NATIONAL PARK SERVICE Research in the Parks NPS Symposium Series No. 1

The Snake River and its tributaries in Wyoming contain one of the few-remaining, native cutthroat trout (Salmo clarki) populations in the Rocky Mountain area. The present distribution of this population is in the Snake River between Jackson Lake and Palisades Reservoir (Baxter and Simon 1970). The Snake River cutthroat trout is the only interior cutthroat trout that has maintained its integrity despite introductions of exotic trouts, including other cutthroat trout (Behnke 1971). Although the taxonomic status of this distinctive population has yet to be determined, the National Park Service has designated the Snake River cutthroat trout as a representative faunal species in Grand Teton National Park.

This trout population supports an important sport fishery which is maintained, for the most part, by natural reproduction. The Snake River in Wyoming has long had a national reputation as a top quality trout fishing stream. Narratives, photographs, and verbal reports of "old timers" indicate that fishing success was good during the 1930-60 period for trophy-sized cutthroat trout. The quality of this fishery, as reflected by rate of catch and numbers of trophy-sized trout creeled has deteriorated since the mid-1950s.

Information on the sport fishery and the ecology of the trout population was lacking or minimal so the Wyoming Game and Fish Commission initiated a long-term study of the Snake River cutthroat in 1964. Adjunct studies on invertebrate organisms and food habits of trout and whitefish in the Snake River and on trout populations in Jackson Lake have been sponsored or conducted by the National Park Service since 1965. Harvest studies on the Snake River below Jackson Lake have been conducted cooperatively by the two agencies since 1967. The overall objectives of these studies were to determine the present status of the Snake River cutthroat trout population and those factors in fluencing its welfare.

THE AREA

The Snake River originates in northwestern Wyoming's Teton Wilderness Area, flows through a portion of Yellowstone National Park, and enters Jackson Lake in Grand Teton National Park. After leaving Jackson Lake, the river flows south for about 25 miles before leaving the park boundary. Some 55 miles south of Grand Teton National Park, the Snake River leaves Wyoming and enters Palisades Reservoir in Idaho.

Within the 80-mile reach of river downstream from Jackson Lake, there are significant physical differences in habitat. A Bureau of Reclamation dam at the outlet of Jackson Lake totally regulates the flow for 4 miles downstream where two major tributaries, Pacific Creek and Buffalo Fork River, discharge into the Snake River. From this point, the Snake River flows south and west through the valley of Jackson Hole for about 47 miles. In this section, the river has cut mostly into glacial out wash of the last (Pinedale) glacial advance and the substrate consists almost entirely of quartzite cobbles (3-10 inches in diameter) firmly em bedded in a sandy-silt matrix. The river is very active on its flood plain in this area and over two-thirds of the entire section consists of braided areas (multiple channels). Extensive levees have been constructed by the Army Corps of Engineers south of Grand Teton Park to prevent damage to adjacent private lands. The Snake River then swings to the south and east (following a fault structure) and enters a steep-walled canyon through which it flows south and then west into Idaho. In this canyon, the river crosses some resistant mountain structures which retard downcutting upstream.

The area has cool summers and cold winters. Precipitation amounts are variable due to topography (from less than 15 to over 60 inches annually) and most falls as snow during the November-April period. This snowpack usually begins to melt and run off in mid-May. Streams and rivers are high and turbid from this time until early July. This pattern is modified in the Snake River (particularly in Grand Teton Park) by the operation of the Jackson Lake dam which reduces the magnitude of the May-June flows and increases July-September flows from 20 to 50%.

Water quality in the upper Snake River is high, being slightly alkaline (pH 7.5-8.0) with relatively small amounts of dissolved material (total dissolved solids varies from 100 to 200 mg/liter). Both alkalinity and dissolved materials increase somewhat downstream. Cultural alteration of water quality is minimal. Primary land use in the area is recreation, and agriculture, mainly hay crops, utilizes only a small proportion of the total acreage.

