| Order Salmoniformes | Subfamily Salmoninae | ||
| Family Salmonidae | Genus Oncorhynchus |
In the Northeastern Pacific Ocean, the genus Oncorhynchus is represented by seven anadromous species. There are five species of salmon: sockeye, coho, chinook, chum and pink; and two species of trout: cutthroat and steelhead (Pearcy, 1992). Of these, the species that have the highest priority by the U.S government are the chinook, coho, chum, and sockeye salmon, and the steelhead; all of which are wide ranging in the Pacific Ocean (U.S. Fish and Wildlife Service, 1991).
The life history of salmon starts in the spawning grounds. All of these species spawn in fresh water streams, where most dig out spawning beds, or redds to deposit their eggs into. The eggs are usually blanketed with gravel and allowed to incubate slowly. As they develop into fry, the length of their stay in the fresh water natal ecosystem varies. Pink and chum fry begin their migration to the sea only a few days after hatching, while other species may remain in the fresh water for many years. The number of years spent in fresh water and the timing of migration varies among salmon according to species, age latitude, and the specific estuarine-coastal area that supports the stock (Pearcy, 1992).
For many of the species, migration to the ocean occurs at the smolt stage (U.S. Fish and Wildlife Service, 1991). In preparation for their actual transition from a fresh water environment to a salt-water environment, the juvenile salmon go through the process of smoltification. In the spring, the juvenile salmonids change into more streamline, silvery smolts, capable of tolerating high salinities. This smolitification process is triggered by many variables such as increased photoperiod, increased stream temperature, decreased flow, increased size, age, and an increase in population density (Pearcy, 1992). An alteration to any of these might throw off the timing of the process.
Not as much is known about the behavior of salmon throughout their lives in the ocean as there is about their freshwater lives. It is known that once in the ocean, the chinook salmon grow and fatten rapidly to a length of 31 to 39 inches in 2 to 3 years. Their diet changing from the drifting aquatic insects of the freshwater systems to a largely piscivorous menu. At the end of their stay in the open ocean, the fish have the necessary energy reserves needed to maintain their survival and gonad development on their migratory return to their natal spawning grounds (U.S. Fish and Wildlife Service, 1995).
In their return migrations, salmon travel long distances up streams in their quest to spawn. The migrating populations entering the rivers historically have consisted of mixed ages, from two to five years old in most species; but now, the majority of the fish in migrating populations are of the same age. As these salmon migrate, they do not feed, but rather rely upon stored body fat reserves from their time at sea for survival and maturation of their reproductive organs. They are fairly faithful to home streams, using visual and chemical senses to locate them. In cases of high-water years, some may ascend other streams (U.S. Fish and Wildlife Service, 1995).
The life cycle is complete when the journey comes to an end, and the salmon have spawned. All the salmon die after spawning, but the steelhead may return to the ocean and spawn again in following years (U.S. Fish and Wildlife Service, 1991).
With such a complex life cycle, alternating between fresh and salty ecosystems, and traveling thousands of miles within their existence, there are many opportunities for a disturbance in their niche. Trying to determine what changes are causing disruptions to their abundance is not a simple task.
The two of the more significant problems facing the salmon in their freshwater migrations are the destruction of their streams and spawning grounds, and artificial barriers obstructing them from even reaching their spawning grounds. These problems appear primarily in the form of dams, logging, pollution, mining, and human development.
This river transformation can be bad for the salmon in many ways. The warmer water is not as suitable for salmon survival as the colder water system was. Because warm water does not hold as much oxygen as cold water does, species that can survive in more anaerobic environments begin to thrive. These species may be predators to juvenile salmon, or may simply out compete them for food. In addition, non-native forms of algae and aquatic macrophytes may thrive and change both the integrity of the water and the biotic structure of the system.
An obvious shortcoming of dams and the stagnant water that they produce is the alteration of the current within the stream. Juvenile salmon may depend upon the high velocity flow to carry them to sea in their initial migration, but when reservoirs are present, this journey is prolonged, increasing the chance that the journey is not completed at all.
Finally, perhaps the biggest adverse quality of a dam is the physical barrier that it represents. Many dams utilize fish ladders as a method of allowing migrating salmon to pass, and continue on their way; however, there are still many dams that do not posses such a mechanism, forcing a premature stopping point. Obviously, juvenile salmon cannot get to the ocean, and the adults cannot get to the spawning grounds. In some instances, the salmon that are hold up above and below the dam are collected and physically trucked back and forth. This method, though better than no action at all, still yields large losses due to stress and predation.
