Oncorhynchus clarki bouvieri
A 16" native yellowstone cutthroat from a stream in Yellowstone National Park.
The Yellowstone cutthroat is a major subspecies of cutthroat trout that is native to the Yellowstone and Upper Snake River drainages. There is some debate as to how far west their original range extended, as populations of cutthroat from Crab Creek, WA and Waha Lake, ID were described as Yellowstone cutthroat. However the spotting pattern of some Cascade Mountain populations of westslope cutthroat closely resembles that of the Yellowstone cutthroat and with both of these populations now extinct there maybe be no way definitively say which type of trout originally inhabited either of these waters. Oddly enough right in the middle of the native range of the Yellowstone cutthroat, dwells another form of cutthroat called the Snake River finespotted cutthroat. As their name suggests, these fish have considerably smaller spots than the Yellowstone cutthroat, which are found both upstream and downstream of them.
Like the westslope cutthroat, Yellowstone cutthroat exhibit three life history strategies, including lacustrine, fluvial and stream resident. Fluvial and stream resident populations of Yellowstone cutthroat occur in a number of drainages across the subspecies' native range. Stream resident fish are typically found in the upper reaches smaller streams and rarely live longer than three to five years of age and typically only attain a maximum size of nine to ten inches (Behnke 2002). These trout are opportunistic feeders with a diet consisting primarily of aquatic and terrestrial insects, although some populations have even been shown to consume large amounts red-osier dogwood berries that fall in the water (Skinner 1985). Fluvial populations are common throughout the native range of the Yellowstone cutthroat, and may even occur alongside lacustrine populations. In the outlet of Yellowstone Lake both fluvial and allacustrine (outlet spawning lake dwelling) cutthroat are present. These two life histories do not appear to be reproductively isolated, as they both spawn in the same sections of the river and during the same time period (Kneading and Boltz 2001). Fluvial Yellowstone cutthroat are also generally opportunistic feeders and in some populations other fish may make up a substantial portion of their diet. Like all cutthroat trout the Yellowstone cutthroat spawn during the spring, with peak spawning typically occurring from between June and July in the Yellowstone River (De Rito 2004). Similarly Yellowstone cutthroat in the Snake River system spawn during June just after spring runoff has subsided, a pattern that appears to be consistent throughout their native range (Thurow and King 1994). Although many populations utilize tributaries as spawning habitat, some populations spawn in the side channels of larger rivers. These trout typically spawn for the first time at an age of four or five years old and repeat spawning may be common in some population. The average spawning female Yellowstone cutthroat in the South Fork of the Snake River is around 15 inches long and carries about 1,400 eggs (Trotter 2008). Fry typically emerge from the gravel in late summer or early fall (WGFD 2005), and spend between one to three years rearing in their natal stream before migrating to the river.
The best studied and strongest population of lacustrine Yellowstone cutthroat occurs in Yellowstone Lake. Yellowstone Lake is a large, deep oligotrophic (low nutrient) lake located at 7,733 feet above sea level, with a short growing season and the lake being frozen over from late December until late May or early June. Between May and July the lake's cutthroat ascend 68 of the 124 known tributaries to spawn (Gresswell and Varley 1988). McCleave and LaBar (1972) showed that the trout of Yellowstone Lake have a homing mechanism and typically return to their natal stream to spawn. Newly hatched juvenile fish generally enter the lake during their first year of life, with the majority of the fish heading directly to the lake after emerging in August as well as some fish overwintering in their natal streams before migrating to the lake (Benson 1960). Once in the lake the cutthroat feed primarily on zooplankton in deep open water, until they reach two to three years old, after which time they spend most of their time in the littoral zone (near shore) feeding on aquatic and terrestrial insects (Trotter 2008). Since the trout of Yellowstone Lake did not evolve alongside prey species, they do not readily feed on other species of fish. With the combination of this and the cold, low nutrient waters, these trout grow slowly attaining maximum size of around 21 inches and four pounds at an age of eight or nine years. In comparison, Henry's Lake in Idaho which is also home to a native lacustrine population of Yellowstone cutthroat, is very shallow and nutrient rich and as such it's cutthroat grow faster attaining a maximum size of 24 inches long and six pounds, with a lifespan of six or seven years (Behnke 2002).
