The Effects of Pacific Decadal Oscillations on Salmon Populations
Kris Gerdeman
Global Climate Change 401
Project
Introduction:
The salmon industry on the Pacific North is among the most lucrative fisheries in the world. Many of the regions towns have their economy closely linked to the salmon populations. Currently the Pacific Northwest region has experienced a sharp decline in salmon catches. Some salmon species have been declared extinct, while many have been placed on the endangered species list. This has caused much controversy in the region regarding projects to bring back the salmon populations. The general consensus is that man is having a significant effect on the decline in salmon populations, through the damming of rivers, over fishing, and pollution. A new theory based on a climate phenomenon called the Pacific Decadal Oscillation (PDO) has shown that there is a significant link between salmon populations and this phenomenon. If this is the main force driving the decline of salmon populations in the Pacific Northwest than the current conservation efforts may be doomed to failure. To get a better picture of how PDO effects salmon populations it is important to look at how PDO affects different aspects of the climate that intern effect the salmon. Since the PDO is a relatively new discovery in climatologic it is not fully understood, but there is enough evidence to show its effects on a variety of aspects that influence salmon. This evidence will need to be used to help create new conservation plans for the salmon if they are to be successful.
Relevance:
PDO has been well documented since its discovery, but there are still many unknown factors surrounding it. Until its discovery there had been many studies regarding the effects of oceanic climate changes on a variety of species. These studies began to show a pattern, which eventually turned into the PDO. In 1996 the PDO was linked to salmon populations for the first time so it is not common knowledge among the people who inhabit the region. These are the people who will be most effected by this knowledge since it creates a dramatic shift in salmon catches from the Pacific Northwest to Alaska and then bask every 20-30 years. This shift has been well documented in fishing records, when one fishery is booming the other is experiencing all time lows. If a better understanding of PDO is gained then fishing operations can become more efficient and local economies can become much more stable.
Methods and Results:
To better understand the dynamics that surround the connection between PDO and salmon population one must first understand PDO. Then the aspects of PDO that related to salmon populations can be analyzed to gain a better picture of what can be done to aid conservation efforts.
PDO:
The Pacific Decadal Oscillation is a climate event not unlike El Nino events; both are reoccurring patterns of ocean atmosphere climate variability. Unlike El Nino events PDO last 20-30 years and is focused on the northern latitudes. The causes of PDO are not yet known nor are we able to predict its occurrence, but it has been shown to have an overwhelming effect on regional climate for 20-30 year periods. This is largely a result in the shift it causes in sea surface temperatures. These temperatures are what drive the rest of the climate effects associated with PDO. The sea surface temperatures of a PDO event are characterized by a pool of cold water in the Northern Pacific and a strip of warm water along the coast of North America (fig.2). This warm strip of water has an impact on the salmon populations for a variety of reasons including providing an environment that is rich in forage. The increased water temperatures are conducive to the growth of algal bloom the bottom level of the ecosystems food chain. From this growth comes a significant growth in the other levels of the food chain including the salmon. The PDO has impacts on other aspects of climate, but they are still being studied.
Ocean Currents:
Ocean currents are one of the areas currently being researched in respect to PDO. They have documented a shift in ocean currents during PDO, but have not yet been able to correlate it with PDO. These shifts contribute to an increase in bottom levels of the food chain due to their ability to transport zooplankton in large quantities to the region. Zooplankton are susceptible to currents in the upper levels of the ocean and are transported with their flow (fig.9). Another result of changes in ocean currents is upwelling and advection. Upwelling and advection result in large quantities of nutrients becoming available to the bottom components of the food chain. These mechanisms of PDO provide a large forage base for the salmon.
Stream Flow :
An increase in area regional stream flow has been recorded during positive PDO years, which indicates that there in increased precipitation during the winter months of these years. This has been explained by the effects that PDO has on regional weather patterns. Atmospheric pressure levels indicate that during PDO positives enhanced cyclonic flows of warm moist air predominate the central and coastal regions of Alaska, while in the Pacific Northwest anticyclonic flows which are dynamically unfavorable for precipitation (fig 4). The warm moist air that is present in Alaska is a result of the warm near coastal waters, which act as a feeding mechanism for the cyclonic pattern, much like the Gulf of Mexico provides moisture for the central United States. The increased winter precipitation results in large snow packs that are released during the spring thaw. Stream flow rates have been recorded at plus eighteen percent of their average flow rates in Alaska, while conversely in the Pacific Northwest stream flow rates fell by eight to fourteen percent (fig.5).
Salmon Populations
Salmon populations in the two regions tend to fluctuate according to PDO, but the Pacific Northwest populations tend to be out of phase with the PDO by a couple of years (fig 6). The Alaska populations tend to be much more compatible with PDO index, which could be due to a variety of factors. Alaskan salmon are mostly wild stocks that are less influenced by human induced factors. The salmon in the Pacific Northwest are mostly hatchery fish that face impediments such as dams and pollution in their environment. The PDO seem s to be the primary driving force in Alaskan salmon populations, while the Pacific Northwest salmon seem to have a variety of influences.
Discussion:
There seem to be a variety of influences on salmon populations, but none more important than PDO. With the signature sea surface temperature shifts, bring increased populations of bottom levels of the food chain. Oceans currents bringing nutrients and zooplankton to the near coastal region enhance this. Stream flow rates are also partially responsible for nutrient loading in the near coastal waters, which are now rich in biological productivity. This creates an ideal habitat for the juvenile salmon who are intent on feeding. There main sources of forage have been dramatically increased in number as the dramatic rise in the bottom levels of the food chain works its way up to the top level predators like the salmon. Stream flow rate increases also result in better reproduction rates due to increased spawning grounds and an increased habitat for the juvenile salmon before the return to the ocean. In the end one can see how PDO has a dramatic effect on salmon populations through enhancing many aspects vital to survival. The juvenile salmon come into the world with many advantages during positive PDO years, which lead to high survival rates and boosts the population.
In undertaking any conservation plans PDO must be considered as a vital part. This phenomenon presents challenges, but also opportunities to manage the salmon fisheries efficiently so that no more extinctions occur in the future. Current plans to save the salmon in the Pacific Northwest region are heavily based on removing four dams on the lower Snake River. These dams a vital to the regions economy because they provide an inland shipping channel that can be used to ship grain fro the Midwest to Asian markets. If the dams are not the major mechanism effecting salmon populations the dam removal could prove to be a costly mistake. Currently the Pacific Northwest is experiencing a negative PDO, which is as negative for salmon populations, as a positive PDO is good for salmon populations. The smaller populations experienced by this are could return to normal levels with a shift in the PDO index, which is near the end of its 20-30 years. It is evident that PDO is a major forcing mechanism on salmon populations, if not the major mechanism, so it needs to be the principle ingredient of any conservation plan.
Figures
Sources:
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