Reports & papers By category Symposium on Ocean Conditions

Long-term climate and ocean trends and salmon populations in the Pacific Northwest

George H. Taylor
College of Oceanic and Atmospheric Sciences
Oregon State University
Corvallis, Oregon

Introduction

Declines in salmon populations in the Northwest have occurred despite many decades of management attempts, and untold millions of dollars. Many of the management strategies focused on a single aspect of the problem, such as overfishing, habitat restoration, or water diversions. Recently a more comprehensive "ecosystem based" approach has been initiated, one that considers a multitude of factors for salmon enhancement. This paper describes some aspects of what may be considered the "backdrop" for salmon survival: climate and ocean conditions in the Northwest and the North Pacific.

There is increasing evidence that salmon populations in the northeast Pacific are significantly influenced by long-term climate and ocean changes. An examination of the historic records for salmon and environmental variables shows variations over a number of time scales. A better understanding of the lengths and causes of these "cycles" will enable decision-makers to make more informed choices regarding salmon recovery strategies.

Precipitation variations

In the Northwest, temperature and precipitation data go back about 100 years. During that time there have been four relatively distinct climatic periods. These are illustrated in Figure 1, which shows annual precipitation (departures from the long-term average) for the Oregon Coast. All stations west of the crest of the Coast Range were averaged together to get a single value each year, and every year's value compared with the long-term average. The Water Year (October through September) was used so that all months from a single winter remained in the same data set.

The four climatic periods were:

1896-1914 Generally wet (and cool)
1915-1946 Generally dry (and warm)
1947-1975 Generally wet (and cool)
1976-1994 Generally dry (and warm)

Some of the data from single stations show variations, which are somewhat different from the multi-station averages in Figure 1. Figure 2 shows annual precipitation at Portland since 1910. While the overall trends are similar to those in Figure 1, there is also evidence of shorter-term variations; in essence, there are 10-year cycles within 50-year cycles.

Ocean currents

It is becoming apparent that surface currents in the northeast Pacific are subject to long-term variations that coincide with the climate variations described above. Since 1946, the Bakun upwelling index, a numeric value based on surface wind speed and direction, has been calculated. Figure 3 shows spring (April-June) values of the Bakun index. Note the high values (strong upwelling) in earlier decades, and generally negative values since 1979.

It appears that the surface current variations result not from local changes but rather from basin-wide variations involving the entire North Pacific. Several scientists have suggested that the position and strength of the Aleutian Low is a key element in North Pacific current variation. During some years (Figure 4, top), the Low is very deep, and strong cyclonic (clockwise) flow around the Low causes surface winds to be generally from the southwest across the northeast Pacific. This results in a strong deflection of surface water to the north, towards Alaska, and relatively weak flow to the south (the California Current). Historically, such periods appear to have been more common during the decades when the Northwest has been warm and dry.

On the other hand, there are years (and decades) when typical wind flow across the North Pacific is from the west of west-northwest, due to a weak or nonexistent Aleutian Low. The bottom half of Figure 4 illustrates this situation. During such conditions, more of the surface water is diverted southward, enhancing the California Current and reducing flow into the Gulf of Alaska. This type of wind/current flow has been much more common during the wet-cool periods in the past.

Since stronger upwelling produces more favorable offshore conditions for salmon, and since salmon thrive onshore during generally wet periods (when river flows are high and water is cool), these correspondences of precipitation and ocean currents cause great variation in the potential for salmon survival. During the generally dry, warm periods, when upwelling is poor, survival potential would appear to be quite low compared with the alternate periods when cool, wet conditions correspond to stronger upwelling. Let us now examine the salmon records to verify whether this is so.

