OVERVIEW

HISTORY

ACTORS

MAPS

DIMENSIONS:

ecosystem

wildlife

economic

policy

recreation/aesthetic

social

STUDY TEAM

REFERENCES

Wildlife

This section will give an overview of the life histories of the Yellowstone Cutthroat trout and the lake trout, followed by case studies of their interactions, and finally a brief overview of other exotic species introduced to the park.

Native Fishes of the Park

The park has 12 fish species native to its waters (http://www.nps.gov/yell/sotp/three/aquatic.pdf). The park has 3 species of salmonid, the Cutthroat trout, Oncorhynchus clarki, with three subspecies the Yellowstone, O.c. bouveri; the westslope, O.c. lewisi; and the snake river finespotted, O. c. carmichaeli subspecies, the arctic grayling, Thymallus arcticus, and the mountain whitefish, Prosopium williamsoni. The park has three suckers of the family catastomous, the Utah, Catostomus ardens; the longnose, Catostomus catostomus; and the mountain, Catostomus platyrhynchus. The park also has 5 minnows or cyprinids, the redside shiner, Richardsonius balteatus; the lingnose dace, Rhinichthys cataractae; speckled dace, Rhinichthys osculus; Utah chub, Gila atraria; and a redside shiner/speckled dace hybrid. Only one cottid species inhabits the park, the mottled sculpin, Cottus bairdi. Of this group only the Yellowstone cutthroat trout and longnose dace are native to the Yellowstone Lake. Introduced species that have succeeded in the park are rainbow trout Oncorhynchus mykiss, brook trout Salvelinus fontinalis, brown trout Salmo trutta, lake trout Salvelinus namaycush, and lake chub Couesius plumbeus.

Yellowstone Cutthroat trout: on left pursuing redside shiners and on right building a nesting redd.

Both the Yellowstone cutthroat trout (Oncorhynchus clarki bouveri) and the westslope cutthroat trout (Oncorhynchus clarki lewisi) have been recently petitioned for listing as threatened on the Endangered Species Act. The Yellowstone cutthroat trout now only occupies 10% of its native range, almost all of the 10% lies within Yellowstone National Park (see maps). The westslope cutthroat trout has also had precipitous declines and their only strong holds of survival in their range, Flathead Basin and Glacier National Park, are being affected by logging and other activities.

Life History of the Lake Trout and Yellowstone Cutthroat Trout

The difference in life histories of the cutthroat trout and the lake trout are important to consider in how they affect their surrounding ecosystem.

The Lake Trout

Important aspects to take away are:

Since lake trout live deep in the water column they are inaccessible as prey to terrestrial predators. The inaccessibility disrupts an important link of energy from the aquatic system to the terrestrial system of the GYE. The spawning activities of the lake trout also leave them inaccessible to terrestrial wildlife since they breed deep in the water. The cutthroat trout on the other hand has a perilous journey up creeks to spawn while avoiding predators like otters and osprey and once they have breed they must run the gauntlet again to return to the lake. The last important aspect of life history is that lake trout are piscivores (fish eaters). Once a lake trout reaches 12 inches it starts eating cutthroat trout that are smaller than it is. A lake trout can eat as many as 3,000 cutthroat trout in its lifetime if not more (Murphy 1999).

US record for sport caught Lake Trout, 71 pounds, in Lake Superior.

Reasons for Concern About Lake Trout

The reason biologists are so scared that lake trout were found in Yellowstone Lake is that the conditions of the lake are perfect for lake trout reproduction. A habitat suitability index for lake trout, by Marcus et al. in 1984, states that the top preferred habitat is one that meets the following five pieces of criteria.

