Salmon: A Keystone Species
The term salmon incorporates several species of fish which are native to freshwater tributaries of the Pacific and North Atlantic Oceans. This fish is considered an anadromous species, which means that it is born and spends its early life in freshwater before migrating to the ocean and finally returning to its place of birth for spawning.
In some areas of North America, specifically the Pacific Northwest and US state of Alaska, the salmon is a keystone species. This means that it plays an important role in maintaining ecological organization and structure. Salmon spend their adult lives in the ocean and consume its nutrients, such as phosphorus and nitrogen. When this fish returns to freshwater tributaries, it becomes a major nutritional food source for bears, birds, and otters. These animals go on to deposit nutrient-rich feces throughout the forest floor, providing an estimated 24% of the nitrogen available in the surrounding soil. Many of these nutrients are then carried downstream to estuary ecosystems, benefitting other species as well.
The Salmon Life Cycle
The life cycle of the salmon begins and ends in the riverbed where it was born (assuming it does not die before or during the migration). After the eggs are left among the gravel in the freshwater tributary, where they incubate for anywhere from 2 to 6 months. Salmon begin their lives as sac fry, or small larvae with the yolk sac still attached. The larvae remain hidden between the rocks, feeding on the yolk sac until it is gone. Without their attached food source, the baby salmon, now referred to as fry, are forced to leave the protection of the rocks in search of plankton. By the August after the Great Salmon Run, the fry have developed to the parr, or young adult, stage. At this point in development, the fish are covered in stripes and spots for camouflage. Their diet graduates to small invertebrates. The parr stage may last for as long as 3 years.
Once their camouflage begins to change, the parr enter the smolt stage. At this point, they are ready to make the journey to the ocean. In order to survive the change to saltwater, the smolt go through physiological changes to their osmoregulation. During this time, smolt stay in the estuary regions of the river because of the lower salt content. Additionally, their physical appearance changes to a silver color, which serves to distract ocean predators.
After reaching between 6 and 8 inches, the smolt finally enter the sea and become post-smolt. For their first year in the ocean, post-smolt travel to the deep, open waters in large schools (groups of fish) for feeding. Post-smolt spend approximately the next 4 years in the ocean before going back to their birthplace.
What Is The Great Salmon Run?
The Great Salmon Run refers to the time when salmon leave their ocean home and return to the freshwater tributaries where they were born. After successfully making the trip, these fish spawn, lay their eggs in the gravel riverbed, and then die. The Great Salmon Run happens every year, generally between September and November. It is a major event for North American indigenous peoples, sporting fishermen, and many wildlife species (particularly bears and eagles).
How Do Salmons Find Their Birthplace Again?
Perhaps one of the most interesting aspects of the Great Salmon Run is how the salmon are able to return to their birthplace after so many years. Several scientific theories attempt to explain this phenomenon. One of these theories is that salmon are able to use geomagnetic signals from the earth’s magnetic field. These magnetic signals point the salmon in the right direction back to the mouth of the river where they were born.
Another theory is based on the fact that salmons have an unusually strong sense of smell. Some researchers believe that once the salmon comes close to their home estuary, they smell their natal freshwater. The theory further explains that salmon remember this smell because it was imprinted in their memory during the smolt phase.
Other possible navigation tools include: the sun, water temperature, salinity levels, and the presence of other chemicals. Some evidence does suggest, however, that the sun plays little to no role in salmon migration. This is because researchers have observed salmon migrating both at night and in cloudy conditions.
Not all salmon make it back to their natal hatching grounds. Sometimes, these fish travel up neighboring tributaries, which contributes to new salmon populations. Researchers have discovered that genetics plays a part in whether salmon find their way home or veer off the correct route.
Difficulties Of The Great Salmon Run
During their time in the ocean, salmon build significant muscle mass, which is extremely important for the difficult migration ahead. It develops both white muscles for bursts of energy needed for jumping and running and red muscles for longer activities. These muscles are necessary for making the long ocean journey and for fighting rapids and jumping waterfalls once in the freshwater tributary.
During the run, salmon do not eat. All of their energy is used in fighting strong currents for hundreds of miles. In the route, salmon are faced with many obstacles, including tall waterfalls. To overcome these, salmon can jump as high as 12 feet into the air. In many places along the river, artificial dams have been built. These dams were found detrimental to the salmon’s success. To combat this, people have now installed fish ladders, which assist these fish and others in crossing the dam. Fishermen, other animal predators, diseases, and parasites also pose a threat to salmon as they make their way up the river.
The End Of The Great Salmon Run
Only the strongest and most persistent salmon reach the end of the run. Statistics show between 3% and 90% of the original salmon migration group are successful. After arriving, the female salmon work on building a nest, called a redd, in the gravel. She does this by using her tail as a shovel to dig away gravel that is then carried down the current. The female salmon then deposits up to 5,000 eggs in as many as 7 different redds. One or more male salmon approach the nest to deposit sperm over the eggs. The female then covers the fertilized eggs with a layer of gravel.
Before re-entering the river, the salmon do not undergo a biological process to readjust to freshwater. After some time, the freshwater begins to destroy the salmon’s immune system and organs. Shortly after spawning, the salmon deteriorate and die.