Salmon returns

Anderson (1995) has studied the effects of climate and ocean conditions on salmon. Figure 5, which was reproduced from that document, compared the "Pacific Northwest Index" (PNI) to Columbia River spring chinook salmon returns going back to 1940 (earlier data are not available). PNI provides a numeric value representing precipitation and temperature; note the similarity with Figure 1. The correlation between spring chinook and PNI is very strong, and indicates that salmon return increase during cool, wet periods and decline during warm, dry ones. While there are undoubtedly human-induced effects on the fish (including dam construction and habitat destruction), Figure 4 indicates that the expected "survival potential" described in the previous section is indeed reflected in salmon returns.

While stocks in the Northwest have shown low numbers in recent decades, Alaska salmon have had a tremendous boom period. Climatologists have known for many years that weather patterns in Alaska and the Northwest are out-of-phase: wet periods in the Northwest tend to be dry in Alaska, and vice-versa. The El Ni?-Southern Oscillation appears to be the major reason for this flip-flop. Interestingly (and perhaps not surprisingly), salmon returns in the Northwest and Alaska are similarly out of phase. In Figure 6, also from Anderson (1995), Columbia and Alaska salmon are shown to be out of phase, with the abundant 1950-1975 period in the Northwest corresponding with a very poor salmon period in Alaska. When Northwest stocks declined in the 1970's, Alaska's were soaring.

Mantua et al. (1996) identified the phase differences between Northwest and Alaska salmon stocks using observational data back to the early 1940s. They also quoted from the Pacific Fisherman Journal to demonstrate that the two areas have been out of phase throughout the century:

It is well known that the El Nino Southern Oscillation (ENSO) has a profound effect on climate in the Northwest. Most of the time, El Nino or "warm" events produce dry, mild winters in the Northwest, while La Nino or "cool" events coincide with wet, cool winters. El Nino winters are characterized by strong Aleutian Lows, while La Ninos are more conducive to westerly or northwesterly winds across the North Pacific, so it can be postulated that ENSO is the cause of the wind and current scenarios shown in Figure 4.

While El Ninos and La Ninas occur with about the same frequency over the historical record (the most reliable records go back about 150 years), there have been periods with far more El Ninos, and others with more La Ninas. The period from 1975-1994, for example, was dominated by El Ninos (six, versus only one true La Nina), whereas the 1947-74 years had more La Ninas. There may be a mechanism that varies over several decades that causes changes in frequency of ENSO phases, and thus in local climate and ocean conditions. Let us examine the most significant cyclical variations in the earth-atmosphere-ocean system to see if we can identify the causes of these variations.

The Global Connection

In the last decade, a great deal of research has focused on global-scale variations in ocean currents and their effects on climate conditions. Gray and Landsea (1993) described the global thermohaline circulation, or "conveyor belt," as a slow, steady movement of warm water from the Pacific, Indian and south Atlantic into the tropical and north Atlantic (see Figure 7). When this "conveyor" is active, the Atlantic warms and the Pacific and Indian oceans cool. During inactive periods, the reverse occurs. A warmer Atlantic should coincide with greater numbers of hurricanes and greater precipitation around the Atlantic rim, including the Sahel region of west Africa. A warmer Pacific would be linked to increased numbers of El Nino events, and thus generally dry conditions in the Northwest, while a cool Pacific should correspond to more La Ninas and wet, cool conditions in our region.

Table 1 is a "scorecard" of variations in hurricanes, precipitation and ENSO over the last century. The matchup with the previously cited periods in the Northwest is quite consistent, and suggests that the "conveyor" effect may be a major reason for the variations seen in our region. The fact that several of the parameters in the table changes suddenly about five years ago, just when the Northwest entered what has been a very wet period, adds further credence to this hypothesis.

Cyclical Variations

There are a number of cyclical or quasi-cyclical variations which are known to affect the atmosphere and oceans. Table 2 lists some of them. It is probable that a combination of the variations shown, as well as others of longer length and some not yet identified, produce the regional climate-ocean variations shown earlier.

Implications for Decision-Making

Over time, our understanding of the role of periodic variations in climate and ocean conditions will improve. Eventually we may even be able to predict these changes in advance. In any case, it is clear that these environmental variables have played a major role in salmon survival rates in this century.

