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Major Topics:

1. Overview

2. El Niņo/La Niņa

3. Aleutian Low

4. Upwelling and Coastal Productivity

5. Pacific Decadal Oscillation

6. Ocean conditions and strategies for increasing NW salmon runs in the next 5-to-7 years

 

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Overview

Salmon were once a staple in the diet and a centerpiece in the economies and cultures of Native Americans and early European-American settlers in the Pacific Northwest (PNW). Although their numbers dwindled in the twentieth Century, salmon were abundant enough to support thriving sport and commercial fishing industries into the 1980s. In spite of the apparent abundance of salmon, numerous distinct PNW salmon populations went extinct in the twentieth century, and in the 1990s many more remained at risk of. By the 1990s most wild PNW salmon stocks were at historically low abundance levels, and the remaining salmon fisheries were heavily dependent upon hatchery reproduction programs rather than natural reproduction in rivers and streams.

The rapid decline in the abundance, diversity, and distribution of PNW salmon is now commonly referred to as "the Pacific Northwest salmon crisis," a crisis that was over a century in the making, and is likely to remain for many decades to come. Despite their presently dire circumstances, salmon remain a cultural and ecological icon for the region, and they continue to have considerable value in a variety of ways. In a few parts of the PNW, salmon remain abundant enough to support fisheries that make significant contributions to Native American tribes for cultural and ceremonial purposes, and to local economies through commercial, Native American and recreational fisheries. Causes for the NW salmon crisis have been widely known for years, and the "short list" has been boiled down to the Four H's: Hydropower, Habitat, Harvest and Hatcheries. More recently, environmental changes associated with climate variability have been added as another important factor in the health and abundance of NW salmon.

Climate affects every part of a salmon's physical environment (Salmon Lifecycle - Figure 1). Incubating eggs in gravel nests are vulnerable to stream-scouring floods, yet require that stream levels are sufficiently high to cover them with water until they hatch. Developing juveniles (fry and parr) require relatively cool, oxygen-rich flows to survive the warm, low streamflow summer and fall seasons typical of PNW streams; high-velocity winter flows or freezing stream temperatures can be problematic for incubating eggs, fry and parr. Migrating smolts require sufficient flows to aid them on their journey from streams, to estuaries and into the ocean. In some years estuaries and the coastal ocean greet migrating smolts with an abundant food supply and relatively light predation pressures, in others food is scarce while predation pressure is intense.

Studies suggest that it is during and shortly after the smolt migration to the marine environment that much of the year-to-year variability in salmon production occurs in response to climate variations.

Though scientists are not certain of all the factors controlling salmon marine survival in the Pacific Northwest, several ocean-climate events have been linked with fluctuations in Northwest salmon health and abundance. These include: El Niņo/La Niņa, the Pacific Decadal Oscillation, the Aleutian Low, and coastal upwelling. Each of these features of the climate system influences the character and quality of marine (and freshwater) habitat experienced by Pacific salmon.

Cooler than average coastal ocean temperatures prevailed from the mid-1940's through 1976, while relatively warm conditions prevailed from 1925-to-1945 and again from 1977-to-1998. The decades-long climate cycles have been linked with the Pacific Decadal Oscillation, an especially long-lived El Niņo-like feature of Pacific climate. In the past century, warm ocean temperature eras coincided with relatively poor ocean conditions for many Pacific Northwest salmon stocks, while cool ocean temperature eras coincided with relatively good ocean conditions for Northwest salmon.

Pacific climate changes beginning in late 1998 indicate that the post-1977 era of unusually warm coastal ocean temperatures may have ended. Coincident with the demise of the extreme 1997-98 (tropical) El Niņo, ocean temperatures all along the Pacific coast of North America cooled to near or below average values, and this situation has generally persisted to date. Recent climate forecasts, largely based on expectations for continued but weakening (tropical) La Niņa conditions, suggest that the cool coastal SSTs are likely to persist through at least the spring, and probably through the summer, of 2000. Beyond the coming summer, there are no strong indications that major changes in the ocean state should be expected. If the recent past is a useful guide to the future, one might surmise that there is a reasonably good chance that cool coastal ocean temperatures will persist for the next twenty to thirty years. On the other hand, there has been no demonstrated skill in North Pacific climate predictions beyond about one year lead times. Thus, a lack of understanding for Pacific interdecadal climate changes bases 20-to-30 year forecasts more on faith than science. With a focus on the next 5-to-7 years, one may be much more confident in predicting that coastal ocean temperatures and coastal marine habitat quality will continue varying within and between seasons, as well as within and between years.

