Great Nutrient Cycle

As salmon grow in the ocean environment, they accumulate marine nutrients, storing them in their bodies. They then transport those nutrients back to their stream of origin when it is their time to spawn, die and decay. Salmon release their eggs and milt back into the freshwater to re-seed the cycle. Eggs that don't get buried in the gravel become immediately available as food for other fish, birds and insects. After spawning the salmon die, and as they decay valuable nutrients are released. These nutrients fertilize the water that feeds the developing salmon, filter-feeding insects, and aquatic and terrestrial plant life. This process of salmon accumulating marine nutrients and returning them to freshwater streams has been referred to as "the great nutrient cycle."

Natal Spawners 

One of the most amazing facts about Pacific salmon is their ability to return to their "natal" or home stream or lake. Salmon are thought to use several navigation aids to find their way back to where they were hatched. Scientists believe salmon use a combination of a magnetic orientation, celestial orientation, the memory of their home stream's unique smell, and a circadian calendar to return to their natal stream to spawn. The memory and smell centers in a salmon's brain grow rapidly just before it leaves its home stream for the sea. A salmon can detect one drop of water from its home stream mixed up in 250 gallons of sea water. Salmon will follow this faint scent trail, with the aid of the other methods mentioned above, back to their home stream to spawn. 

High Mortality

Although a single female salmon can lay 1,000 to 17,000 eggs, very few of those eggs actually survive from fertilization to maturity. An average of 3 fish returning for every parent fish that spawns would be considered good production. Many natural and human-related factors cause this high mortality. During spawning eggs may not be fertilized, or may not get buried in the gravel before they are either eaten by predators (birds and fish) or become damaged as they bounce along the river bottom. Some eggs may not mature and hatch due to freezing, drying out if the water level drops too low, being trapped in the gravel, or being smothered by silt. 

Those eggs that successfully hatch to "alevin" stage continue to grow, and then emerge from the gravel as "fry." Fry become subjected to a whole new batch of obstacles and predators, since salmon at this stage are near the bottom of the food chain. Pink and chum salmon juveniles head out to sea immediately. The other species may spend as many as two years in freshwater before they head out to sea. During times of these seaward migrations you can find corresponding concentrations of predators, such as beluga whales, arctic terns, gulls, and other fish species. 

Spawning

Salmon reach sexual maturity at 2 to 8 years old. Different species mature at different rates. See below for information on the spawning of each of the five salmon species on Togiak Refuge. When the adult salmon are ready to spawn, after their long journey homeward, they select spawning sites with water flow through the gravel which will provide oxygen for their eggs and carry away carbon dioxide. 

Once a female salmon selects a spawning site, she rapidly pumps her tail to wash out a depression in the stream gravel. After the eggs are laid, the female uses the same tail movements to completely cover the eggs with gravel. These gravel nests in which the salmon deposit their eggs are known as redds. Over several days, females may lay several more redds in a line upstream. A single spawning Chinook female can lay up to 17,000 eggs! 

• Chinook: mature after 3-8 years; spawn July - August in large gravel and deep water with a strong current.  
• Sockeye: mature after 4-5 years; spawn in August in fine gravel (2-7 cm in diameter) on lake shoals or slack water in rivers.  
• Chum: mature after 3-5 years; spawn late July - August; spawn in gravel 2-3cm+ and upwelling currents in rivers or some shallow ponds or lakes.  
• Pink: mature at 2 years; spawn August - September over coarse gravel and sand, in riffles with moderate to fast currents.  
• Coho: mature at 4 years; spawn late September - December; utilize a wide range of spawning sites and currents, often in the farthest reaches of drainage.  

Egg

Although thousands are laid, up to 85% of the eggs can be lost before hatching. The eggs hatch after 6-20 weeks. Hatching times are influenced by water temperature, levels of oxygen and carbon dioxide, and vary for the different species. 