THE FISH FAUNA

The fish fauna of the Snake River is typical of cold waters and is relatively species-poor. Glaciers of the Pinedale advance existed in the valley of Jackson Hole as recently as 9000 years ago (Love and Reed 1968) and the indigenous fishes inhabiting the lakes and streams of the area undoubtedly became established since then. In addition to the cutthroat trout, the native fish fauna includes the mountain whitefish (Prosopium williamsoni), five minnows (Cyprinidae), three suckers (Catostomidae), and two sculpins (Cottidae). A single specimen of the June sucker (Chasmistes liorus) was collected from the Snake River below Jackson Lake in 1927, but is now probably extinct (Baxter and Simon 1970). Four introduced fishes presently inhabit portions of the upper Snake River drainage in Wyoming—lake trout (Salvelinus namaycush), brown trout (Salmo trutta), brook trout (Salvelinus fontinalis), and rainbow trout (Salmo gairdneri). Lake trout have become the dominant fish in Jackson, Jenny, and Leigh lakes in Grand Teton National Park, and in Shoshone, Lewis, and Heart lakes in Yellowstone National Park. Brown trout are abundant in the Lewis River drainage and are common in the Snake River north of Jackson Lake and in Jackson Lake. Brook trout are common in tributary streams, while localized populations of rainbow trout and rainbow cutthroat hybrids occur in the Snake and Gros Ventre rivers.

THE INVERTEBRATES

National Park Service-sponsored studies involving classification, life history, and downstream drift of invertebrate organisms in the Snake River were initiated in 1965. These studies indicated that the Snake River is relatively productive—quantitative collections averaged 11,399 mg of organisms per square meter. Caddisflies, mayflies, stoneflies, and dipterans composed over 98% of the total biomass of invertebrates collected. The caddisflies (particularly the genera Hydropsyche and Arctopsyche) were the most abundant and important group found. The Snake River invertebrate fauna is fairly complex—170 species were collected and identified. Major differences in species composition were observed between the two major sections of river in the park. The invertebrate fauna inhabiting the portion between the Jackson Lake dam and the mouth of Pacific Creek contained only 7 major species, while below Pacific Creek at least 23 major species were present (Kroger 1967).

Major groups of insects in the downstream drift were also important in the benthos (stream bottom community). Behavioral differences were observed in the drift among major groups—mayflies were active drifters while caddisflies were relatively inactive. Most benthic forms showed nocturnal drift patterns, but considerable daytime drift of immature midge (Diptera) larva was observed. Emerging forms occurred in the drift at all times, but mostly at sunset and sunrise. Spent adults had peak periods corresponding with oviposition and terrestrial insects occurred sporadically (Good 1971).

Aquatic invertebrates are the most important component of the diet of cutthroat trout in the Snake River. They also compose nearly the entire diet of the abundant (and possibly increasing) population of mountain whitefish. Studies are planned or are underway to evaluate the ecological relationships between these two native fishes. A present hypothesis is that selective angling (for trout) may have altered these relationships.

Studies of the water-release patterns from the Jackson Lake dam indicated that nearly all invertebrate organisms and most sculpins in some areas of the river were left stranded when volume flows were reduced suddenly. Recommended changes in water-release practices to reduce this problem have been followed by the Bureau of Reclamation.

GROWTH AND REPRODUCTION OF CUTTHROAT TROUT

Estimates of age composition, total mortality, and age-growth relationships of the trout population were obtained from analysis of samples of cutthroat trout scales collected during harvest studies. Snake River cutthroat trout have slightly better than average rates of growth compared to other river cutthroat populations in the western United States. Average length of various age groups was: age I—211 mm (8.3 inches); age II—267 mm (10.5 inches); age III—351 mm (13.5 inches); age IV—394 mm (15.5 inches); and age V—470 mm (18.5 inches) (Hagenbuck 1970). Few Snake River cutthroat trout live longer than 5 years. Annual mortality rates of cutthroat trout for the entire study area were 66%, 71% for areas within the park and 60% for areas south of the park.

