Chinook salmon in California have an array of life history patterns allowing them to take advantage of California variable environments. Migration to fresh water occurs at different times for different spawning runs of adult Chinook salmon. In the Sacramento River basin of the California Central Valley the migration period of the late fall run ranges from October to April peaking in December. The winter run ranges from December to July peaking in March. The migration timing of the spring run ranges from March to September peaking in May and June. The fall run ranges from June to December peaking in September and October.
The fall run of the San Joaquin River basin of the California Central Valley occurs between October and early January peaking in November.
Other river systems in California support mainly fall and spring-run Chinook though there are local versions of each life history variation as a result of their capacity to adapt to local conditions.
Spawning and Egg Development
Fall-run chinook are adapted for spawning in lowland reaches of big rivers and their tributaries and typically spawn within a few days or weeks of arriving on the spawning groups. This strategy takes advantage of the high-quality spawning and rearing areas in valley reaches which are often too warm to support salmon in summer.
Late-fall run chinook typically reside in the river to 1 to 3 months before spawning and are adapted for spawning in the reaches of mainstem rivers that remain relatively cold and deep in summer.
Winter-run chinook salmon are unique to the Sacramento River and typically wait several months to spawn in the early summer. They are adapted for spawning in the clear, spring-fed rivers of the upper Sacramento basin where summer temperatures are typically 10 to 15°C.
Spring-run chinook salmon enter rivers as immature fish and migrate as far upstream as they can go where they will spend several months in deep, cold pools and then spawn in early fall. This strategy allows them to take advantage of midelevation habitats that inaccessible during summer and fall due to high temperatures and low flow in lower reaches and difficult to use during high-flow periods when holding pools are scoured.
The majority of fish return to the same stream in which they were hatched but some do stray and wind up in a different stream close to their natal stream. Depending on the run of chinook they either select an area for holding or spawn without delay in the case of fall-run chinook.
Spring-run fish typically select large and deep (>2m) pools with bedrock bottoms and moderate velocities. They often hold under ledges, in deep pockets, or under bubble curtains formed by water plunging into pools. Spawning areas are often near holding areas including the tails of holding pools. Chinook have been observed digging redds and spawning at depths from several centimeters to several meters. Winter-run chinook are the exception to his rule because they usually spawn at depths in the range of 1 to 7 meters.
When a female digs a redd the area cleaned measures between 2 and 10 m². Low silt content is necessary in the course substrate the eggs are buried in. To ensure adequate flow around the developing embryo, which is essential for successful spawning, the fine sediments are mobilized as the gravel is loosened. In addition redd sites are chosen in areas with good subsurface flow to ensure the constant delivery of oxygen containing water.
Ideal water temperatures for maximum embryo survival are between 5 and 13°C with oxygen levels close to saturation.
Each female produces somewhere in the range of 2000 to 17000 eggs with the number of eggs produced having some correlation with the female body size. Spawning males include large adults that have returned from the ocean in addition to mature 1 year olds that have never gone to sea. More than 90 percent of the eggs are thought to be fertilized.
Fry and Juveniles
After approximately 40 to 60 days embryos hatch. The hatched embryos are referred to as alevins and remain in gravel for 4 to 6 weeks until their yolk sac is fully absorbed. After leaving the gravel the juvenile chinook are referred to as fry and are usually washed downstream into areas with low velocities, dense cover, and an abundance of small food items. As the fry grow larger and stronger they move into deeper and faster water.
The time that juvenile chinook spend in fresh water before entering the ocean varies greatly depending on the run of chinook they are part of and the conditions of the river. This rearing time can be anywhere in the range of 3 months to over a year. Fall-run chinook are thought to have the shortest rearing time of just a few months before they move the river mainstem or estuary. Late-fall-run chinook spend 7 to 13 months in fresh water before entering the ocean at a size of about 150-170 mm. Winter-run chinook spend approximately 5 to 10 months in streams followed by an indeterminate time in estuaries before entering the ocean. Spring-run chinook rear in streams for 3 to 15 months depending on the flow conditions. Juvenile social behavior ranges from schooling to territorial.
While in fresh water juvenile chinook salmon eat a wide variety of terrestrial and aquatic insects.
Smolt Emigration and Chinook in the Ocean
Timing of juvenile outmigration occurs at a wide variety of times. Outmigration of spring-run juveniles tends to peak in January and February and then again in April. Fall-run outmigration peaks in March and April. Winter-run outmigration peaks from September to January.
The size at which outmigration occurs also depends on the run and can vary from 30 to 150 mm fork length.
Once in the ocean juvenile chinook from California rivers tend to stay along the California coast, although a general northern movement of fish may occur. Juvenile chinook salmon off the coast of California take advantage of the food rich waters caused by the upwelling generated by the California Current. When the California Current does not flow as strongly and upwelling decreases declines in ocean survival of salmon has also been recorded highlighting the importance of ocean productivity. When juvenile chinook enter the ocean they become predators of small fish and crustaceans. Smaller juveniles feed on invertebrates such as crab larvae and amphipods. When they grow larger fish become dominant in their diet which leads to rapid growth in size.
Chinook are thought to swim in schools at depths in the range of 0 to 100 meters. Their ocean stage of life lasts 1 to 5 years.
When salmon return to their home stream they select an area for holding. These holding areas are usually large deep pools, typically with bedrock bottoms and moderate velocities. Within these pools they often stay under ledges, in deep pockets, or under bubbles formed by water plunging into pools. Spawning females have been observed to dig redds at depths from a few centimeters to several meters and at water velocities in the range of 15 to 190 cm/sec. Most seem to spawn at depths in the range of 25 to 100 cm and velocities of 30 to 80 cm/sec though. The redds occur in course substrate which is typically a mixture of gravel and small cobbles with a low silt content.
Parr (juveniles) move progressively from shallow stream margins where cover is dense and water velocity is low to progressively deeper and faster waters as they grow until they are large, strong, and agile enough to inhabit the deeper pools.
Smolts emigrating from freshwater to the ocean will spend a variable amount of time in estuarine habitat. “Ocean-type” chinook may stay longer in estuary waters as opposed to “stream-type” chinook that typically pass through estuaries more quickly. Upon entry into the ocean they will tend to stay along the California coast, although some fish may move in a general northward direction. They stay at depths that are typically in the range of 20 to 45 meters although the range can vary to 0 to 100 meters depending on the season.