The Peace/Williston Fish and Wildlife Compensation Program (PWFWCP)

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Rainbow Trout
Oncorhynchus mykiss

What do they look like?

A trout with black spots on the back, sides, and dorsal and caudal fins. The hind margin of the anal fin (8 to 12 bony rays) is almost perpendicular to the long axis of the body. Bluish to greenish dorsally, sides silvery to yellowish-green and sometimes with a broad reddish lateral band. In fry (25-50 mm) and parr (50-500 mm), the adipose fin is edged with dark pigment.

Where do they live?

The rainbow trout is one of the most extensively studied fish in the world. Generally, rainbow trout are a cool water species. They prefer temperatures between 7 and 18 °C. Since the anadromous rainbow trout (steelhead) does not occur in our area, only fluvial and lacustrine populations will be described.

During the summer, in streams and rivers, adult rainbow trout occupy riffles, runs, glides, and pools; however, they tend to occur in deeper and faster water than juveniles. Adults use runs with depths of 0.5-1.0 m, cobble-boulder substrates, and average water velocities of 40 to 80 cm/s. In small streams overhead cover (primarily riparian vegetation and large woody debris) is an important component of "good" trout habitat. Lake-dwelling adult rainbow trout typically stay below the 18 °C isotherm and in areas where the oxygen concentration is above 3.0 mg/l. In small lakes, rainbows use all parts of the lake but are often associated with cover (e.g., logs and other large woody debris) in the lower littoral zone.

In streams and rivers, juvenile rainbow trout also occupy riffles and runs; however, in these habitats they are found in shallower and slower water than adults. In lakes, juveniles remain inshore during winter and early spring and are associated with cover (e.g., cobble and boulder substrates or woody debris).

In flowing water during their first summer, the fry establish territories in shallow water along stream edges. Both the level of aggression and territory size are influenced by fry density and prey abundance. Fry of most lacustrine populations migrate to their lake late in their first summer or in early fall. In lakes, fry remain in shallow water, typically about 2-5 m offshore and are associated with cover (e.g., cobble, boulders, and woody debris).

What is their life like?

In the wild, rainbow trout breed in the spring and normally spawn in flowing water. Migrations into spawning streams are triggered by rising water temperatures (above 5 °C) and rising water levels. Egg deposition usually occurs from late April to July. The female excavates a nest (redd) by turning on one side and strongly beating the caudal fin. This digging motion creates a vortex that loosens and lifts gravel that is then moved downstream by the current. After spawning the female moves slightly upstream and begins gentle digging movements which do not move any gravel but force the eggs into interstices at the bottom of the nest. The digging movements gradually become more vigorous and gravel is then displaced downstream over the nest.

Typically, lake-dwelling populations in central B.C. produce about 1,000-3,000 eggs per female. The optimum incubation temperature is around 10-11 °C. Depending on temperature, emergence occurs from 42 days (at 5 °C) to 32 days (at 14 °C) after hatching (alevins range from 11-13 mm in fork length). Temperature, food availability, and population influence growth rate in fry (0 +). In general, fish at 1 + are about 100-120 mm and at 2 + 155-170 mm. Size and age at maturity varies among populations: small-bodied, stream-resident populations can mature as early as 1 + and at < 150 mm. Rainbow trout are relatively short-lived and, in B.C., fish older than 10 years (9 +) are rare.

What do they eat?

In streams, fry and juveniles forage primarily on the drifting stages of aquatic insects. As they grow, terrestrial and emerging aquatic insects are added to the diet. Permanent stream- or river-resident, adults retain this diet throughout life. Persistent piscivory is rare in river-resident populations and in the Peace system stomachs of large (>300 mm) river-resident adults sometimes contain fish (usually sculpins).

Once in a lake, young-of-the-year forage on both bottom organisms (e.g., amphipods, snails, and the nymphs of aquatic insects) and water column organisms (especially chironomid pupae and cladocerans). Typically, lacustrine juveniles and adults have similar diets but take larger prey and more adult insects (terrestrial and aquatic) than young-of-the-year. During the summer, some populations are primarily planktivores, while other populations are primarily benthivores, and in large lakes (especially if kokanee are present) individuals over 400 mm often become piscivores.

What is their distribution? In North America, except for the headwaters of a few Mackenzie tributaries (e.g., the Athabasca, upper Peace and, perhaps, the upper Liard), the original distribution of rainbow trout was restricted to areas west of the Continental Divide. Here, it ranged from central Baja California north to the Kuskokwim River, Alaska. On the Asian side of the North Pacific the original distribution was from rivers flowing into the Sea of Okhotsk in the south to the Kamchatka Peninsula in the north. In British Columbia the original distribution of non-anadromous rainbow trout has been obscured by extensive, and often unrecorded, introductions. Still, except for some native populations in the upper Peace (Parsnip and Finlay rivers) and Athabasca drainages, most of the native range was in west-flowing rivers. There are also isolated populations in the upper Liard system.

View Peace Williston distribution map.