As previously stated, the cutthroat trout population in the Snake River is maintained by natural reproduction. Sporadic stocking of hatchery-reared cutthroat trout in the river proper has been done in the past, but the present policy restricts stocking to a few tributary streams. All known natural spawning of Snake River cutthroat trout occurs in tributary streams. Apparently, suitable spawning habitat does exist in some side channels of the river, but evidence of use by cutthroat trout has never been reported. In Grand Teton Park, cutthroat trout are presently known to spawn in only two tributary streams. Two other tributaries in the park which once supported large spawning runs have no known spawning today.

Snake River cutthroat trout become sexually mature at age groups III or IV, with females tending to mature later than males. There is evidence that the river population may consist of subpopulations which spawn in different tributaries. Timing of spawning runs in three tributaries occurred from March through June with little overlap and appeared to be influenced by inherent rather than environmental factors. Mortality of post-spawning trout was about 50% in one stream, but a high proportion (17% in one year) of the survivors may return to spawn again the following year. Mortality of newly hatched trout is high, less than 5% survive the first 2 months. Young-of-the-year cutthroat trout migrate into the Snake River from September throughout the following winter although some may remain in tributary streams for over one year (Hayden 1968).

THE ANGLER HARVEST

The Snake River trout fishing season runs from 1 April through 31 October. Fishing for mountain whitefish is permitted throughout the year. Angler use is most intensive from July through September, coinciding with the peak visitation period and subsidence of high water conditions. Most people fish from the shore near one of the vehicle access areas along the river, but increasing numbers of anglers are using boats to gain access to the entire river.

The first investigation of the Snake River sport fishery was a harvest study conducted in 1955 by the Wyoming Game and Fish Commission. A portion of the area studied was in Grand Teton National Park. As a result of this study, it was concluded that the sport fishery was having no detrimental effects on the trout population. Angler success rates were considered satisfactory (0.69 trout caught per hour) since many anglers contacted were either inexperienced or were fishing during adverse water conditions (Rasmussen 1956).

Annual harvest studies have been conducted cooperatively since 1967, the National Park Service being responsible for that portion of the Snake River within Grand Teton National Park and the Wyoming Game and Fish Commission, for that portion south of the park. No trends in the cutthroat trout population (as reflected by angler harvest) were indicated during the 1967-70 harvest studies. Analysis of the 1971 data is still incomplete. There were variations in harvest estimates between years of considerable magnitude which were ascribed to variations in water conditions and sampling methods.

Catch rates for the entire study area were 0.26, 0.53, 0.31, and 0.30 trout caught per hour for the 1967 through 1970 seasons, respectively (Wiley 1969; Kiefling 1971). With an exception in 1967, in one study subsection, angler success rates within Grand Teton National Park were lower than in those areas south of the park. During the period, 59% of the total number of persons fishing the Snake River actually fished in the park while catching 41% of the total trout harvested. There was no apparent correlation between catch and fishing effort on any section of the river. Differences in catch rates appeared to be a function of water conditions—better fishing was related to an early run-off and low water in late summer (Kiefling 1971).

Cutthroat trout are not fully vulnerable to capture by the sport fishery until age group II (Hagenbuck 1970). Average length of trout in the angler harvest has been near 11 inches although this appears to reflect angler selectivity rather than any factor in the trout population. In 1967 and 1968, when comparable data were collected, 79% of the trout harvested within the park were less than age group III, while 59% of the trout harvested from areas south of the park were less than 3 years old.

THE CUTTHROAT TROUT POPULATION

Efforts to estimate stock density of cutthroat trout in Grand Teton Park in 1968 and 1969 were not successful due to an inability to sample enough fish. Using hook and line methods to capture and mark trout and harvest information to recover the marked fish, the Wyoming Game and Fish Commission was able to make population estimates in a 10-mile river section south of the park. Because of angler selectivity, these estimates (4002 in 1969 and 9919 in 1970) were only of those trout in the population over 8 inches long (Kiefling 1971). Although strict comparison is not possible, estimated stock density of trout in the Madison River in Montana has averaged about twice as much (Vincent 1970).