An expanded discussion of the impact of varying ocean conditions on Pacific salmon follows.

 

El Niņo/La Niņa

El Niņo has received a lot of bad press for causing warm biologically unproductive conditions in the coastal waters of the Northeast Pacific Ocean. Especially intense El Niņo events in 1982/83 and 1997/98 were connected with exceptionally warm coastal waters from Baja California to the Gulf of Alaska. Scientists have determined that El Niņo plays an important role in North Pacific climate, but it is only one piece of a more complicated climate-ecology puzzle.

El Niņo is Earth's dominant source of year-to-year climate variations. This phenomenon is understood to be a natural part of this planet's climate that spontaneously arises from interactions between Pacific Trade Winds and ocean surface temperatures and currents near the equator. It is important to keep in mind that the "essence" of El Niņo is contained within the tropics, thousands of miles to the south of where any North Pacific salmon ever swims. However, swings between El Niņo, and its cold counterpart La Niņa, have consequences for climate around the world. Simply put, massive changes in the distribution of tropical rainfall, which are directly related to changing ocean temperatures in the tropical Pacific, influence atmospheric pressure patterns, winds and storm tracks thousands of miles away. These changes over the North Pacific and North America are especially strong in the months from October through March. During these months, El Niņo influences the character of the dominant feature of North Pacific weather, the Aleutian Low pressure cell.

 

 

Aleutian Low

The Aleutian Low (Fig.2) is a semi-permanent atmospheric pressure cell that settles over much of the North Pacific from late fall to spring. The exact position and intensity of the Aleutian Low varies greatly from week-to-week, year-to-year, and even decade-to-decade.

An intense Aleutian Low favors northward winds along the Pacific coast, and causes relatively dry, mild winter and spring weather. In the left panel of Figure 1 is a map with contours for atmospheric sea level pressures from October 1997-March 1998, at the height of the 1997/98 El Niņo. This was a period with an exceptionally intense Aleutian Low, which can be identified as the bulls-eye of low pressure values centered over the Aleutian Islands. Northern Hemisphere surface winds blow in a direction that almost parallels the contour lines but angled slightly toward lower pressures, counter-clockwise around the lows and clockwise around the highs. Of special significance to the Pacific Northwest's coastal ocean is the fact that relatively warm northward blowing near-shore winds caused by a strong Aleutian Low tend to drive surface waters onshore (to the right of the wind direction), piling up relatively warm nutrient poor water in the coastal zone.

On the other hand, periods with a relatively weak Aleutian Low favor onshore coastal winds that move surface currents to the south. In the right panel of Figure 1 is a contour map for sea level pressures from October 1971-March 1972, a La Niņa period with a weak Aleutian Low. Notice that in this year there were two relatively weak low pressure centers in the North Pacific, one near the coast of Asia and the other in the Gulf of Alaska. Also note the strong high pressure cell located off the coast of Northern California. Periods with a weak Aleutian Low typically bring relatively wet and cool winters to the Pacific Northwest region. In weak circulation periods the coastal ocean surface waters are cooler, less stratified and richer in nutrients because onshore currents are relatively weak. Off the coast of Northern California the strong high pressure cell causes southward upwelling winds even in the winter months.