• Chinook: hatching occurs at 12 weeks  
• Sockeye: hatching occurs after 8 to 20 weeks  
• Chum: hatching occurs after 8 to 16 weeks  
• Pink: hatching occurs after 8 to 16 weeks  
• Coho: hatching occurs after 6 to 7 weeks  

Alevin

A newly hatched salmon is called an alevin. At this stage, it looks like a thread with eyes and an enormous yolk sac. Alevin remain in the redd until the yolk sac is absorbed. At this point, they work their way up through the gravel and become free-swimming, feeding fry. Alevins must have cool, clear, oxygen-rich water to remain healthy. Excessive sediment or extreme water temperatures can kill the fish. Aquatic insects and other fish are an alevin's primary predators. 

Fry

Salmon fry may go to sea shortly after they hatch or may spend several years in freshwater. Most species of salmon fry have parr marks (bars and spots along their sides) that act as camouflage to help to avoid predators and hide among the cover provided by rocks, stumps, undercut banks and overhanging vegetation. Parr markings vary for fry of different species. As salmon fry grow larger, they move out into more open, faster moving water. During their fresh water residence, fry feed chiefly on terrestrial insects. Fry may form into schools during their freshwater residence. 

• Chinook: fry have bar-shaped parr marks which are larger than the spaces between.  
• Sockeye: fry have short, oval parr marks, extending a little below the middle of the body; silver in color, with a tinge of blue in the back.  
• Chum: fry have small, distinct parr marks and slight spots on the body; go to sea almost immediately upon emergence and migrate at night.  
• Pink: fry have no parr marks (silvery).  
• Coho: fry have bar-shaped parr marks; rarely have spots on upper half of dorsal fin.  

Smolt

Many physiological and morphological changes occur in a young salmon to help it make the transition from a freshwater to saltwater existence. This process is called smoltification. As the time for migration to the sea approaches, the salmon acquire the dark back, light belly, and silvery coloration typical of fish living in open water. They seek deeper water, avoid light, and their gills and kidneys begin to change so that they can process salt water. The young fish remain in estuaries and tidal creeks feeding on small fish, insects, crustaceans and mollusks. They gradually move into deeper, saltier water, until they enter the ocean.

Life in the Marine Environment

Alaskan salmon can stay at sea for up to 7 years, although this varies by species. During their ocean existence, salmon primarily eat fish, invertebrates, and crustaceans.  

Salmon can undertake extensive ocean migrations of over 3,000 miles, and average approximately 18 miles per day depending on the species. Generally, juvenile salmon from southwestern Alaska streams migrate from the Bristol and Kuskokwim bays through the Aleutian Island chain into the northern Pacific Ocean and the Gulf of Alaska. Some salmon populations may use the Bering Sea extensively. Movement of salmon in the ocean is thought to be timed to take advantage of seasonal food availability and ocean conditions.  

Salmon are all bright silver while in the ocean environment, however when the they return to freshwater to spawn, they undergo many physiological and morphological changes. First they must switch from using saltwater to freshwater. Returning to freshwater, they change body color from a silver to a brown, green or red depending on the species. The males of some species may change their body shape and develop a hooked snout, humped back, and elongated teeth, which are used to attract a mate and defend spawning territory. Salmon stop feeding once they enter freshwater, but they are able to travel many miles to spawning grounds by using the stored energy from their ocean residence. All adult salmon die after spawning, and their bodies decay, thus providing nutrients to future generations of salmon. 

Resources

• The USFWS Fairbanks Fishery Resources Office's website "cybersalmon" was the source for much of the above information.  
• Morrow, James E. 1980. The freshwater fishes of Alaska. Alaska Northwest Publishing Company. Anchorage Alaska.  
• Groot, C. and L. Margolis (ed.). 1991. Pacific salmon life histories. University of British Columbia Press. Vancouver British Columbia.  
• Alaska Department of Fish and Game  
• List of fish species in the Togiak National Wildlife Refuge.