While the populations of many other subspecies of cutthroat have diminished throughout the Western United States, Yellowstone cutthroat have continued to hold their own in many parts of their native range. This is largely due to the creation of Yellowstone National Park in 1872, as well as the remoteness of much of the area that they inhabit. At the heart of Yellowstone lies Yellowstone Lake, which is supports the greatest lacustrine population of cutthroat today, with an estimated 7.5 to 10 million trout inhabiting the watershed (Trotter 2008). Today the Yellowstone cutthroat are the most abundant and extensively distributed inland subspecies of cutthroat and are thought to inhabit around 7,500 miles of stream habitat (Varley and Gresswell 1988; May et al. 2003). Even though the Yellowstone cutthroat has not been pushed as close to the brink of extinction as other subspecies, they have experienced substantial declines due to a variety of issues.
The biggest threat to the Yellowstone cutthroat today continues to be the introduction of non-native fish. Although stream type Yellowstone cutthroat still inhabit around 7,500 miles of streams they originally occurred in around 17,400 miles, around a 67% decrease in the amount of habitat that they once occupied. Of 457 sites in their native range in Idaho about half of them were found to contain non-native species (Meyer et al. 2006). The Yellowstone cutthroat were once the dominant species in some of the region's most famous rivers, such as the Henry's Fork of the Snake and the Yellowstone River. Today both brown and rainbow trout are the predominant species in both of these waters as well as many others and hybridization between the rainbow trout and cutthroat has led to the loss of many populations. Although hybridization has been a major cause the decline the Yellowstone cutthroat, some populations such as fluvial fish in the Yellowstone River have managed to maintain their genetic integrity. According to De Rito (2004) the major mechanism for this is a difference between the spawning time of the cutthroat and rainbow trout, which spawn five to nine weeks earlier than the cutthroat do. However other sources report that this reproductive isolation has begun to break down in recent years and the rainbow trout are becoming increasingly dominant throughout the drainage, especially in the lower river (Trotter 2008).
No discussion on the status of Yellowstone cutthroat would be complete without mentioning the problems now facing the trout of Yellowstone Lake. In 1994 illegally introduced lake trout were discovered in Yellowstone Lake and now pose a serious threat to these fish. Lake trout are highly piscivorous (fish eaters) predators and are expected to reduce the cutthroat population by 60 to greater than 90 percent over the next 20 to 100 years, if left uncontrolled (Kaeding et al. 1996; Stapp and Hayward 2002). The national park service as adopted a required catch and kill policy for lake trout caught by anglers in Yellowstone Lake and has been gill netting the lake trout in known spawning areas in an effort to confront this issue. As of 2007 the park service has netted around 272,000 lake trout, with 74,000 fish caught in 2007 alone (NPS 2008). However recent trends indicate that the lake trout population may still be growing and the park service is researching alternative techniques for lake trout removal, such as attempting to kill lake trout eggs before they can hatch. As if the lake trout didn't already present a big enough threat, in 1998 whirling disease was discovered in the lake and is also expected to have an impact on the cutthroat population. The impact that these issues are having on the lake's cutthroat population is already clear, as Koel et al. (2005) showed that number of adult spawners returning to two of the lake's tributaries, Bridge and Clear Creeks have decline about 90 percent in the last five years. With the future for these fish uncertain, environmental groups have been pushing for an ESA listing, but all petitions to have these fish listed have been denied by the U.S. Fish and Wildlife Service.
The coloration of the Yellowstone cutthroat is generally less intense than that of other inland forms of cutthroat (Behnke 2002). These fish are typically a yellowish-brown golden-olive or bronze color, which becomes paler near the belly; although the bellies of some mature fish may become a brilliant golden-yellow. Some individuals may display a rose color along the lateral line and on the gill plates. The coloration of the lower fins ranges from a bright orange to rose color on some specimens. There is considerable variation in the spotting pattern of Yellowstone cutthroat, but overall the spots tend to be fairly large, round and concentrated toward the posterior region of the fish. These spots are found above and below the lateral line, as well as on the dorsal and caudal fins. Like other cutthroat, these fish exhibit a red cutthroat mark below the lower jaw.
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