Rainbow Trout

Oncorhynchus mykiss

Profile / Morphology 2

Oncorhynchus mykiss, called Steelhead or rainbow trout exhibit perhaps the greatest diversity of life history patterns of any Pacific salmonid species (Barnhart 1986), including varying degrees of anadromy, differences in reproductive biology, and plasticity of life style types between generations. O. mykiss can be anadromous, or can be found as a permanent freshwater resident (and under some circumstances, apparently produce young of the opposite form). Anadromous O. mykiss are called Steelhead trout, and non-anadromous forms are usually called Rainbow trout. However, east of the Cascade Mountains in the Fraser and Columbia River basins, they are called redband trout. Rainbow trout are usually dark-olive in color, shading to silvery-white on the underside with a heavily speckled body and a pink to red stripe running along their sides. Rainbow trout that migrate to the ocean develop a much more pointed head, become more silvery in color, and typically grow much larger than the rainbow trout that remain in fresh water.

Diet 3

Young animals feed primarily on zooplankton. Adults feed on aquatic and terrestrial insects, mollusks, crustaceans, fish eggs, minnows, and other small fishes (including other trout).

Average lifespan 3

up to 11 years.

Size / Weight 3

Rainbow trout can reach up to 45 inches (120 cm) in length and 55 pounds (25 kg) in weight, though average size is much smaller.

Habitat 3

Rainbow are capable of surviving in a wide range of temperature conditions. They do best where the dissolved oxygen concentration is at least 7 parts per million. In streams, deep low-velocity pools are important wintering habitats. Critical habitat for 10 west coast steelhead listed under the Endangered Species Act was designated on September 2, 2005 (see ESA link below).

Critical habitat is specific areas that contain physical or biological features essential to the conservation of ESA listed species, and those features may require special management considerations or protection for actions by Federal Government agencies. rainbow are euryhaline, since they do not spend a large part of their life in estuaries and migrate between the sea and freshwater habitats.

Range 3

The present spawning distribution of rainbow extends from the Kamchatka Peninsula in Asia, east through Alaska, and south to southern California, although the historical range extended at least to the Mexico border. They have been introduced worldwide.

Reproductive / Life span 3

Anadromous rainbow can spend up to 7 years in fresh water prior to smoltification, the physiological transition process of adapting from freshwater to ocean living conditions. Then the species can spend up to 3 years in salt water prior to first spawning.

Males mature generally at two years of age and females at three. Adult female rainbow will prepare a redd (or nest) in a stream area, with suitable gravel type composition, water depth, and velocity. The female may deposit eggs in 4 to 5 "nesting pockets" within a single redd, which are simultaneously fertilized by males. Spawning habitat consists of gravel substrates in fast-flowing, well-oxygenated rivers and streams free of excessive silt. The eggs hatch in 3 to 4 weeks. Some populations actually return to freshwater after their first season in the ocean, but do not spawn, and then return to the sea after one winter season in freshwater. Timing of return to the ocean can vary, and even within a stream system there can be different seasonal “runs”.

Rainbow can be divided into two basic reproductive types: stream-maturing or ocean-maturing. This classification is based on the state of sexual maturity at the time of river entry and the duration of spawning migration. The stream-maturing type (summer-run steelhead in the Pacific Northwest and northern California) enters freshwater in a sexually immature condition between May and October and require several months to mature and spawn. The ocean-maturing type (winter-run rainbow in the Pacific Northwest and northern California) enters freshwater between November and April, with well-developed gonads, and spawns shortly thereafter. Coastal streams are dominated by winter-run steelhead, whereas inland steelhead of the Columbia River basin are almost exclusively summer-run steelhead.

With rainbow trout, another life history trait with high variation is the ability of this species to spawn more than once (called iteroparity). All other species of Oncorhynchus, except cutthroat trout, spawn once and then die (called semelparity).

Rainbow trout can live for up to 11 years.

Relatives 3

Rainbow trout are in the genus that includes other pacific salmon including Chinook, king, coho, pink, chum and other salmonids. They are important food fishes, and play a tremendously important role in the cultures of native peoples of the West coast.

Found in the following Estuarine Reserves 3

They occur in the following NERRs: Padilla Bay (WA), South Slough (OR), San Francisco (CA), and Elkhorn Slough (CA).

Water quality factors needed for survival 3

•Water Temperature: cool water preference below 18 °C
•Turbidity: low to moderate
•Water Flow: moderate
•Salinity: euryhaline - spawn in freshwater streams, adults live in ocean
•Dissolved Oxygen: need higher levels above 7 mg/L

Threats 3

Factors for decline for rainbow trout have been extensively examined in the National Marine Fisheries Service Factors for Decline document (NMFS 1996). The status review emphasized that freshwater steelhead habitat is far from pristine. Factors highlighted by this report include: •Hydropower development and dams: Dams and water development projects can block trout access to spawning habitat, alter water temperature and sediment distribution, and prevent migration
•Logging, mining and agricultural development: . Logging, agriculture, mining and other development can lead to increased stream sediments, loss of the protection, shade, and food sources provided by adjacent riparian forests, etc.
•Over harvesting: Rainbow trout are relatively small fish. They were less targeted by commercial fisheries. However, they have been a very popular recreational fishing species and were over harvested in many areas.
•Ocean conditions: Ocean conditions including El Niño, global warming, and other conditions may have altered ocean conditions and food sources.
•Genetic mixing with hatchery fish: Steelhead trout in streams with hatchery fish may lose some of their genetic adaptations for living in those conditions if they crossbreed and produce young with hatchery fish originating from, and adapted to, other stream systems.
•Water withdrawal and flood control facilities

Conservation notes 3

Importance to Humans and Estuaries
Rainbow trout are a very important recreational fishery throughout their native and introduced range. This is no longer the case in many areas of their native range because of their decline and subsequent Endangered Species Act listings. As relatively large, long-lived predators, and because of their migratory lifestyle, they were a significant source of nutrients being brought into and out of estuaries.

A variety of conservation efforts have been undertaken. Some of the most common initiatives include:
•Captive-rearing in hatcheries
•Removal and modification of dams that obstruct salmon migration
•Restoration of degraded habitat
•Acquisition of key habitat
•Improved water quality and in-stream flow

In 2000, the Pacific Coast Salmon Recovery Fund (PCSRF) was established by Congress to support the restoration of salmon species. The fund is overseen by the National Marine Fisheries Service and carried out by state and tribal governments. The 2007 PCSRF report summarizes their work in detail (click here to view report).

How to Help Protect This Species
Since Rainbow trout use estuaries and freshwater areas, they are susceptible to water pollution, plus damage to and alteration of stream channels and riparian zones. Suggested efforts to help this species include:
•Minimize runoff of neighborhood pollutants, fertilizer, and sediment into local streams are helpful to this species or other stream and estuary dwelling species.

•Join a stream or watershed advocacy group in your area to protect your local estuary ecosystems.
•Advocating the implementation of effective fish passage solutions, so fish can bypass dams and artificial barriers
•Advocate the restoration of more natural water flow regimes.

•Support conservation programs like the Species of Concern program and other non-governmental organization programs that work to protect the endangered populations.

Sources and Credits

  1. (c) Brian Yap (葉), some rights reserved (CC BY-NC), http://www.flickr.com/photos/30265340@N00/3067858402
  2. Adapted by GTMResearchReserve from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Oncorhynchus_mykiss
  3. (c) GTMResearchReserve, some rights reserved (CC BY-SA)

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