the basis of future research activities in the region.

- The program will advance our ability to make predictions on the future composition of marine communities which could be utilized in simulation models to assess the impact of human activities in the region.

#### Recommendations for Initial Activities

Workshop participants focused on the four broad research questions proposed by PICES. These questions could form the basis of Announcements of Opportunity for research at a later date. A first AO would probably emphasize retrospective data analysis, modeling, and monitoring studies. (Anne Hollowed is a fisheries scientist with the NOAA Alaska Fisheries Science Center in Seattle and a member of the U.S. GLOBEC Steering Committee)

#### References

Baumgartner, T. R., A. Soutar, and V. Ferreira-Bartrina. 1992. Reconstruction of the history of Pacific sardine and northern anchovy populations over the past two millenia from sediments of the Santa Barbara Basin. CalCOFI Rep., 33, 24-40.

Chelton, D. B., and R. E. Davis. 1982. Monthly mean sea-level variability along the west coast of North America. J. Phys. Ocean., 12, 757-784.

Francis, R. C. and S. R. Hare. 1994. Decadal-scale regime shifts in the large marine ecosystems of the North-east Pacific: a case for historical science. Fish. Oceanogr., 3, 279-291.

Hollowed, A. B. and W. S. Wooster. 1992. Variability of winter ocean conditions and strong year classes of Northeast Pacific groundfish. ICES mar. Sci. Symp., 195, 433-444.

Kawasaki, T. 1992. Mechanisms governing fluctuations in pelagic fish populations. So. Afr. J. Mar. Sci., 12, 873-879.

Royer, T.C. 1993. High latitude oceanic variability associated with the 18.6 year nodal tide. J. Geophys. Res., 198, 4639-4644.

Trenberth, K. E. and J. W. Hurrell. 1994. Decadal atmosphere-ocean variations in the Pacific. Climate Dynamics, 9, 303-319.