Pacific climate events in the past few years have followed an often observed pattern: the 1997/98 tropical El Niņo favored an intense Aleutian Low, while the 1998-2000 La Niņa has favored a relatively weak Aleutian Low. Additionally, El Niņo sends coastal currents from the tropics that travel northward along the coast of North America. These also warm and stratify the near-shore coastal waters, reinforcing the wind-driven warming and stratification brought by the intense Aleutian Low. Likewise, La Niņa produces coastal currents that cool and weaken the stratification in the surface waters, reinforcing the La Niņa-influenced, wind-driven cooling. In both El Niņo and La Niņa, the Pacific Northwest's coastal ocean is affected by changes in the oceanic and atmospheric circulation that can be traced to the equatorial Pacific--a long-range double whammy.

The maps shown in Figure 3 highlight some of the dramatic year-to-year changes that El Niņo and La Niņa can bring to the west coast's ocean. In the left panel are observed sea surface temperatures in December 1997, near the peak of the last El Niņo event. The contour lines and shading depict temperatures as deviations from the long term average. Actual temperature values are shown with the larger numbers. West coast sea temperatures were 3-to-5 degrees Fahrenheit above average in a thick layer of warm water that extended to depths of 50-to-100 meters below the surface. The wide belt of warm and sharply stratified surface waters had been present since the previous summer.

In May and June of 1998 the tropical El Niņo was quickly replaced by La Niņa conditions, a climatic switch that set the stage for a dramatic ocean cooling along the west coast of North America. Coastal ocean temperatures in December of 1998 (shown in the center panel) were actually a bit colder than the long term average, some 3 to 5 degrees Fahrenheit lower than those observed 12 months prior. An important factor behind this cooling was the prevalence of a weak Aleutian Low from October 1998 through April 1999. Throughout this period, North Pacific barometric sea level pressures often resembled those in the right panel of Figure 2 . During December of 1999 (right panel of Figure 3 ) ocean temperatures were again mostly near to below the long-term average. This second year of cool coastal ocean temperatures is clearly related to a second fall and early winter with a weak Aleutian Low, which in turn has been influenced by the second consecutive year of tropical La Niņa conditions.

 

 

Upwelling and Coastal Productivity

As the spring/summer upwelling season approaches, the coastal ocean is often primed for either rich or poor biological productivity. Clearly, the coastal ecosystem will be strongly influenced by the presence or lack of upwelling winds, but it will also depend upon the character of the preceding winter/spring Aleutian Low circulation and related ocean conditions. Following a weak Aleutian Low, cool and weakly stratified surface waters favor an especially productive food-web because upwelling winds are able to tap into the nutrient rich subsurface waters with little resistance. Conversely, following an intense Aleutian Low, warm and sharply stratified surface waters tend to have poor biological productivity even in the presence of strong upwelling winds. The warm stratified upper ocean effectively caps the nutrient rich waters at depth. Upwelling in a sharply stratified ocean simply recycles the same depleted water in the surface layer over and over again, never replenishing the nutrients that are quickly used up by phytoplankton.

Low phytoplankton production cascades through the marine food-web (Fig. 4). Zooplankton and small fish that feed on plankton become scarce, resulting in low food production for salmon. For juvenile salmon, this low productivity may result in slow growth which can also make them more vulnerable to predation, leading to lower smolt survival rates. Also, during warm years many fish from subtropical waters, such as mackerel, migrate into coastal waters of the Pacific Northwest from the south. These fish may compete with young salmon for food, and in some cases even target juvenile salmon as prey.

 

 

Pacific Decadal Oscillation

Typically, individual El Niņo or La Niņa events play out over the course of 8 to 14 months. However, climate records kept over the past century document decades-long warm and cool eras in the Pacific Northwest's coastal ocean that are superimposed upon the year-to-year changes associated with El Niņo and La Niņa. Recent research points to a second important player in North Pacific climate, the recently named Pacific Decadal Oscillation, or PDO.

The PDO has been described as a long-lived El Niņo-like pattern of Pacific climate variability. Extremes in the PDO pattern are marked by most of the same Pacific climate changes caused by El Niņo and La Niņa. Two main features distinguish the PDO from El Niņo. First, typical PDO "events" are much longer-lived than a typical El Niņo - in the past century major PDO regimes have persisted for 20-to-30 years. Second, evidence of the PDO is most visible in the North Pacific/North America sector, while secondary signatures exist in the tropics - the opposite is true for El Niņo. In short, warm and cool eras of the PDO do most of the same things to Pacific climate that swings between El Niņo and La Niņa do, but the PDO does them for 20-to-30 years at a time.

