Salvelinus confluentusBull Trout

Last updated:

Geographic Range

Salvelinus confluentus, also called the bull trout or inland dolly varden, is most closely associated with pristine mountainous areas of the northwestern United States and Canada where cold, clean waters flow. The geographic range of S. confluentus is confined to northwestern North America from Alaska to northern California (Bjornn, 1991). The species is generally considered to live within the Arctic, Pacific and Missouri River drainages in mountain and coastal streams (Page and Burr, 1991). More specifically, the southern limit of the species has historically been within the McCloud River drainage system of northern California. East of this limit, S. confluentus occurs in the Columbia River drainage in northern Nevada and north to the extreme southern Yukon Territory (Page and Burr, 1991). Some drainage systems in Montana, particularly the Flathead system located on the western slope of the continental divide, sustain S. confluentus as well. In the United States, Idaho, Nevada, Montana, Oregon and Washington all contain some native stocks of bull trout. In Canada, the provinces of British Columbia and Alberta have existent populations located on both sides of the Continental Divide (Nelson and Paetz, 1992).

Habitat

S. confluentus is usually found in medium to large river systems but can also occur in large lakes and reservoirs when conditions are adequate. In the fluvial setting the bull trout favors deep pools where it usually sits on or near the bottom (Bjornn, 1991). In the lacustrine setting the species frequents the cold, deeper sections of lakes as well as the shallows. The particular location of Salvelinus confluentus within a lake is dependant on the time of year and water temperature. Within their habitat the species generally prefers temperatures below 50 degrees Fahrenheit (Bjornn, 1991).

Although most representatives of the species are of the inland form, some populations of bull trout are anadromous and can co-exist with dolly varden char along the coast. These bull trout begin life in tributaries draining to the ocean and spend only a short period of their juvenile life in the stream. After entering the marine environment they spend up to three years feeding and maturing. When sexual maturity is reached, they return to freshwater for the sole purpose of spawning.

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • coastal

Physical Description

The bull trout, although similar in appearance, is not a true trout (Oncorhynchus sp.). Several characteristics differentiate them and likewise all of the char. The primary attributes that distinguish S. confluentus are the lack of teeth on the roof of the mouth and the presence of light spots on a dark background versus dark spots on a lighter background which true trout possess (Bjornn, 1991; USFWS, 1998). The general non-spawning coloration of the species is an olive to blue-gray back with some gray to silver tones on the fish's sides (Nelson and Paetz, 1992). Spot coloration can be red, yellow or orange and can often times be a combination of the three (Nelson and Paetz, 1992). Another trait that is often present and distinguishes the bull trout from other genera is the presence of a white margin on the leading edge of the ventral fins (Bjornn, 1991).

The species, as do all salmonids, display sexual dimorphism. In preparation for spawning, the breeding male can possess magnificent coloration that is characterized by red to orange lower sides and a belly of similar coloration. The appearance of the female is similar to the male during the non-spawning season but is generally more gray to silver in coloration. During the spawning period the female retains these colors with little or no change.

Individuals living in streams do not often grow larger than 4 kg, but the lake inhabitants, which have a longer migration to spawing sites, can acheive more than 9 kg (USFWS, 1998).

  • Range mass
    4-9 (high) kg
    lb

Development

The life cycle of the bull trout can be separated into five periods of development (Moyle and Cech, 2000). As with most salmonids the rate of development for bull trout embryos is primarily temperature driven(Bjornn, 1991). After egg deposition in the fall, relatively warm temperatures facilitate rapid embryonic development (Bjornn, 1991). These warmer temperatures, usually 40-50 degrees F, are followed by much colder winter temperatures that approach freezing and subsequently decrease the rate of embryonic development (Bjornn, 1991). As winter progresses into spring and temperatures increase, the rate of development is again increased.

The first developmental period is the embryonic stage in which the fish's growth is reliant on the yolk sac. The embryonic stage can be further broken down. The first part of the embryonic period is the cleavage stage and is accepted to be the time between the first occurrence of cell division and the development of pre-organ systems (Moyle and Cech, 2000). This is followed by the embryo stage in which the organs are formed (Moyle and Cech, 2000). This stage ends with hatching, at which time the animal is no longer confined to the shape of the egg. Most bull trout throughout the range hatch from the egg between 100-120 days. The free-embryo stage follows and is the period in which the fish is still dependent on the protein of the egg sac but is more fish-like in nature (Moyle and Cech, 2000).