Also apparent is that management/enhancement strategies will be more effective if this environmental "backdrop" is considered. A strategy that works quite well during periods of favorable climate and ocean conditions, such as occurred in the 1950s and 1960s, may be a dismal failure in the dry, warmer regime. In addition, the evaluation of the success of salmon management should consider these environmental conditions. What might be deemed a very poor year for salmon returns might in actuality be a successful one if it occurred during truly unfavorable climate/ocean conditions. On the other hand, a slight increase in salmon might not be cause for celebration if it occurred during a truly outstanding climatic year. Rather than simply report salmon returns, we must evaluate them in light of what the potential returns might have been.

References

Anderson, J.J. 1995. Decline and Recovery of Snake River Salmon. Information based on the CriSP research project. Testimony before the U.S. House of Representatives Subcommittee on Power and Water, June 3.

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

Ebbesmeyer, C. C., D. R. Cayan, D. R. McLain, F. H. Nichols, D. H. Peterson, and K. T. Redmond. 1991. 1976 step in the Pacific climate: forty environmental changes between 1968-1975 and 1977-1984, p. 120-141. In Proceedings Seventh Annual Pacific climate (PACLIM) Workshop, April 1990. Edited by J.L. Betancourt and V.L. Sharp. California Dept. of Water Resources. Interagency Ecological Studies Program Technical Report 26.

Ebbesmeyer, C.C., and R.M. Strickland. 1995. Oyster Condition and Climate: Evidence from Willapa Bay. Publication WSG-MR 95-02, Washington Sea Grant Program, University of Washington, Seattle, WA. 11p.

Gray, W.M., and C.W. Landsea. 1993. West African rainfall and Atlantic basin hurricane activity as proxy signals for Atlantic conveyor belt circulation strength. Conference on Hydrology, American Meteorological Society, Anaheim, California, January 1993.

Mantua, N.J., S.R. Hare, Y. Zhang, J.M. Wallace, and R.C. Francis. 1996. A Pacific interdecadal climate oscillation with impacts on salmon production. Submitted to the Bulletin of the American Meteorological Society.

Ware, D. M., and R. E. Thompson. 1991. Link between long-term variability in upwelling and fish production in the northeast Pacific Ocean. Can. J. Fish. Aquat. Sci. 48: 2296-2306.

Ware, D. M. 1995. A century and a half of change in the climate of the NE Pacific. Fish. Oceanogr. 4: 267-277. 

Figure 1. Water Year Precipitation (Oct.-Sept.), Oregon Coast climate division, 1896-1998, showing annual departures from 103-year average.

Figure 2. Annual precipitation, downtown Portland, 1910-1996 (5-year running average).

Figure 3. Bakun upwelling index, April-June average, at 45? N, 125? W.

Figure 4. Typical wind direction (small arrows) and surface currents (large arrows) during different types of conditions. (Top) strong Aleutian Low, southwesterly winds, strong Gulf of Alaska Current, weak California Current; (Bottom) westerly or northwesterly winds, west-to-east trans-Pacific Current, strong California Current, weak Alaska Current.
 

Figure 5. The correlation between the Pacific Northwest Index (PNI) and abundance of Columbia River bright spring chinook salmon (5-year running averages), reproduced from Anderson (1995).

Figure 6. Comparison of Columbia River spring chinook and Bristol Bay, Alaska sockeye salmon counts since 1940 (reproduced from Anderson 1995).

Figure 7. Schematic of the global thermohaline circulation, or "conveyor belt" (adapted from Gray and Landsea 1993).

Table 1. Evidence for multi-decadal shifts in weather and ocean conditions worldwide, including precipitation in the Pacific Northwest (PNW). These patterns are consistent with apparent changes in the global thermohaline circulation, or "conveyor belt." 

Table 2. Some of the major short-term cycles in the atmosphere and oceans.

Additional illustrations displayed during this presentation

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