The record of coastal sea temperatures shown in Figure 5 illustrates some of the impacts of PDO climate cycles. This data comes from the west coast of Vancouver Island near a lighthouse at Amphitrite Point . The record is presented in two ways: monthly deviations from the long term mean are shown with the thin line, and 5-year running averages are shown with the thick line. The month-to-month temperature fluctuations can be as large as a few degrees, while decade-to-decade variations are more typically about +/- 1 degree Fahrenheit. Temperature records from stations along most of the Pacific Coast show the same prolonged periods of above average temperatures in the early 1940's, then again from 1977-1998. Coastal temperatures were mostly lower than average from the mid-1940's through 1976. Since the fall of 1998, sea surface temperatures at Amphitrite Point have been near or below average in most every month to date.

Several independent studies find evidence for just two full PDO cycles in the past century: cool coastal ocean regimes for the PNW prevailed from about 1890-1924 and again from 1947-1976, while warm coastal ocean regimes dominated from 1925-1946 and from 1977 through 1998. Climate reconstructions based on tree-rings from the Pacific Northwest suggest that the PDO has been an important player in Pacific climate for at least the past few centuries, and that 20-to-30 year climate regimes are normal.

Because causes for PDO climate cycles are not understood, it is now impossible to predict a PDO change before it occurs, or to accurately detect a PDO change while it occurs. The recent shifts to cooler ocean temperatures along the Pacific coast are one of the signals we expect to see with a shift from a warm to cool PDO regime. However, no one is certain if the recent cooling will fade away when the current La Niņa leaves us--which is expected sometime in the summer or fall of 2000--or whether this coastal ocean cooling will stick around for the next 20 or 30 years as part of a cool PDO regime.

A number of recent studies find evidence for important decade-to-decade climate impacts on Pacific salmon. Essentially, the El Niņo and La Niņa impacts described above appear to play out over 20-to-30 year periods because of PDO climate cycles. An interesting finding is the that the biologically unproductive periods in the Pacific Northwest coincide with production booms in the Gulf of Alaska (Fig.6). Likewise, periods with especially high coastal ocean (and salmon) production in the northwest have coincided with low-production eras in Alaska. This north-south "inverse" production pattern is thought to arise in part because a warmer, more stratified ocean in the coastal waters of Alaska benefits phytoplankton and zooplankton production. The cool waters in the north are most always nutrient rich, but strong stratification is needed to keep phytoplankton near the surface where the energy from the high-latitude sunshine is limited. In the Pacific Northwest's coastal ocean, lack of nutrients from increased stratification is most often the limiting factor in phytoplankton production.


 

Ocean conditions and strategies for increasing NW salmon runs in the next 5-to-7 years

Given the growing body of evidence that ocean conditions play an important role in regulating salmon health and abundance, what management steps might be taken to improve NW salmon populations in the next 5-to-7 years and beyond? It seems that climate insurance for NW salmon would be provided by adopting management strategies aimed at restoring some of the characteristics possessed by healthy wild salmon populations. Although the mechanisms are not completely understood, wild salmon evolved behaviors that allowed them to persist and thrive under variable stream and ocean conditions. Excessive harvest of individual stocks, the widespread development of salmon hatcheries in the region, and habitat loss and degradation, have combined to greatly simplify the complex population structures and behaviors that salmon evolved over millennia. In short, management actions taken to restore some of the wild salmon characteristics that have been lost in the past century are likely to be the best routes for minimizing the negative impacts of poor ocean conditions, and may also prove beneficial during periods of especially good ocean conditions. There should be little doubt that the ocean environment will continue to vary between favorable and unfavorable conditions for NW salmon at both year-to-year and decade-to-decade time scales.

 

 


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