Communication Channels: visual ; acoustic
Perception Channels: tactile ; chemical
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).
Known Predators:
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.
Other Physical Features: bilateral symmetry
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.
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.
Habitat Regions: freshwater
Terrestrial Biomes: forest ; mountains
Aquatic Biomes: lakes and ponds; rivers and streams; coastal
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).
Biogeographic Regions: nearctic (Native )
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
Primary Diet: carnivore (Piscivore , Insectivore )
None known
Positive Impacts: research and education
None known
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).
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).
US Federal List: threatened
IUCN Red List of Threatened Species: vulnerable
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 gestation period: 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.
Key Reproductive Features: iteroparous ; seasonal breeding ; sexual ; fertilization (External ); broadcast (group) spawning; oviparous
Parental Investment: no parental involvement
The bull trout (Salvelinus confluentus) is a char of the family Salmonidae native to northwestern North America.[4] Historically, S. confluentus has been known as the "Dolly Varden" (S. malma), but was reclassified as a separate species in 1980.[5] Populations of bull trout in the lower 48 states are listed as threatened under the U.S. Endangered Species Act,[6] and bull trout overall are listed as vulnerable to extinction on the IUCN Red List of Threatened Species.[1] The Saskatchewan-Nelson Rivers population in Alberta, Canada is listed as threatened under the Species at Risk Act.[7]
Like other species of char, the fins of a bull trout have white leading edges. Its head and mouth are unusually large for salmonids, giving it its name. Bull trout have been recorded measuring up to 103 cm (41 in) in length and weighing 14.5 kg (32 lb).[4] Bull trout may be either migratory, moving throughout large river systems, lakes, and the ocean, or they may be resident, remaining in the same stream their entire lives.[6] Migratory bull trout are typically much larger than resident bull trout, which rarely exceed 2 kg (4.4 lb).[5] Bull trout can be differentiated from brook trout (S. fontinalis) by the absence of distinct spots on the dorsal fin, as well as yellow, orange, or salmon-colored spots on the back as opposed to red spots with blue halos on the brook trout. Bull trout lack the deeply forked tail fin of lake trout (S. namaycush, another char).[6]
S. confluentus is found in the cold, clear waters of the high mountains and coastal rivers of northwestern North America, including Yukon, British Columbia, Washington, Oregon, Idaho, and western Montana, as well as the Jarbidge River of northern Nevada and perhaps Alaska.[5][6] A population of bull trout exists east of the Continental Divide in Alberta, where it is the provincial fish. The historical range of bull trout also included northern California, but they are likely extirpated.[2][9]
Bull trout have exacting habitat demands, requiring water temperatures generally below 55 °F (13 °C), clean gravel beds, deep pools, complex cover such as snags and cut banks, and large systems of interconnected waterways to accommodate spawning migrations.[9] Thus, they favor the deep pools of cold lakes and large rivers, as well as high, cold mountain headwaters. Bull trout may be anadromous in coastal rivers, and individual bull trout have been found to have migrated from one coastal river to another by the ocean.[10]
Bull trout are known for their extensive movements within river systems. They exhibit short-distance and long-distance movements, which are crucial in maintaining healthy populations and genetic diversity.[11] During the early life stages, bull trout are often found in small, low-gradient streams or near the shores of larger rivers and lakes.[12] As they grow and mature, they start to disperse in search of suitable spawning grounds and feeding habitats.[13]
One of the primary reasons for bull trout dispersal is the need to find appropriate spawning areas. Bull trout rely on clean, cold water and gravels of specific sizes for successful reproduction.[14] Once mature, they migrate upstream to spawn in the same streams where they were born.[15] This behavior ensures that the population remains connected and allows for the exchange of genetic material.[16] The dispersal distances can vary greatly, with some individuals traveling only a few miles while others undertake extensive migrations of over a hundred miles.[17]
Long-distance dispersal is also an essential aspect of bull trout ecology.[18] These movements often occur during the non-spawning season when the fish search for suitable feeding grounds or escape unfavorable conditions such as high water temperatures or low oxygen levels.[19] In some cases, bull trout have been observed moving between different river basins, crossing over mountain ranges, and even traversing large lakes.[20] These long-distance dispersal events contribute to gene flow between isolated populations, maintaining genetic diversity and increasing the species’ resilience.[21]
Understanding bull trout dispersal patterns is crucial for effective conservation and management strategies.[22] Conservation efforts can be focused on protecting critical habitats and maintaining connectivity between populations.[23] Conservation biologists use various techniques to study bull trout dispersal, including radio telemetry, genetic analysis, and mark-recapture studies.[24] By tracking the movements of individual fish and analyzing their genetic makeup, researchers can gain valuable insights into dispersal patterns, population dynamics, and potential barriers to migration.[25]
Bull trout dispersal is a complex and vital aspect of their life history. Their movements are critical to maintaining population connectivity, genetic diversity, and ecosystem health.[26] Protecting and restoring essential habitats, ensuring the connectivity of river systems, and managing anthropogenic influences in bull trout habitats are all crucial steps in conserving this iconic species. [27]
Young bull trout feed on zooplankton and zoobenthos, especially chironomids. As they grow larger, they begin to feed heavily upon other fish. In coastal Washington, some of the southernmost populations of bull trout feed heavily on salmon eggs and fry, as well as fish.