Estimated fishing mortality in the river section studied south of the park was 46% in 1969 and 22% in 1970. No correlation between angler success and stock density was detected. It was concluded that the correlation between harvest rates and water conditions so distorted any relationships between catch, effort, and stock density that reliable estimates of exploitation rates were impossible (Kiefling 1971).

ALTERATION OF HABITAT BY MAN

The most obvious cultural changes in the Snake River habitat, regulated flows and levees, have undoubtedly had some influence on the trout population. Flow regulation due to the operation of the Jackson Lake dam has occurred since 1916. The dam itself is a barrier to fish migration, the biological consequences of which probably occurred long ago and are now only a matter of speculation. Prior to 1956, when Palisades Reservoir was completed, there were abrupt changes in flow (over 5000 cfs or 141.6 m³/s a day) during the irrigation season, as dictated by the water needs of the irrigation districts in Idaho, and water was stored in Jackson Lake during the winter resulting in dewatering of the first 4 miles of river. At the present time, minimum flows are maintained throughout the winter and summer releases are adjusted gradually to simulate natural flows. The one persistent problem concerns required periodic shut down for inspection of the dam. Both the National Park Service and the state of Wyoming are programming funds for construction of a by-pass to maintain minimum flows below the dam at these times.

The apparent effect of the levees (constructed between 1951 and 1964) is the permanent dewatering of over 8 miles of side channels, which are otherwise excellent trout habitat, and the increased hydraulic activity between levee structures (Wiley 1969). There has also been significant loss of habitat for other wildlife due to levee construction. It should be mentioned that fishing success in those river sections entrained by levees has been better than in Grand Teton National Park where no levees exist.

NATIONAL PARK SERVICE POLICIES

The basic management objective in those National Park Service areas classified as natural areas (including Grand Teton National Park) is to limit modern man to nonconsumptive uses (Houston 1971). Recreational angling violates this objective but is a traditional use established in the creation of Yellowstone National Park in 1872. Recreational fishing in some form will undoubtedly be continued in the foreseeable future.

The objectives of management of aquatic resources in national parks are to protect, perpetuate, and restore natural environments, native fishes, and the associated fauna and flora. Where fishing is encouraged, it is governed by the conservative and controlled use of native and non native sport fish species and by regulations and measures that are designed to encourage high-quality angling as part of the park experience without impairing the basic fish populations or other park values (Wallis 1971). High quality angling is defined as that in which a person has the opportunity to fish for and catch rare native fish or wild trout in a pristine setting under conditions where angler removals do not exceed natural replenishment rates or materially alter the population structure of the fish being harvested. A basic requirement of this definition distinguishes between catching and killing fish. Concepts such as maximum fishing opportunity and maximum sustained yield tend to upset ecological relationships and should not guide management in national parks.

CONCLUSIONS, HYPOTHESES, AND FUTURE PLANS

Condition of the cutthroat trout fishery in Grand Teton National Park is mediocre when compared to what it was in 1955 and to present conditions south of the park. We interpret this to mean that condition of the trout population in the park is worse than it was previously and is presently poorer than in areas downstream. Lacking evidence of significant habitat deterioration in recent years, we conclude that recreational angling is the major cause of this decline. Cutthroat trout are quite vulnerable to capture by sport angling and removals from the population by anglers may have exceeded natural replenishment for some time. Imminent collapse of the trout population in the park is very unlikely, but survival of trout to spawning age is low. Low numbers of spawning trout coupled with periodic adverse environmental factors and/or man-caused habitat destruction may have already eliminated at least two spawning runs in the park.