In Californiaís Sacramento River, a barrier presented by a dam blocks the passage of all salmon, forcing them to spawn below the dam in conditions that may not always be ideal for such and event. Spring and fall chinook salmon typically breed at different times of the year, and in different elevations of the river. Normally interbreeding is not a problem, but because the salmon are forced to breed in the same place, this has become more common. As a result, the characteristics that distinguished the two from each other are disappearing, and a new hybrid species is being formed (U.S. Fish and Wildlife Service, 1995).
Recently, efforts have been made to combat these effects by the removal of dams, or allowing the water in the reservoirs to be drawn down when the salmon are migrating. This is good for the salmon, however, it creates its own set of problems by destroying the non-native biodiversity that has established itself and has thrived for many of years as a result of the dams.
As the climate warms, there will be a higher demand for water for both agriculture and domestic purposes. This will increase the necessities for reservoirs to draw water from. Current efforts to restore salmon populations will likely dwindle as the human necessity for water increases, and becomes a higher priority to many people.
In this process of clear cutting, a whole unit of timber is cut, regardless of size or age, leaving a barren slope of stumps and exposed ground. Essentially, everything is cut, but only some of the harvest is used. This practice affects the physical characteristics of both the rivers and the spawning grounds of salmon, contributing to their lower production.
The exposed slope that results from clear cutting has little vegetation to slow water flowing toward the rivers when the rain comes, allowing a higher incidence of flash floods, landslides, and in general, a high amount of sediment loading in the streams. The salmon need a gravel substrate to reproduce, and runoff from clear-cutting may often disrupt this condition. By contributing silt and organic material, the water quality of the stream deteriorates. This is unfavorable to the salmon and many of the organisms that the juveniles feed upon.
In addition to the change in water quality and riverbed substrate, the shade that the forests once provided helped to reduce warming of pools by the sun, and gave refuge for juveniles to hide in when threatened. When these are removed, the water warms quickly in the shallow spawning habitat, possibly exceeding the optimal temperatures for development or even drying the streambed completely.
The demand for timber will likely always exist, but there are different ways of harvesting that are less destructive to aquatic and terrestrial habitats. In the Pacific Northwest, logging practices are changing from the popular clear cutting method to a selective cutting method. With this practice, instead of cutting everything, the timber is cut in sections, taking, for the most part, only what is needed, leaving the rest in clumps for the wildlife. Ideally this method reduces erosion and leaves enough habitat so that the ecosystem is not altered too much. In actuality, this is better than clear cutting, but some of the same problems with runoff are experienced, and the practice is more expensive and time consuming.
In addition to the use of selective cutting, laws now require that riparian boundaries be established. This means that timber cannot be cut within a certain number of feet of a wetland. This is good for wildlife as well as the streams.
With the warming of the atmosphere, this region of North America may begin to receive more precipitation than usual due to the warming of the ocean. If this occurs, the runoff from the disturbed watersheds will be intensified. For the most part, however, the effects that timber harvesting have on the salmon will not likely be the greatest concern in the future. Because timber in the northwest is depleted so quickly, and takes so long to grow back (approximately 1 foot per year for douglas fir), the ability of these forests to absorb carbon will be greatly reduced. The rate of harvest seems to exceed the rate of regeneration, while at the same time the amount of carbon in the atmosphere is increasing.
The practice of mineral extraction also tends to leave a large disturbance in the ecosystem, which can have an effect, much the same as logging practices did. Increased runoff and a higher frequency of landslides may occur.
Pollution does not necessarily only change the pH of the system. Inputs of many other types of chemicals may prove lethal to all organisms, or may induce genetic defects, as is currently occurring frogs in many of the western states.
Farming has a great impact on aquatic systems for many reasons. In many cases, the most productive land comes from draining wet lands. Wetlands act as sponges to absorb pollutants and excess runoff. In general, water that leaves a wetland is of higher quality than water that enters the wetland. Many times lush vegetation or forests are cut down to expose the soil for agricultural purposes. Practices such as these invite the same problems of runoff and lack of shading that are characteristics of logging.
