Weight: 40 pounds (18 kg), but can be up to 120 pounds (55 kg)
Length: considered mature at about 3 feet (1 m)
Appearance: at sea, they are blue-green back with silver flanks
Lifespan: they spend 3 months-2 years in freshwater and about 2-4 years at sea
Diet: insects, amphipods, and other crustaceans while young, and mainly fish as adults
Behavior: adults migrate from a marine environment into the freshwater streams and rivers of their birth in order to mate (“anadromous”)

Adults migrate from a marine environment into the freshwater streams and rivers of their birth in order to mate, a process known as anadromy. Salmon experience semelparity, where they spawn only once and then die.

Populations exhibit considerable variability in size and age of maturation, and at least some portion of this variation is genetically determined. There is a relationship between small size and long distance of migration that may also reflect the earlier timing of river entry and the cessation of feeding for Chinook salmon stocks that migrate to the upper reaches of river systems. Body size, which is related to age, may be an important factor in migration and spawning bed, or redd, construction success.

Juvenile Chinook may spend from 3 months to 2 years in freshwater before migrating to estuarine areas as smolts and then into the ocean to feed and mature. Chinook salmon remain at sea for 1 to 6 years (more commonly 2 to 4 years), with the exception of a small proportion of yearling males, called jack salmon, which mature in freshwater or return after 2 or 3 months in salt water.

There are different seasonal (i.e., spring, summer, fall, late-fall or winter) “runs” in the migration of Chinook salmon from the ocean to freshwater, even within a single river system. These runs have been identified on the basis of when adult Chinook salmon enter freshwater to begin their spawning migration. However, distinct runs also differ in the degree of maturation at the time of river entry, the temperature and flow characteristics of their spawning site, and their actual time of spawning. Freshwater entry and spawning timing are believed to be related to local temperature and water flow regimes.

Adult female Chinook will prepare a nest called a redd in a stream area with suitable gravel type composition, water depth and velocity. The adult female Chinook may deposit eggs in four to five “nesting pockets” within a single redd. Spawning sites have larger gravel and more water flow up through the gravel than the sites used by other Pacific salmon. After laying eggs in a redd, adult Chinook will guard the redd from just a few days to nearly a month before dying.

Chinook salmon eggs will hatch, depending upon water temperatures, three to five months after deposition. Eggs are deposited at a time that ensures young salmon fry emerge during the following spring when the river or estuary productivity is sufficient for juvenile survival and growth.

As the time for migration to the sea approaches, juveniles lose their parr marks, the pattern of vertical bars and spots useful for camouflage. They then gain the dark back and light belly coloration typical of fish living in open water. Chinook salmon seek deeper water, avoid light, and their gills and kidneys begin to change so that they can process salt water.

Two distinct types or races among Chinook salmon have evolved.

One race, described as a “stream-type” Chinook, is found most commonly in headwater streams of large river systems. Stream-type Chinook salmon have a longer freshwater residency, and perform extensive offshore migrations in the central North Pacific before returning to their birth, or natal, streams in the spring or summer months. Stream-type juveniles are much more dependent on freshwater stream ecosystems because of their extended residence in these areas. A stream-type life history may be adapted to areas that are more consistently productive and less susceptible to dramatic changes in water flow. At the time of saltwater entry, stream-type yearling smolts are much larger, averaging 3 to 5.25 inches (73-134 mm) depending on the river system, than their ocean-type sub-yearling counterparts, and are therefore able to move offshore relatively quickly.

The second race, called the “ocean-type” Chinook, is commonly found in coastal streams in North America. Ocean-type Chinook typically migrate to sea within the first three months of life, but they may spend as much as a year in freshwater prior to emigration to the sea. They also spend their ocean life in coastal waters. Ocean-type Chinook salmon return to their natal streams or rivers as spring, winter, fall, summer, and late-fall runs, but summer and fall runs predominate. Ocean-type Chinook salmon tend to use estuaries and coastal areas more extensively than other pacific salmonids for juvenile rearing. The evolution of the ocean-type life history strategy may have been a response to the limited carrying capacity of smaller stream systems and unproductive watersheds, or a means of avoiding the impact of seasonal floods. Ocean-type Chinook salmon tend to migrate along the coast. Populations of Chinook salmon south of the Columbia River drainage appear to consist predominantly of ocean-type fish.

Chinook salmon are easily the largest of any salmon, with adults often exceeding 40 pounds (18 kg); individuals over 120 pounds (55 kg) have been reported. In contrast, Chinook mature at about 36 inches and 30 pounds.