Historical Salmon Production in Lake Clark National Park and Preserve: Relevance to Emerging Subsistence Use Issues

Bruce Finney, Carol Ann Woody, and Patricia Heiser

Problem Statement: Recent dramatic declines in sockeye salmon returns (2.3% of the previous eight-year average; Rogers et al., 1997) to Bristol Bay impacted subsistence and commercial fishers in the region such that the governor declared the region an economic disaster area (Anchorage Daily News, Sunday July 12, 1998).

Alaska Natives of the Lake Clark region have relied on annual sockeye salmon returns for their subsistence since prehistoric times (Unrau, 1992), as do many contemporary users. Lake Clark originating sockeye salmon also contribute a significant portion (~ 30% of the Kvichak return) to the world’s most lucrative commercial salmon fishery in Bristol Bay, Alaska. 

Management of fishery resources on Alaska Federal Lands switched from State to Federal control 1 Oct. 1999. Personnel at Lake Clark National Park and Preserve (LCNPP) are now charged with managing salmon fishery resources for a rural subsistence priority within the Park. Defensible data on current and historic salmon production trends are currently lacking for this 6000 mi² watershed. Production information is critical to managers in terms of evaluating and justifying future subsistence management decisions; decisions that will likely be challenged as the politics regarding salmon resources of the region are highly charged due to numerous user groups. Subsistence management is a high priority for LCNPP personnel and evaluation of historic and contemporary salmon population trends is imperative for the decision making process. Therefore, the purpose of this project is to provide technical assistance to LCNPP personnel by providing a measure of historic salmon production trends within the Park. Current efforts by Park personnel to document historic and anticipate future subsistence needs will be complemented by such a database and will provide facts with which to justify future management decisions. A salmon productivity database is also scientifically valuable, as it will allow researchers to evaluate production trends relative to natural climate fluctuations, the advent of commercial fisheries, and recent escapement estimates.

Procedures and Methods

Recent studies at the Institute of Marine Science, University of Alaska Fairbanks have led to the development of a method to reconstruct long-term changes in salmon abundance from sediment core analysis. This method is based on the observation that salmon impact freshwater environments via input of marine-derived nutrients released from carcasses after spawning. This input, which can be quantified through analysis of the stable isotopes of nitrogen (d¹⁵N), will vary depending on escapement. Therefore, downcore changes in the abundance of d¹⁵N will reflect changes in the number of returning adult salmon. The sedimentary nitrogen isotope technique is summarized in Finney (1998).

The sediment ¹⁵N method is well suited for sockeye salmon productivity analysis because adults spawn then die within lake systems and lake sediments are ideal for high-resolution paleoenvironmental studies due to rapid and continuous sedimentation.  Studies combining 15N analysis with standard paleolimnological techniques and historic records provide an opportunity to address the following objectives relative to LCNPP:

1.      To collect and analyze cores from several sites within LCNPP.  Analyses include core dating, and downcore measurements of d¹⁵N, organic carbon, nitrogen, biogenic silica (diatom abundance) and d¹³C.

2.      To assess this data in terms of temporal changes in salmon escapement (d¹⁵N) and lake primary productivity (organic carbon, nitrogen, biogenic silica and d¹³C). 

3.      To examine relationships between salmon abundance (d¹⁵N) and climate.

4.      To assess escapement trends and goals for these systems based on long-term data.

5.      To determine whether relationships exist between salmon productivity trends and commercial harvest.

6.      To determine relationships between lake productivity and salmon abundance. This information, coupled with measurements of lake biota d¹⁵N, will be used to assess the importance of salmon carcass-derived nutrients in controlling salmon production. 

Cores will be obtained from several sites within LCNPP (Kijik, Sixmile, one or more locations within Lake Clark) using a gravity corer designed for sampling unconsolidated sediments and an undisturbed sediment-water interface. The cores will be described and analyzed for standard sedimentological properties including water content, and wet and dry bulk density. Magnetic susceptibility will be measured on each sample to determine the distribution of volcanic ash layers. The dating of the gravity cores will be determined by both 210Pb analysis and tephrachronology (ash stratigraphy).   Ash layers may be common given the close proximity to active volcanoes; chronologies will be refined if ashes of known events are found.  Based on previous experience, the shorter gravity cores probably span the past 200 - 500 years. Downcore analyses will be conducted for organic carbon, nitrogen, d¹³C, d¹⁵N, and biogenic silica. Changes in the input of salmon-derived N will be determined by analysis of d¹⁵N. We will attempt to calibrate sedimentary d¹⁵N with historical records of escapement to estimate pre-historic escapement. Paleoproductivity will be assessed from downcore analyses of organic carbon, biogenic silica and organic d¹³C.  Lake biota (zooplankton and juvenile salmon) will also be analyzed for d¹⁵N to determine the proportion of marine derived nutrients in freshwater food webs.  Stable N and C isotopes, and organic C and N content will be measured using standard techniques on a Europa 20/20 mass spectrometer.

Paleoclimatic trends will be compiled from published and ongoing work using tree ring indices, glacial advances, and marine and lake sediment core proxies. Recorded measurements of coastal air and sea surface temperatures (SST) go back about 150 years for these regions (Ware, 1995; Roden, 1989). Coastal tree rings record coastal air temperatures, which are strongly correlated with the SST of a large area of the adjacent ocean (Wiles et al., 1996). Tree ring-based temperature reconstructions extend beyond 400 years before present for the Northern Gulf of Alaska (Wiles et al. 1996, 1998). Other sources of paleoclimatic change are available from records of neoglacial advance for both regions (e.g., Calkin, 1988; Wiles and Calkin, 1994; Mann and Hamilton, 1995). Finally, data from the lake sediments obtained as part of this study, and from other lake cores (e.g., Overpeck et al., 1997), will provide paleoclimatic information for the time period of interest. Thus there is an abundant and diverse set of paleoclimatic/paleoceanographic data to compare with the records of change in salmon abundance. Time series analysis will be used for the analysis using SCA Statistical System software.

Commercial salmon harvest records from Alaska Department of Fish and game databases and escapement estimates (Fisheries Research Institute counts) from the Kvichack and Newhalen river systems will be compiled and examined for current trends. LCNPP personnel are currently compiling historic subsistence use records and will estimate future use based on current use and determination of number of residents who can be categorized as rural.

The study is expected to generate 3 main products:

1.       Data sets of: historic production of salmon based on d¹⁵N analysis, climate (SST), commercial harvest, and escapement estimates.

2.      Generation of a database that will allow evaluation of the relationship among and trends within: lake productivity, climate, and commercial harvest of salmon and salmon escapement. 

3.      Peer-reviewed journal article sharing results and conclusions with the scientific community. 

These results will be important to LCNPP for future subsistence management decisions and will provide a backdrop whereby impacts of climate and commercial harvest may be evaluated relative to lake productivity.  Data will be produced to evaluate the importance of marine/salmon-derived nutrients in freshwater ecosystems.  The importance of carcass-productivity feedbacks is unknown in this system, and can be important in controlling sockeye nursery lake production (Schmidt et al., 1998).  The study will also complement an ecological study by BRD documenting migration corridors and spawning habitats of sockeye salmon within LCNPP.  Results will also be relevant to researchers, state and federal managers working on other salmon systems, as the data should produce basic knowledge with respect to understanding salmon population variability.   

Technology/ Information Transfer