A criterion that is important to lake trout not listed above is that they like big, clear lakes. The environment of Yellowstone Lake closely matches these criteria. Yellowstone Lake has a surface area of 341 square kilometers and an average depth of 48.5 m (Kaeding et al. 1995). A thermocline forms in July and may persist to mid-September; the hypolimnion is well oxygenated during stratification (Kaeding et al. 1995). The surface temperature rarely exceeds 18 degrees Celsius (Kaeding et al. 1995), so one can logically assume that the hypolimnion stays at 16 degrees of less throughout the year. In the lake the lake trout can choose a variety of forage fishes to consume. It has been known to eat long nose suckers and cutthroat trout (Murphy 1999). The lake has miles and miles of shoreline where cobble, rubble, or boulder substrate occurs (Kaeding et al. 1995). If the winds were right than all of those miles of shoreline is potential breeding habitat for lake trout. The last aspect of the optimal environment for lake trout is that the reservoir is not dropped more than 15 cm a year. Since Yellowstone Lake is a natural system with no dams it is unlikely that the lake level would drop by as much as 15 cm in one year. So, Yellowstone Lake makes a perfect habitat for lake trout to expand their numbers rapidly.

Lake trout produce 628 to 1710 eggs per kg of body weight. On average, a single female lake trout in Jackson Lake, Wyoming yield 6000 eggs (Marcus et al. 1984). When forage fish are the primary source of food, a slowing in growth of lake trout is not apparent (Marcus et al. 1984). These two aspects line up for a very rapid growth of lake trout populations in Yellowstone Lake. It is possible that the lake trout in Yellowstone Lake will produce more than 6000 eggs per female due to their rapid growth potential.

Case Studies of Lake Trout and Cutthroat Trout Interactions

Introduced lake trout often lead to the demise of other trout. For instance, introductions into Lake Tahoe led to the elimination of the native Lahontan cutthroat O. c. henshawi (McAffee 1966; Moyle 1976). Cordone and Frantz (1966) reported a drastic decline and eventual extinction of native cutthroat in Lake Tahoe after the introduction of lake trout. Lahontan cutthroat were abundant in the lake in 1907, with only an occasional lake trout reported. By 1938, the cutthroat had virtually disappeared, and by 1966, lake trout comprised 70% of angler catch in the lake (Cordone and Frantz 1966). Stocked lake trout have replaced native cutthroats in deep Rocky Mountain lakes (Benson et al. 1961). This trout has also virtually eliminated cutthroat and bull trout in Flathead Lake, Montana, and Pend Orielle Lake, Idaho. Lake trout introduced into Flaming Gorge Reservoir were found to prey heavily on the Utah chub Gila atraria (Teuscher and Luecke 1996). So, it is understandable that fisheries biologists, in the Yellowstone National Park, are very worried over the introduction of lake trout into fragile Yellowstone Lake ecosystem.

This figure represents an idealized interaction of cutthroat trout and lake trout in Yellowstone Lake. (www.nps.gov/yell/planvisit/todo/fishing/laketroutweb/)

Other Important Introduced Species

Another introduced species that could possibly cause the demise of the Yellowstone cutthroat trout is Myxobolus cerebralis, which is commonly known as whirling disease. The disease is of European origin. It was first found in the US in 1956 in Pennsylvania and now occupies 21 states. The disease may have been transported to Yellowstone Lake by dirty fishing gear or boats. A fall study in 1998 revealed that of 41 cutthroat trout sampled from the lake, 27% showed an infection of whirling disease (Hudson and Mahoney 1998). Whirling disease is caused by microscopic parasite that attacks the cartilage of some fish, like rainbow and cutthroat trouts. Ironically this disease does not affect lake trout so the effects of the disease are additive to the losses caused by introduced lake trout. A fish affected with whirling disease is unable to feed normally or swim normally and are therefore more susceptible to predation and starvation. When a fish dies and decays due to whirling disease the parasite turns into spores that can live up to 30 or more years.

Introduced species having an affect on Yellowstone Lake or a possible effect of Yellowstone Lake is a reoccurring theme. In 1995 the zebra mussel was found in Yellowstone Park. This mussel has caused millions of dollars of damage to the great lakes. These filter feeders can clean a lake out of its primary production of phytoplankton and zooplankton. If they get a foothold in Yellowstone Lake they will affect the ecosystem from the bottom up. Cutthroat trout will not have food to survive to adulthood, nor would lake trout. The whole ecosystem of the lake and all linkages to the terrestrial environment could change. Even the insect structure around the lake and the rest of the park could be vastly different if zebra mussels are able to get a foothold in Yellowstone Lake or any other lake in the park.

Zebra Mussels colonizing on a crawdad.