Bonneville Cutthroat

Oncorhynchus clarki utah

Native Trout Fly Fishing

A Bonneville Cutthroat from a beaver pond in the Rocky Mountains.

Introduction

The Bonneville cutthroat is one of the minor subspecies of Yellowstone cutthroat and is native to the Bonneville basin of Utah and parts of Wyoming, Idaho, and Nevada. These trout adapted to life in ancient Lake Bonneville which was the largest of the Great Basin lakes and was at its greatest extent during the last ice age. Today all that is left of the Lake Bonneville is the Great Salt Lake, Bear Lake and Utah Lake and most extant populations of Bonneville cutthroat are found in small isolated streams along the fringes of the basin. Due to their long isolation from each other, there are three genetically distinct forms of Bonneville cutthroat found across the basin. The first group includes the fish of the Bear River drainage and its tributaries, the second is the fish from the Snake Valley area and the last group includes fish from the south and eastern parts of the basin.

Life History Information

Like most populations inland cutthroat, the Bonneville cutthroat are found in lacustrine, fluvial and stream resident populations. However due to long isolation between the different populations of these trout, there may be quite different adaptations to environmental conditions across their native range. Lacustrine populations of Bonneville cutthroat were once found in several large lakes across the basin and were known for their large fish. As Lake Bonneville desiccated at the end of the last ice age, its remnant: Great Salt Lake became to salty to support cutthroat trout. However freshwater influxes in Bear and Utah Lake were large enough maintain cutthroat populations in both lakes. The trout of Utah Lake went extinct in Utah Lake in the early 1900's, before any formal life history studies could be completed and as such not much is known of this population.

Today the only large lake population of Bonneville cutthroat that still persists are those from Bear Lake in Utah and Idaho. The trout of this lake have developed a specialized predator-prey relationship with the lake's endemic whitefish and cisco and historically have had the ability to attain weights of 25 pounds. This large size can be correlated to the relatively old age of Bear Lake cutthroat at maturity. Nielsen and Lentsch (1988) showed that the Bear Lake cutthroat usually don't reach sexual maturity until they are five years old, with some not spawning until the age of ten. Bear Lake cutthroat are dependant on tributaries for spawning, which typically occurs between April and June. In one of Bear Lake's tributaries, St. Charles Creek lake-run fish typically utilize the tailouts of pools are spawning habitat (Kershner 1995). There is a high mortality rate among spawners, as is indicated in the number of repeat spawners, which typically comprise only 4% of the run. Fry typically emerge from the gravel from mid to late summer depending on when they were spawned and typically will spend one to two years in their natal stream before migrating to the lake (Nielsen and Lentsch 1988). Upon entering the lake aquatic and terrestrial insects make up the majority of the cutthroat's diet, but once the reach a large enough size they feed primarily on fish. According to Wurtsbaugh and Hawkins (1990) trout under 10" fed mostly on chironomid pupae in the spring and terrestrial insects in the summer. Once the fish reached a size of 14" they became almost complete piscivorous, with the Bear Lake cisco being the preferred prey item.

Fluvial and stream resident populations of Bonneville cutthroat were historically found in most cooler streams along the fringes of the Bonneville basin. The size of these fish varies greatly depending on the productivity of the streams that they reside in. Stream resident fish isolated in headwater creeks generally reach sizes of 8 to 12 inches, while fluvial fish reach sizes of 16 to 20 inches. Fluvial fish have been found to be highly mobile and fish from a population in the Thomas Fork River of Idaho and Wyoming were found to range over a rather large territory, with some fish having home ranges of more than one mile (Colyer et al. 2005). These fish were also observed to make long spawning migrations with some individuals travelling over fifty miles. For fluvial and stream resident populations spawn timing varies with altitude, but may occur from April through July. Young of year cutthroat from fluvial populations emerge from the gravel in late summer and studies have shown that they typically drop down to the river to over winter by the end of September (Trotter 2008). These trout will feed in the river for the next two to three years and their primary diet is composed of aquatic and terrestrial insects, with some minor piscivory.