Our basic hypothesis is that stock density and age structure of the cutthroat trout population in the Snake River will be improved by eliminating or greatly reducing angling mortality. In Yellowstone National Park, where angler success in the Yellowstone Lake cutthroat trout fishery was declining, two year classes (ages VI and VII) had been eliminated from the population in the more heavily fished parts of the lake and the fishery was largely supported by 4-year-old trout. In 1970, regulations were imposed placing a 14-inch minimum size on cutthroat trout kept by anglers and eliminating the use of bait. As a result of these regulations, the total harvest was reduced about two-thirds in 1970. Angler success, however, increased by 47% over 1969 (Dean and Mills 1971). Part of this increase in success rate was attributed to the presence of a strong year class of 4-year-old trout, but in 1971, when a weak age IV year class entered the fishery, success rates increased 35-40% over 1970 to nearly the highest in 21 years of record (J. L. Dean 1971, pers. comm.).

In Grand Teton National Park, we are recommending enactment of regulations to reduce angling mortality on cutthroat trout. Our specific recommendations are the complete protection of cutthroat trout in the Snake River between Jackson Lake and the south park boundary for a 5-year period and the prohibition of the use of bait. Our research efforts during this period will be to evaluate any response of the trout population. Consideration was given to the alternative of establishing a minimum size limit. Experience in Yellowstone National Park indicated that most anglers accept size limit regulations only if the fishing is good. When fishing is poor, there is a tendency to keep trout smaller than the legal size. If, as expected, angler success improves under these regulations, then a size limit on cutthroat trout may be appropriate.

To summarize, the management in Grand Teton National Park of this irreplaceable and unique resource, the Snake River cutthroat trout, has not been up to the standards outlined in National Park Service resource policies. If we are to protect these fish and " leave them unimpaired for the enjoyment of future generations," then we must improve present conditions.

REFERENCES

BAXTER, G. T., and J. R. SIMON. 1970. Wyoming fishes. Wyo. Game Fish Comm. Bull. 4.

BEHNKE, R. 1971. The zoogeography, systematics and management of cutthroat trout. Am. Fish. Soc. Exhibit. 101st Annual Meeting, Salt Lake City, Utah.

DEAN, J. L., and L. E. MILLS. 1971. Annual progress report, Yellowstone fishery management program for 1970. U.S. Bur. Sport Fish. Wildl. 107 p.

GOOD, W. R. 1971. Downstream drift of aquatic invertebrates in the Snake River, Grand Teton National Park. Final Report, Segment 2, NPS Research Contract 14a 10-2:920-18. 75 p.

HAGENBUCK, W. W. 1970. A study of the age and growth of the cutthroat trout from the Snake River, Teton County, Wyoming, M. S. Thesis. University of Wyoming, Laramie. 68 p.

HAYDEN, P. S. 1968. The reproductive behavior of the Snake River cutthroat in three tributary streams in Wyoming. M.S. Thesis. University of Wyoming, Laramie. 144 p.

HOUSTON, D. B. 1971. Ecosystems of national parks. Science 172:648-651.

KIEFLING, J. W. 1971. An analysis of stock densities and harvest of the cutthroat trout of the Snake River, Teton County, Wyoming. M.S. Thesis. University of Wyoming, Laramie. 184 p.

KROGER, R. L. 1967. A study of the classification and ecology of the aquatic invertebrates in the Snake River, Grand Teton National Park, Wyoming, M.S. Thesis. University of Wyoming, Laramie. 161 p.

LOVE, J. D., and J. C. REED, Jr. 1968. Creation of the Teton landscape. Grand Teton Nat. Hist. Assoc. publication. 120 p.

RASMUSSEN, D. H. 1956. A creel census and fisherman expenditure study on Snake River, Wyoming. Wyo. Game Fish Comm. Fish. Tech. Report No. 4. 26 p.

VINCENT, E. R. 1970. Evaluation of river fish population. Job Progress Report, Federal Aid in Fish and Wildlife Restoration Acts, Montana Project No. F-9-R-18.

WALLIS, O. L. 1971. Management of aquatic resources and sport fishing in national parks by special regulations. Presented at 51st Annual Meeting, Western Div., Amer. Fish. Soc., Aspen, Colo.

WILEY, R. W. 1969. An ecological evaluation of the Snake River cutthroat trout fishery with emphasis on harvest. M.S. Thesis. University of Wyoming, Laramie. 106 p.