Size and Weight Range
Spawning adult size range: 35 to 65 cm (13.75 to 25.5 inches) fork length (FL).
Spawning adult weight range: 1.4 to 5.4 kg (3 to 12 lbs)
Dorsal fin: 10 to 12 principal dorsal fin rays
Anal fin: 8 to 12 anal fin rays
Pectoral fin: 11 to 17 pectoral fin rays
Pelvic fin: 9 to 10 pelvic fin rays
Small with 110 to 160 pored scales along the lateral line, 18 to 35 scale rows above the lateral line, and 14 to 29 scale rows below the lateral line.
The area around the lateral line is iridescent pink to red.
Number from 9 to 13
Number from 16 to 22
Juvenile coloration is similar to the adult coloration described below except for the addition of 5 to 13 oval parr marks centered on the lateral line. The parr marks are widely spaced with the interspaces wider than the parr marks. Juveniles also possess 5 to 10 dark marks on the back between the head and dorsal fin and white to orange tips on the dorsal fin and anal fins. Finally juveniles have few or no black spots on the tail that are found in adult fish.
Generally silver in color with pinkish cheeks, green backs, and sides and belly silver, white or yellowish. As these fish remain in freshwater, the silvery coloration darkens and these anadromous fish begin to more closely resemble freshwater resident O. mykiss in appearance. Black spots on tail, adipose fin, dorsal fin and back. The tail spots are typically in radiating lines. The lateral line is iridescent pink to red. The mouth is large with a maxillary bone that usually extends to the rear of the eye. Teeth are well developed on the upper and lower jaws. Basibranchial teeth are absent.
Steelhead are the anadromous form of rainbow trout. Like Pacific salmon, steelhead are born in fresh water, emigrate to the ocean where most of their growth occurs, and then return to fresh water to spawn. Unlike Pacific salmon, steelhead do not necessarily die after spawning and may return to spawn multiple years. Steelhead post-spawning survival rates vary considerably between populations but are generally quite low.
Rainbow trout are separated from steelhead on the basis of anadromy. Little or no morphological or genetic differentiation has been found between anadromous and resident forms inhabiting the same stream system. The conversion of anadromous forms that have become isolated upstream of dams to resident populations is a further indication of the close genetic and taxonomic relationship of these two forms.
Lack of genetic differences between anadromous and resident forms indicated that there is substantial gene flow between anadromous and resident forms. It is not uncommon for male anadromous steelhead to mature as parr and then assume a resident life style. Mature male parr rainbow trout have also been observed spawning with female steelhead. This variability in life history patterns probably results in a survival advantage especially in unstable, variable climatic and hydrographic conditions found in areas such as southern California.
Most California steelhead spawn from December through April.
Spawning and Egg Development
There are two basic life history types of steelhead. Stream-maturing steelhead enter fresh water with immature gonads and consequently must spend several months in the stream before they are ready to spawn. Stream-maturing steelhead are also referred to as summer steelhead. Stream-maturing steelhead typically enter fresh water in spring, early summer, and possibly fall. They ascend to headwater tributaries, hold over in deep pools until mature and spawn in late fall and winter. Ocean-maturing steelhead mature in the ocean and spawn relatively soon after entry into fresh water. Ocean-maturing steelhead are also referred to as winter steelhead. Ocean-maturing steelhead typically begin their spawning migration in fall and winter and spawn within a few weeks to a few months from the time they enter fresh water. Ocean-maturing steelhead generally spawn in January through March, but spawning can extend into spring and possibly early summer months.
Steelhead spawn in small streams and tributaries where cool, well oxygenated water is available year-round. The female selects a site where there is good intergravel flow, then digs a redd and deposits eggs while an attendant male fertilized them. The eggs are then covered with gravel when the female begins the excavation of another redd just upstream.
An average of 550 to 1,300 eggs are deposited in each redd. Steelhead redd sizes range from 22.5 to 121 square feet and average 56 square feet.
The length of time it takes for eggs to hatch depends mostly on water temperature. Fry emerge from the gravel usually about 4 to 6 weeks after hatching but this timing is effected by factors such as redd depth, gravel size, siltation, and temperature.
Fry and Juveniles
Fry move to shallow, protected areas associated with the stream margin. To establish feeding locations they soon move to other areas of the stream. Most juveniles inhabit riffles but some larger juveniles will inhabit pools or deeper runs.
Juveniles typically rear in fresh water from one to three years.
Smolt Emigration and Steelhead in the Ocean
Steelhead do not necessarily migrate to the ocean at any set age. Some individuals will remain in a stream, mature, and even spawn without ever going to sea while others will migrate to sea at less than a year old. For steelhead that do migrate to the ocean they typically remain at sea for 1 to 4 growing seasons before returning to fresh water to spawn.
Information for the ocean distribution of the different steelhead stocks is not abundant but it is likely that most California fish do not wander far from the California coast.
It has been reported that 7 inches is the minimum depth required for successful migration of adult steelhead although the distance fish must travel through shallow water areas is also a critical factor. Excessive water velocity and obstacles which impede the swimming and jumping ability are significant in hindering or blocking migration. Water velocities of 10 to 13 ft/s begin to hinder the swimming ability of adult steelhead and may delay migration. Optimum temperature requirements vary based on stock but generally fall in the range of 46 to 52°F.
The preferred depth for spawning ranges from 6 to 24 inches with an average of 14 inches. Steelhead spawn in areas with water velocities ranging from 1 to 3.6 ft/s but prefer velocities of about 2 ft/s. Larger steelhead have the ability to establish redds and spawn in faster currents than smaller steelhead.
Adult steelhead have been reported to spawn in substrates from 0.2 to 4.0 inches in diameter. Based on the Bovee (1978) classification, steelhead utilize mostly gravel-sized material for spawning but will also use mixtures of sand-gravel and gravel-cobble.
Optimum temperature requirements vary based on stock but generally fall in the range of 39 to 52°F.
Eggs and sac fry in the gravels interstitial spaces require highly permeable gravels to keep the incubating eggs and sac fry well oxygenated and should contain less than 5% sand and silt. Once fry emerge from the gravel, they utilize water in the range of 2 to 14 inches in depth and prefer water approximately 8 inches in depth. Fry prefer substrate categorized as cobble/rubble which is slightly larger than that preferred by adult steelhead for spawning. Optimum temperature requirements vary based on stock but generally fall in the range of 45 to 60°F.
Parr prefer a water depth of 10 inches but utilize water 10 to 20 inches deep. Juveniles prefer substrate categorized as cobble/rubble which is slightly larger than that preferred by adult steelhead for spawning.
Optimum temperature requirements vary based on stock but are generally less than 57°F.
Bovee KD. 1978. Probability-of-use criteria for the family Salmonidae. Instream flow
information paper 4. US Fish and Wildlife Service, FWS/OBS–78/07. 79 p.