Perhaps one of the larger consequences of agriculture is approached by the use of fertilizers and pesticides. Fertilizers input nitrogen and phosphorus into aquatic systems. This provokes change because aquatic systems are typically limiting in phosphorus, meaning that an input of phosphorus will allow domination by certain plants and algae. This can easily turn a pristine stream into a mixture of pea soup from an explosion of primary producers. The same effects can occur from sewage that may runoff from animals, since manure is high in phosphorus.
The development of roads that require culverts can create barriers to salmon migrations. High in the mountains, there are few spots that are not accessible by roads. This means that streams need to have culverts to allow their flow to remain constant. Often, however, the culvert is installed at too high a gradient for fish to pass, or there contains a large drop that is too high for the fish to navigate. Developments such as these are not often considered as problems, but can have a huge impact on salmon migrations.
Along the southern coast of California, where a high amount of nutrient upwelling occurs, the ocean temperature has already increased from 12.5 to 14.1 degrees Celsius. This warming has acted to increase the thermocline, reducing the contact that the nutrient rich, cold lower layers of the ocean have with the atmosphere, in turn reducing the upwelling. This has ultimately decreased the zoo plankton populations in this zone of the subarctic region (Climate Change and its Effects on Salmon in the Pacific Ocean).
Salmon have a unique migration behavior. They have been shown to migrate north in the winter toward the colder waters, and to migrate south in the summer toward the warmer waters. This reverse migration raises some interesting questions about what will happen as the ocean warms. It is predicted that doubling the CO2 concentration of the atmosphere will warm the ocean 3 degrees Celsius, reducing the size of the subarctic region of the Pacific, which encompasses the habitat suitable for salmon survival (Welch, 1998). Knowing this, the question is raised as to what will happen to those stocks of salmon that migrate up the rivers on the southern edge of the subarctic region? Will they be forced to migrate up streams that are higher in latitude and abandon their own natal streams? A reaction like this will have drastic effects to the Pacific Northwest through its economy, culture and biodiversity.
Changes in current patterns and mixing of the water column due to changes in the wind pattern in the atmosphere will also directly effect fish in general. A change in currents will affect the transfer of eggs and larval fish from the spawning grounds to the nursery grounds in non-salmon species. The lack of these species will increase the frequency of predation upon salmon juveniles. The mixing in the water column effects the concentration of nutrients, in particular nitrogen and phosphorus, which are essential in photosynthesis by primary producers (Climate Change and its Effects on Salmon in the Pacific Ocean).
There are many recent studies that deal with this very phenomenon, calling it the Pacific Decadal Oscillation (PDO). In these studies, PDO has been shown to affect sea surface and land temperatures, sea level pressure and stream flow from Alaska to California (Hare 1996, Zhang 1996, Mantua et al. 1997); the coastal range that encompasses the migrations of the salmon (see figure 1). It has been further shown that PDO occurs independently of the El Nino Southern Oscillation phenomenon.
There is a recent study in which an apparent opposite response in salmon production was noted between the California and Alaska Current domains (Francis & Sibley, 1991). This demonstration coincides with the historical inversely favorable salmon production experienced by the coasts of Alaska and California (Chelton & Davis, 1982). In the past 20 years, Alaska has enjoyed high salmon productivity, while the Pacific Northwest has suffered losses. In the era before that, California enjoyed high productivity and Alaska suffered.
An observation has been made that zooplankton biomass and its distribution around the Subarctic Gyre has provided as favorable feeding condition for smolts. At the same time, the opposite is occurring off the coast of the Pacific Northwest, probably due to stratification of California Current waters and the lack of products imported by wind drift. Within a period of 43 years, 80% of the biomass of zooplankton off the coast of California has disappeared, in conjunction with the warming of the ocean. This presents a major disturbance to the food web that smolts need to survive (Climate Change and its Effects on Salmon in the Pacific Ocean).
In the open ocean salmon keep a constant movement, believed to enable the salmon to make the best use of the abundant food resources available in its major ocean ecosystem (see figure 2). Because of this behavior, it is likely that salmon survival and growth are a good indicator of the ocean conditions on a large scale. It is probable that large changes in ocean temperatures and productivity would be indicated by changes in the salmon abundance (Climate Change and its Effects on Salmon in the Pacific Ocean). This suggests that salmon tend to move toward areas of the ocean that are rich in food sources, explaining why the salmon production is unusually high off the coast of Alaska, and unusually low off the coast of California (qtd. in Taylor, 1997).