The larval period of S. confluentus is not unlike other salmonids in that at this time the fish leaves the spawning site and begins to feed on it's own. In mid spring, after the larval fish has absorbed its yolk sac, it emerges from the redd as an alevin and proceeds to find suitable habitat for the summer period. This usually occurs 200-230 days after spawning (Bjornn, 1991).

The juvenile period follows the larval period and is generally demarcated by the presence of fully formed organ systems and fins (Moyle and Cech, 2000). Other than coloration, the bull trout at this stage is essentially a miniscule version of the adult form (Moyle and Cech, 2000).

With the maturation of bull trout, gonads are fully capable of performing sexual reproduction and the fish is considered to be in the adult stage. Behavioral changes in the species and the development of spawning colors are characteristic of this stage (Moyle and Cech, 2000).

The senescent period which is the last and final stage in the life of bull trout as well as other salmonids occurs when the fish is sexually degenerative. With the onset of this period of "old age" the fish has essentially stopped growing (Moyle and Cech, 2000).

Reproduction

Salvelinus confluentus, whether in a lake or stream setting, become sexually mature between four and ten years old (Bjornn, 1991; USFWS, 1998). Bull trout begin to spawn in the late part of August or early September and often lasting through October (Bjornn, 1991). Spawning times for specific populations is governed largely by water temperature. Generally, however, spawning activities are triggered when the water temperature drops below 10 degrees Centigrade (Bjornn, 1991). Many populations of bull trout are migratory in nature whether they are of the lake or river form. Those population segments living in lakes will migrate up natal rivers as spawning is not possible in still water. Those segments that are river dwelling will often migrate within the system from smaller headwaters to larger mainstream sections of river (Bjornn, 1991).

As is typical of all trout and salmon, the female, after finding a suitable spawning area, fans a section of gravel substrate thereby producing a large depression in the stream bed. After this process is complete a male bull trout (usually only one) will fertilize eggs deposited by the female. Depending on the size of the individual, fecundity can vary. For average size bull trout around 5000 eggs can be typical (Nelson and Paetz, 1992), however many more can be found in larger specimens and some bull trout in the fifteen pound range have been documented as having approximately 12,000 eggs (Bjornn, 1991).

It is possible for male bull trout to spawn with more than one female. Although the species is not monogamous, some evidence has been documented that male and female bull trout can pair up during migration (Bjornn, 1991).

  • Range number of offspring
    200 to 12,000-13,000
  • Average number of offspring
    5000
  • Range time to hatching
    100 to 120 days
  • Average age at sexual or reproductive maturity (female)
    4-10 years
  • Average age at sexual or reproductive maturity (male)
    4-10 years
  • Parental Investment
  • no parental involvement

Lifespan/Longevity

The lifespan and longevity of the bull trout is not well documented but seems to be approximately 10-12 years (USFWS, 1998). In the lacustrine setting, where the species attains much greater size, it would seem that bull trout would have a longer lifespan.

  • Average lifespan
    Status: wild
    10-12 years

Behavior

Behavioral characteristics other than feeding and reproduction have not been well documented for the bull trout. For bull trout populations in lakes, no literature was found as to whether they are primarily solitary or whether they tend to form pods. However, S. confluentus in the stream setting behave as most stream resident trout for most of the year. While feeding in the non-spawning season, the bull trout is likely to be a solitary species occupying a particular piece of structure such as a deep hole. More than one large individual may occur in the same hole but at a good distance from the other.

During the breeding season the bull trout will become more social for the purpose of spawning. Larger bull trout during the course of spawning are likely territorial and will keep other inferior males off of the same redd.

Communication and Perception

Food Habits

After departing from the spawning redd in spring the young bull trout begins looking for suitable habitat for protection and food sources. As a juvenile, the first year in the life of the bull trout is spent eating small aquatic invertebrates (Bjornn, 1991; USFWS, 1998). These aquatic larvae, often ephemeropterans or dipterans (Bjornn, 1991), are readily available in the lower water column and interstitial spaces that these juveniles inhabit. As the bull trout grows larger, their diet, in addition to aquatic invertebrates, consists of other fish species (Bjornn, 1991). This increased piscivorous behavior increases as the fish grows. Of all salmonids, namely the salmon and trout, S. confluentus is more inclined to feed on fish. In certain parts of their range the mountain whitefish (Prosopium williamsoni) comprise a large part of the bull trout diet (Bjornn, 1991). In addition to whitefish bull trout will feed on sculpins, darters or other trout and where applicable, salmon fry (USFWS, 1998).

