Rainbow Trout - Threats and Conservation

Threats and Conservation

Steelhead trout populations have declined due to human and natural causes. Steelheads (Oncorhynchus mykiss) historically occurred around the North Pacific Ocean from northwestern Mexico in North America to eastern Russia in Asia.

Two West Coast Evolutionarily Significant Units (ESUs) are endangered under the Federal Endangered Species Act (Southern California and Upper Columbia River) and eight ESUs are threatened. The U.S. National Marine Fisheries Service has a detailed description of threats. Southern California (south of Point Conception) ESU steelheads have been affected by habitat loss due to dams, confinement of streams in concrete channels, water pollution, groundwater pumping, urban heat island effects, and other byproducts of urbanization.

Several studies have shown almost all California coastal steelheads are of native origin, despite over a century of hatchery stocking. Genetic analysis shows that South Central California Coast (SCCC) distinct population segment (DPS) and Southern California (SC) DPS from Malibu Creek north, and including the San Gabriel River, Santa Ana River and San Mateo Creek, are not hatchery strains. However, steelheads from Topanga Creek and the Sweetwater River were partly, and from San Juan Creek completely, of hatchery origin. Genetic analysis has also shown the steelheads in the streams of the Santa Clara County and Monterey Bay basins are not of hatchery origin, including the Coyote Creek, Guadalupe River, Pajaro River, Permanente Creek, Stevens Creek, San Francisquito Creek, San Lorenzo River, and San Tomas Aquino Creek basins. Natural waterfalls and two major dams have isolated Russian River anadromous steelheads from its freshwater rainbow trout form above the impassable barriers; however, a recent genetic study of fin samples collected from steelheads at 20 different sites both above and below passage barriers in the watershed found that despite the fact that 30 million hatchery trout were stocked in the river from 1911 to 1925, the steelheads remain of native and not hatchery stock.

The rainbow trout is susceptible to enteric redmouth disease. A considerable amount of research has been conducted on redmouth disease, given its serious implications for rainbow trout farmers. The disease does not affect humans.

The U.S. National Marine Fisheries Service has identified 15 populations, called distinct population segments (DPSs), in Washington, Oregon and California. Eleven of these DPSs are listed under the U.S. Endangered Species Act (ESA). One DPS on the Oregon Coast is designated a U.S. Species of Concern. Species of Concern are those species that lack sufficient data to determine whether to list the species under the ESA.

Rainbow trout, and subspecies thereof, are currently EPA-approved indicator species for acute fresh water toxicity testing.

In 2010, the Oregon Department of Fish and Wildlife hatchery expects to more than double its take over 2009. The 2009 population grew 60% over 2008. Hatchery-taken fish will spawn tens of thousands of juvenile "smolts" that will be released to swim downstream and mature in the Pacific.

In March 2010, the Los Angeles Times reported the New Zealand mud snail had infested watersheds in the Santa Monica Mountains, complicating efforts to improve stream-water quality for the steelheads. According to the article, the snails have expanded "from the first confirmed sample in Medea Creek in Agoura Hills to nearly 30 other stream sites in four years." Researchers at the Santa Monica Bay Restoration Commission believe the snails' expansion may have been expedited after the mollusks traveled from stream to stream on the gear of contractors and volunteers.

Hatcheries have also been demonstrated to present a risk to wild steelhead populations. Releases of conventionally reared hatchery steelheads pose ecological risks to pre-existing wild steelhead populations. Hatchery steelheads are typically larger than the wild forms, and can displace wild-form juveniles from optimal habitats. Dominance of hatchery steelheads for optimal microhabitats within streams may reduce wild steelhead survival as a result of reduced foraging opportunity and increased rates of predation.