The stream dwelling populations of the Bear River drainage have adapted to life in a highly unstable arid environment similar to that of the Humboldt cutthroat. Their superior adaptation to this environment is the main reason that they continued to be the dominant trout in the watershed despite a century of introductions of non-native trout. It has been shown that stream resident populations of Bonneville cutthroat are able to survive daily water temperatures in the range of 65 to 79 degrees Fahrenheit, although a chronic exposure to temperatures over 77 degrees Fahrenheit resulted in the death of all of the studied fish (Johnstone and Rahel 2003). The diet of stream resident Bonneville cutthroat varies depending on habitat type to some degree. Fish in beaver ponds tend to feed primarily on midges, while fish in high gradient stream sections tend to depend on terrestrial insects for the majority of their diet (Hilderbrand and Kershner 2004).

Status

The Bonneville cutthroat have suffered great declines across their native range since the arrival of Euro-Americans and are only now starting to make a come back thanks to more enlightened fisheries practices. A wide variety of issues contributed to the decline of these fish including, over-harvest, habitat destruction and the introduction of non-native fish. The introduction of non-native fish has been especially detrimental to many populations of cutthroat around the basin. This is especially true for the Snake Valley populations and the fish of the south and eastern parts of their range. Brook trout and brown trout both often out compete the Bonneville cutthroat, while rainbow trout not only compete with they but also hybridize with them. Agriculture has also been particularly detrimental to the Bonneville cutthroat, due to habitat degradation and a loss of habitat connectivity. Diversion dams for irrigation purposes often block migration corridors and access to spawning habitat, while also isolating populations from each other. On the Bear River none of the dams that have been erected were built with fish ladders leading to highly fragmented habitat. During summer many of the tributary streams of Bear Lake completely dry up when Bonneville cutthroat eggs are still incubating, leading poor survival rates (Lentsch et al 2000). Irrigation also leads to an increase in the sediment load in streams, lowering the survival of eggs due suffocation. Like irrigation, grazing by livestock also contributes to stream sedimentation by destabilizing stream banks, but also leads to a loss of riparian vegetation. This means higher stream temperatures, which often exceed the tolerance of the cutthroat.

Due to a combination of these issues by the 1960's populations of Bonneville cutthroat were so rare across the basin that these fish were believed to be extinct. However because the Bonneville Cutthroat native to the Bear Lake drainage had evolved in an arid and highly fluctuating environment, they were better adapted and persisted in streams that nonnative trout could not survive in. Today there are around 50 pure populations of Bonneville Cutthroat and recovery are steadily increasing that number (Behnke 2002). Due to this reason a recent petition to list the Bonneville cutthroat as threatened under the ESA was rejected by the U.S. Fish and Wildlife Service.

Description

Due to the long isolation from each other there is a high degree of variability in the coloration between Bonneville cutthroat populations. Stream resident and fluvial populations of Bonneville cutthroat have very similar coloration to that of the Yellowstone Cutthroat, although the coloration of the Bonneville cutthroat does tend to be a bit more drab. Generally Bonneville cutthroat exhibit a grayish-olive to greenish-olive color on their back, which transitions to a grayish-brown or bronze color on the sides. Some mature fish may exhibit quite vivid spawning colors an orange to red color on their gill plates and some may even display a red band along their lateral line. The lower fins in some populations, especially those of the Bear River may be a bright yellow color, while populations from the southeastern part of their range often have reddish colored fins. The Bonneville cutthroat have fairly large spots that are sparse, yet fairly evenly distributed across their body. Lacustrine populations adopt a much more silvery blueish-green coloration, which transition to white on the belly. The spots on these fish tend to finer yet still sparse distributed across the body like their stream brethren.

Stream Resident Form

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Native Range

Native Trout Fly Fishing

A map of the native range of the Bonneville Cutthroat trout. Data Source: Behnke (2002) and Trotter (2008).