Importance
Steelhead trout are an important component of California´s diverse wildlife heritage. They are a good indicator of the health of aquatic systems because they use all portions of a river system, and require cool, clean water.
Steelhead are a sport fish, with about 100,000 steelhead anglers throughout the state. If the current population of steelhead in California were to double, the state’s economy from fishing revenue would increase by an estimated 37.5 million dollars.
Life Cycle
Steelhead trout (Oncorhynchus mykiss) belong to the family Salmonidae which includes all salmon, trout, and chars. Steelhead are the anadromous form of rainbow trout, a salmonid species native to western North America and the Pacific Coast of Asia. The term anadromous refers to fish species born in the stream that migrate to the ocean for their adult phase. Steelhead are similar to some Pacific salmon in their life cycle and ecological requirements.
They are born in fresh water streams, where they spend their first 1-3 years of life. They then emigrate to the ocean where most of their growth occurs. After spending between one to four growing seasons in the ocean, steelhead return to their native fresh water stream to spawn. Unlike Pacific salmon, steelhead do not necessarily die after spawning and are able to spawn more than once.
In California, most steelhead spawn from December through April in small streams and tributaries where cool, well oxygenated water is available year round. The female selects a site with gravel substrate where there is good flow through the gravel. She then digs a nest, called a redd, and deposits eggs, which the male then fertilizes. The eggs are covered by gravels and cobbles when the female excavates another redd just upstream.
The length of time it takes for eggs to hatch is heavily dependent on water temperature. In hatcheries with carefully controlled conditions, steelhead eggs hatch after 30 days at a temperature of 51° F. The optimal temperature for egg incubation is between 44 and 50° F (7-10° C). Eggs hatch sooner in warmer water, but the young fish are smaller and generally have lower survival rates. If the temperature goes too high, eggs will not hatch at all. After hatching, the developing steelhead will remain in the gravel for another four to six weeks. During this time, they are called alevins and obtain nutrients from a yolk sack attached to their body. When they emerge from the gravel, they are called fry, and are able to catch their own food.
Newly emerged fry move to shallow, protected areas of the stream (usually in the stream margins). They establish feeding areas which they defend. Most juveniles can be found in riffles, although larger ones will move to pools or deep runs.
Steelhead vs. rainbow trout
Rainbow trout and steelhead are the same species of fish; the two names reflect two distinct life history patterns. The name rainbow trout is used for the non-anadromous life history. Rainbow trout do not leave the stream to go to the ocean. They spend their entire life in the stream. The name steelhead refers to the anadromous life history described above.
Anadromous steelhead and resident rainbow trout did not arise from two distinct evolutionary lines. There is a close genetic and taxonomic relationship between these two forms. Anadromous forms of the trout can convert to resident populations when drought events or damming of rivers blocks their access to the ocean. Conversely, resident trout populations can become anadromous if ocean access becomes available. It is typical to have both life history patterns occurring in the same stream. In fact, resident and anadromous parents can produce offspring of both varieties. It has been speculated that there is a food availability related trigger which determines whether a particular fish emigrates to the ocean or remains in the stream. It may be that if there is abundant food in the stream and a fish is growing at a rapid rate, it will remain in the stream. If food is limited and growth is slow, the fish will have a tendency to emigrate.
This dual life history pattern of steelhead and rainbow trout makes the species more adaptable to changing environmental conditions. At the southern most limits of steelhead distribution this is particularly important due to unstable, variable climatic and hydrographic conditions.
Habitat
Steelhead habitat requirements change as they go through different life phases. Adult steelhead need to have access to their natal streams. This means that streams must be free of barriers to migration, as the majority of spawning occurs in the upper reaches of tributaries. Adults also need access to spawning gravel in areas free of heavy sedimentation with adequate flow and cool, clear water. Steelhead utilize gravel that is between 0.5 to 6 inches in diameter, dominated by 2 to 3 inch gravel. Escape cover such as logs, undercut banks, and deep pools for spawning adults is also important.
For steelhead eggs and pre-emergent fry, the most important consideration in terms of habitat is cool water with adequate dissolved oxygen. Fine sediment will smother developing eggs, so the area must not have excessive fine silt or sand. During their first summer, juvenile steelhead are typically found in relatively shallow areas with cobble and boulder bottoms. They reside at the downstream end or in riffles less than two feet deep. Juvenile steelhead prefer areas including woody debris accumulation such as logs or tree roots. Cover structures such as boulder clusters and root wads provide both summer and winter rearing habit. Surface turbulence (or white water) provides another source of cover during the summer months.
As juvenile steelhead grow, pools become an important habitat component. The best pools for habitat are those with abundant escape cover in the form of large woody debris, undercut banks, root masses, and large boulders. Cool, clean water is essential for the survival of steelhead during all portions of their life cycle. Elevated water temperatures (>70° F) can greatly impair growth rates of juvenile steelhead if adequate food is not available. Warmer water also holds less dissolved oxygen and increases a fish´s susceptibility to disease.
Distribution and Current Status
In North America, steelhead are found in Pacific Ocean drainages from southern California through Alaska. In Asia, they are found on the east and west coast of the Kamchatka Peninsula, with a few populations on the mainland. In the state of California, known populations occur in coastal rivers and streams from Malibu Creek in Los Angeles County up to the Smith River near the Oregon border, and in the Sacramento River system.
Historically, large runs of steelhead trout made their way up the Napa River to spawn in its tributaries. The local steelhead population has been greatly reduced. The National Marine Fisheries Service (now called NOAA Fisheries) listed steelhead as a threatened species in Napa County in August 1997. However, adult steelhead are still observed spawning in many of the Napa River’s tributaries on a yearly basis, and juvenile steelhead can be seen in the summer months. The Napa River watershed is still considered one of the most significant anadromous fish streams within San Francisco Bay.
In California, steelhead were once abundant in coastal and Central Valley rivers and streams. A rough estimate of the total statewide steelhead population is 250,000 adults. This is less than half the population of 30 years ago. The major factor causing steelhead population decline is freshwater habitat loss and degradation. This has resulted from three main factors: inadequate stream flows, blocked access to historic spawning and rearing areas due to dams, and human activities that discharge sediment and debris into waterways.
Sources
Steelhead Restoration and Management Plan for California, by Dennis McEwan and Terry Jackson, California Department of Fish and Game
California Salmonid Stream Habitat Restoration Manual, by Gary Flosi et al., California Department of Fish and Game, Inland Fisheries Division
The Napa Watershed Owner’s Manual, Napa County RCD