Fish Facts » Bull Trout
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).
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.
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).
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
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 eastern slope of the continental divide, sustain S. confluentus as well. In the United States, it is known that 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).
University of Michigan Museum of Zoology Animal Diversity Web
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