In the occurrence of El Nino, the major oceanic changes occur in the form of elevated sea surface temperatures, reduced offshore flow, and reduced primary productivity (Climate Change and its Effects on Salmon in the Pacific Ocean). This is consistent with the longer scale hypothesis described above, but on a much more temporary scale. As noted, there are many similarities derived between PDO and the El Nino phenomenon. Knowing that El Nino increases the layer of warm surface water, it can be assumed that this probably increases the size of the thermocline, limiting nutrient circulation in much the same manner as PDO does. This explains why PDO was initially thought to have been related to El Nino events, because they have much the same effect on salmon production.
Another proposed explanation for Alaska's rise in salmon production in the late 1970s was derived from an increase in marine survival of migrating salmon during their final winter at sea. Unusually warm temperatures in Alaskan waters may have altered both the migration paths and timing, reducing the vulnerability to attack by predators (qtd. in Hare & Francis, 1994).
The greatest concern appears with the loss of native salmon. Although there are many hatchery fish released, there are still significant losses to native populations from sport and commercial fishing. Hatchery stock can introduce disease to native stocks, and can themselves have weaker immunities to some diseases. The loss of a native run one year will result in poor returns 3 years later. Losses of runs in many consecutive years can result in the potential population extinction (U.S. Fish and Wildlife Service, 1995).
To reflect just how the industry is suffering, here are some numbers. In 1980, the commercial harvest of salmon for Washington, Oregon and California was valued at $200,000,000, but in 1990 was only valued at $120,000,000 (U.S. Fish and Wildlife Service, 1991). Figures for Alaska would of course show improvements.
Botsford and Brittnacher,1998: Chinook and Salmon Viability. Conservation Biology. 12, 65-78.
Chelton, D. B. and R. E. Davis, 1982: Monthly mean sea level variability along the western coast of North America. J. Phys. Oceanogr 12, 757-784.
Climate Change and it's Effects on Salmon in the Pacific Ocean. Online: http://gladstone.uoregon.edu/~joolee/
Francis, R. C. and S. R. Hare, 1994: Decadal-scale regime shifts in the large marine ecosystems of the Northeast Pacific: A case for historical science. Fish. Oceanogr 3, 279-291.
Francis, R. C. and T. H. Sibley, 1991: Climate change and fisheries: what are the real issues? NW Env. Journal. 7, 295-307.
Hare, S. R. and R. C. Francis, 1994: Climate change and salmon production in the northeast Pacific Ocean. In R. J. Beamish (ed.) Climate Change and Northern Fish Populations. Can. Spec. Publ. Fish. Aquat. Sci. 121. Online: http://www.iphc.washington.edu/PAGES/IPHC/Staff/hare/html/papers/hare-francis/harefran.html
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.
Hollowed, A. B. and W. S. Wooster, 1995: Decadal-scale variations in the eastern subarctic pacific. II. Response of Northeast Pacific fish stocks. p. 373-385 in R.J. Beamish [ed.] Climate change and northern fish populations. Can. Spec. Publ. Fish. Aquat. Sci. 121.
Mantua, N. J., S. R. Hare, Y. Zhang, J. M. Wallace, and R. C. Francis, 1997: A Pacific interdecadal climate oscillation with impacts on salmon production. Bull. Am. Met. Soc. 78, 1069-1079.
Pearcy, W. G., 1992: Ocean Ecology of North Pacific Salmonids. University of Washington. Seattle, WA.
Taylor, G. H., 1997: Long-term Climate Trends and Salmon Population. Online: http://www.ocs.orst.edu/reports/climate_fish.html
U.S. Fish and Wildlife Service, 1991: Pacific Salmon Management. U.S. Fish and Wildlife Service: Portland, Oregon.
U.S. Fish and Wildlife Service, 1995: Recovery Plan for the Sacramento/ San Joaquin Delta Native Fishes. U.S. Fish and Wildlife Service: Portland, Oregon. pp. 9-11, 96-106.
Welch, D. 1998. Seminar: Oceanographic controls on the distribution of Pacific Salmon and possible impacts of future climate changes on salmon production. Online: http://www.ios.bc.ca/ios/seminars/feb29
Figure 1
Generalized currents and upper zone domains in the subarctic Pacific Ocean.
(qtd. in Pearcy, 1992)
Figure 2
Inferred February distribution of sockeye, chum, pink and coho salmon in the North Pacific:
dotted area indicates species present; slanted lines indicate main concentration.
(Pearcy, 1992)