The bull trout is listed as a threatened species under the U.S. Endangered Species Act throughout its range in the contiguous United States.[6] In 1998, the Klamath River distinct population segment (DPS) and Columbia River DPS were federally listed as threatened.[28] This finding followed a legal challenge[29] to the U.S. Fish & Wildlife Sevice's (USFWS) decision in 1994[30] to assign a lower priority to the species' listing. The Jarbidge River DPS was listed as endangered under an emergency rule in 1998,[31] and was subsequently downgraded to threatened in 1999.[32] Finally, in determining that the Coastal-Puget Sound and St. Mary-Belly River DPS were threatened, the USFWS issued a threatened listing for all bull trout in the lower 48 states in 1999.[33]
In the United States, bull trout are used as a management indicator species for several national forests, including Boise National Forest and Sawtooth National Forest (Sawtooth National Recreation Area). They can also be found in the Glacier National Park.[34] Bull trout reproduction requires cold water and very low amounts of silt, both of which are negatively impacted by road building and logging. Additionally, its need to migrate throughout river systems may be hindered by impassible fish barriers, such as dams.[10] Bull trout populations are also in danger from hybridization with non-native brook trout.[9] Several of these issues were raised in a long-running lawsuit where in 2003, the Oregon Natural Desert Association and the Center for Biological Diversity sued the U.S. Forest Service claiming they violated the National Forest Management Act and Wild and Scenic Rivers Act by approving grazing plans in Oregon's Malheur National Forest. In April 2018, U.S. District Judge Michael W. Mosman dismissed the complaint.[35]
They are a prized game fish in northern Canada. It was once maligned out of fear they threatened populations of other native species more prized by anglers. Some jurisdictions publicize the requirement to release with the slogan "No black, put it back".[36]
Within Canada, bull trout have been designated as a "species of Special Concern" by both the Government of Alberta and the Government of British Columbia.[37]: 71–72 By the recommendation of COSEWIC,[37]: iv the Saskatchewan-Nelson Rivers population in Alberta was listed as threatened under the Species at Risk Act in 2019.[7]
Historically, confusion has existed between S. confluentus and Salvelinus malma malma, today commonly called the Dolly Varden trout. This was likely due to overlapping ranges and similar appearances among members of the two species.
The first recorded use of the name "Dolly Varden" for a fish species was applied to members of S. confluentus caught in the McCloud River in northern California in the early 1870s. In his book Inland Fishes of California, Peter Moyle recounted a letter from Mrs. Valerie Masson Gomez:
My grandmother's family operated a summer resort at Upper Soda Springs on the Sacramento River just north of the present town of Dunsmuir, California. She lived there all her life and related to us in her later years her story about the naming of the Dolly Varden trout. She said that some fishermen were standing on the lawn at Upper Soda Springs looking at a catch of the large trout from the McCloud River that were called 'calico trout' because of their spotted, colorful markings. They were saying that the trout should have a better name. My grandmother, then a young girl of 15 or 16, had been reading Charles Dickens' Barnaby Rudge in which there appears a character named Dolly Varden; also, the vogue in fashion for women at that time (middle 1870s) was called 'Dolly Varden', a dress of sheer figured muslin worn over a bright-colored petticoat. My grandmother had just gotten a new dress in that style and the red-spotted trout reminded her of her printed dress. She suggested to the men looking down at the trout, 'Why not call them "Dolly Varden"?' They thought it a very appropriate name and the guests that summer returned to their homes (many in the San Francisco Bay area) calling the trout by this new name. David Starr Jordan, while at Stanford University, included an account of this naming of the Dolly Varden trout in one of his books.
In 1874, Livingston Stone, a naturalist working for the U.S. government, wrote of this fish:[38]
Also called at (Upper) Soda Springs the 'Varden' trout. … The handsomest trout, and, on the whole, having the most perfect form of all the trout we saw on the McCloud. Also, the only fish that had colored spots. This one was profusely spotted over most of the body with redish [sic?] golden spots. ... The local name at (Upper) Soda Springs is the Dolly Varden.
It is currently unknown whether the name "Dolly Varden" was later applied to S. m. malma because of its similar appearance to S. confluentus; the two may have even been believed to be the same species. The name "Dolly Varden" may have also been given to S. m. malma independent of the McCloud River fish.
Ironically, the original "Dolly Varden" trout (i.e., S. confluentus) apparently likely became extirpated in the McCloud River in the 1970s, although reports continue of its being caught. Other fish species, typically introduced trout, outcompete S. confluentus, and can interbreed with them, resulting in sterile hybrids. An attempt to reintroduce S. confluentus to the McCloud was unsuccessful, and no additional attempts are expected.[39]
The "Dolly Varden" name is also applied to the other subspecies of S. malma, the S. m. krascheninnikova, and S. m. miyabei, found in Lake Shikaribetsu on the island of Hokkaidō in Japan.[40]
The name has also been applied to S. alpinus, today more commonly known as Arctic char.
The name "bull trout" was also given in the past to some of the large sea trout that run the River Tweed and other rivers in Scotland and North East England. Victorian anglers and others classified these as a separate race, but today they are biologically classified along with all other UK brown and sea trout as Salmo trutta. This does not deny that populations of S. trutta can differ appreciably in habits, size, and appearance from place to place, or indeed in the same river or lake.
The bull trout (Salvelinus confluentus) is a char of the family Salmonidae native to northwestern North America. Historically, S. confluentus has been known as the "Dolly Varden" (S. malma), but was reclassified as a separate species in 1980. Populations of bull trout in the lower 48 states are listed as threatened under the U.S. Endangered Species Act, and bull trout overall are listed as vulnerable to extinction on the IUCN Red List of Threatened Species. The Saskatchewan-Nelson Rivers population in Alberta, Canada is listed as threatened under the Species at Risk Act.
