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

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Bull Trout
Salvelinus confluentus

What do they look like?

The bull trout is a trout-like char with pale pink, lilac, or red spots along the flanks. It has an olive-green to tan body with a broad flattened head and a down turned long upper jaw. Fins are generally transparent and without black spots on the dorsal fin; pelvic and anal fins often have a whitish leading edge, not followed with black. The caudal fin is moderately forked. The spots on the back are larger and more well-spaced than those on the Dolly Varden, while Dolly Varden have a rounder head shape.

Bull trout and Dolly Varden are difficult to distinguish in the field. Where they coexist, the life history of adults is usually strikingly different. Typically, adult bull trout are large (55-95 cm), migratory, and piscivorous; adult Dolly Varden are small (10-20 cm), stream-resident, and feed on drift. As far as is known, bull trout and Dolly Varden hybridize (interbreeding of two different species) wherever they coexist. Despite this persistent hybridization, the two species maintain themselves as separate entities with life histories adapted to two alternative ecologies. In the Williston area, bull trout coexist with Dolly Varden for part of their life cycle in the upper Finlay drainages (tributaries to Thutade Lake).

Where do they live?

Bull trout are a coldwater species and are rarely encountered in environments where the temperature exceeds 15°C for prolonged periods. Habitat use in adult bull trout is influenced by life-history type and the presence of other fish species. In lakes, during the day, adult bull trout are rarely found in littoral areas but at night appear to move into shallower water. In large rivers they are commonly associated with the tail-out of pools and are usually close to overhead cover.

In B.C., two species that influence bull trout habitat use are Dolly Varden and lake trout. In areas of contact, Dolly Varden are typically stream residents and bull trout are riverine or adfluvial. Habitat use by juvenile bull trout and adult Dolly Varden has also been examined in Thutade Lake tributaries, where there was considerable overlap between the species. For example, they used the same hydraulic habitat types (main channel pools, main channel riffles, side channel pools, side channel riffles) in similar proportions during both day and night. At night, however, juvenile bull trout were found in shallower and, perhaps, slower water than adult Dolly Varden. There is also evidence of an interaction between bull trout and lake trout. Adults of both species are piscivores and small northern lakes tend to contain one or the other species but not both. Large lakes, however, often contain both species.

Regardless of life-history type, most bull trout rear in streams for two to four years. As they grow, juveniles shift to deeper water and are strongly associated with riffles and pools, although they also use deep side-channels. During the day they are associated with cover, such as large rocks, woody debris, root wads, and undercut banks, but at night they disperse and are less strongly associated with cover. Juveniles in adfluvial populations migrate to a lake in their fourth (3+) or fifth (4+) year.

Newly emerged bull trout fry are denser than water and associate with shallow (<5cm deep) stream edges. Here, they are found in and around gravel (20-100 mm in diameter) interspersed with boulders, especially in low water velocities (<20cm/s) areas such as side-channels and shallow bays. In most populations the fry remain in this habitat during their first summer but shift to deeper, faster water as they grow.

What is their life like?

Like many char, bull trout display a bewildering array of different life-history patterns. In the upper Peace system, three life-history patterns are common: a fluvial form that spends its entire life in flowing water but often makes extensive migrations within large river systems; an adfluvial form that lives in both rivers and lakes and, again, makes major migrations between these habitats; and a stream-resident form that spends its entire life in rivers and streams and, in many cases, is isolated above barriers.

Like all char, bull trout spawn in the fall. In the Williston area, fluvial populations usually begin migrating from large rivers like the Parsnip and Finlay into smaller (but still substantial rivers) in late August when temperatures are high and water levels low. These spawning migrations can involve round trips of over 200 km. Similarly, in late summer or early fall, the adfluvial life-history form migrates from lakes or reservoirs into spawning streams. Some of these migrations are short (<10 km) but others are substantial (50-100 km).

As far as is known, all bull trout spawn in flowing water with the peak of spawning usually occurring at around 6°C. Typically, bull trout spawn during the day, however, in some disturbed systems spawning occurs at night. Females select the spawning site and dig a nest (redd). Redd size can be large (up to 2m in diameter) but varies with female size.

Egg number in bull trout is a function of female body size and varies within and among populations. Large (typically 350-650mm) fluvial and adfluvial females produce up to 9,000 eggs. Small (typically <250 mm) stream-resident populations produce fewer eggs (<1,000). The eggs incubate in the gravel over winter and fry emerge near the beginning of June. Newly emerged fry range from 20-25 mm in length.

In the upper Finlay, young bull trout reach 30-50mm by the end of their first summer. In Thutade Lake tributaries, the majority of juvenile bull trout remain in streams for three growing seasons (age 2+). In adfluvial populations in southern B.C., sexual maturity is reached at about 350mm (5+) in males and at about 450mm (6+) in females. Maximum age in the upper Peace is unknown but ages up to 24 years are recorded elsewhere in the province.

What do they eat?

Adult bull trout are piscivorous - riverine and adfluvial populations prey on trout, whitefish (especially mountain whitefish), kokanee, grayling, and a variety of suckers, minnows, and sculpins. Juveniles and adults of stream-resident populations feed primarily on the nymphs and larvae of aquatic insects (e.g., mayflies, caddisflies, stoneflies, and chironomids). Juveniles over 150 mm also take small fish (including young of their own species). Fry feed primarily on chironomid larvae but gradually shift to larger prey (predominately insect nymphs) as they grow.

What is their distribution? Bull trout are native to western North America. Their range extends farther south than that of Dolly Varden and they were originally distributed from northern California to the extreme headwaters of Yukon system in British Columbia and parts of the Liard system (Mackenzie drainage) in the Northwest Territories. In B.C., bull trout are distributed throughout the interior of the province but are absent from most short coastal rivers and from both Vancouver Island and the Queen Charlotte Archipelago. The exceptions are the major, west-flowing rivers (e.g., the Fraser, Homathko, Klinaklini, Skeena, Nass, Stikine, and Taku rivers) that cut through the Coast Mountains.

View Peace Williston distribution map.