  • Animal Foods
  • fish
  • insects

Predation

In the natural and unaltered fluvial or lacustrine setting, S. confluentus does not have many predators. As would be assumed with juvenile or young fish, some predation could occur from piscivorous birds. The greatest threat to the species however is most likely other fish species, specifically introduced non- indigenous species. Other fishes, such as trout, that are introduced into a drainage can either prey on young bull trout or compete with them directly for food (Bjornn, 1991).

One particular scenario that has been documented occurred in Priest Lake, Idaho. In this instance both brook trout (S. fontinalis) and lake trout (S. namaycush) were introduced into the drainage. In tributaries feeding the lake, brook trout competed with young bull trout for food and in the lake, lake trout preyed on bull trout (Bjornn, 1991). This eventually took a toll on the bull trout population as this type of predation and competition prevents many from reaching maturity and subsequently reduces number of future spawning stock (Bjornn 1991).

Another threat to the preservation and viability of the bull trout is the incidence of capture by humans. Being easily caught by the angler makes the species quite vulnerable to overfishing (Bjornn, 1991).

Economic Importance for Humans: Positive

None known

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

None known

Conservation Status

Over the past century habitat degradation, introduction of non-native fishes and overfishing has caused the bull trout to become threatened across most of its range (Bjornn, 1991). S. confluentus is currently extinct in California with all of the native stock gone (USFWS 1998). Throughout its range, bull trout have been extirpated from many streams once inhabited by the species (Bjornn, 1991). Viable populations do exist, but the population density of the species has been severely diminished.

It is imperative at this stage that measures continue to be taken to ensure the future of S. confluentus as a unique, indigenous western species. Some steps have already been implemented to reduce mortality, such as habitat restoration and protection, catch and release fishing in some areas, and prohibitions on fishing in others.

Habitat degradation, facilitated largely through increased siltation due to unsound land use practices needs to be further curtailed. The bull trout is highly intolerant of polluted waters and likewise requires the coldest and cleanest water of all salmonids (USFWS, 1998). Implementation of better land use practices in or around drainages that contain bull trout are vital to the existence of the species. According to the IUCN red list the species is vulnerable and has seen a decline of at least 20% over the last ten years, primarily for the reasons stated above. A reduction of an additional 20% is projected for the next ten years if the current trend continues (IUCN, 2000).

In some areas where populations have been affected by anthropogenic impacts, bull trout culture in hatcheries could prove to be a successful means of restoring the population (Bjornn, 1991).

Contributors

William Fink (editor), University of Michigan-Ann Arbor.

Timothy O'Brien (author), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

freshwater

mainly lives in water that is not salty.

insectivore

An animal that eats mainly insects or spiders.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

threatened

The term is used in the 1994 IUCN Red List of Threatened Animals to refer collectively to species categorized as Endangered (E), Vulnerable (V), Rare (R), Indeterminate (I), or Insufficiently Known (K) and in the 1996 IUCN Red List of Threatened Animals to refer collectively to species categorized as Critically Endangered (CR), Endangered (EN), or Vulnerable (VU).

visual

uses sight to communicate

References

Bjornn, T. 1991. Bull trout, *Salvelinus confluentus*. Pp. 230-235 in J Stolz, J Schnell, eds. Trout. Harrisburg, PA: Stackpole Books.

IUCN, 2000. "The IUCN Red List of Threatened Species" (On-line). Accessed November 14, 2002 at http://www.redlist.org/info/categories_criteria1994.html#categories.

Moyle, P., J. Cech. 2000. Fishes: An Introduction to Icthyology. Upper Saddle River, NJ: Prentice Hall.

Nelson, J., M. Paetz. 1992. The Fishes of Alberta. Edmonton and Calgary, Alberta, Canada: The University of Alberta Press and The University of Calgary Press.

Page, L., B. Burr. 1991. Freshwater Fishes. NY, New York: Houghton Mifflin.

USFWS, 2000. "Trout, bull/Salvelinus confluentus" (On-line). Accessed November 11, 2002 at http://ecos.fws.gov/servlet/SpeciesProfile?spcode=E065.