John J. Fraley and Bradley B. Shepard, MontanaDepartment of Fish,Wildlife and Parks P.O.Box 67, Kalispell,Montana 59903 Life History,Ecology and PopulationStatus of MigratoryBull Trout (salvelinusconfluentus) in the FlatheadLake and RiverSystem, Montana Abstract Life history, ecology, and population trends ofmigratory bull trout (Sabelinus confluentus) were investigated in the Flathead Lake and River system of northwest Montana and southeast British Columbia. We conducted these studies to obtain informa- tion to manage the species in light of threats posed by timber harvest, hydropower development, and a proposed coal mine. We estimated that about half the adult bull trout in Flathead Lake enbarked on a spawning migration from May through July, swimming 88-250 km to reach tributaries of the North and Middle Forks of the Flathead River. Bull trout entered the tributaries when water temperatures dropped below l2oC, and spawned from late August through early October after water temperatures were below 9oC. They spawned in areas of tributaries with low gradient, loosely compacted gravel, groundwater influence, and cover. After spawning, females left the tributaries and returned to the lake sooner than males. Most spawners were six or seyen years old and they averaged 628 mm in length. Juveniles were found close to the substrate in streams with summer maximum temperatures less than l5oC. Juveniles migrated out of the tributaries to the river system from June through August, at age I (18%), lI(49yo),IlI(32%), and IV (l%). Population status was monitored through redd counts and estimates of juvenile abundance in natal tributaries. The population may be limited by quantity and quality of rearing and spawnrng habitat, and spawning escapement. Specific requirements for spawning and rearing habitat, and general sensitivity of each life stage, make the bull trout an excellent indicator of environmental disturbance. Introduction Flathead River Basin Studies (Graham et al. 1980, Fraley et al. 1981, Shepard et al. 1982, The bull trout (Salttelinus confluentus) is the 1984b, Graham et al. 1982, Fraley and Graham largest species of fish native to the Flathead 1982, Graham and Fredenbergl9S2,Leathe and drainage,attaining a length of nearly one meter Graham 1982). We studied bull trout age and and a weight of l0 kg. The bull trout inhabiting growth both in the lake and in the river system, the inland waters of northwestern North America harvest by anglers, the adult spawning migration, is considered a separate speciesfrom the smaller, spawning site selectionand use, and the densities, coastal Dolly Varden (Saluelinus malma) habitat selection, and emigration of juveniles (Cavender 1978).The bull trout population in the growing in tributaries. Methods included tagging, Flathead system is largely migratory, growing to gillnetting, stream trapping and electrofishing, maturity in lakes and migrating through the river creel survey, otolith and scale analysis, redd system and into the tributaries to spawn. Juve- counts, and substrate analysis (Graham and niles live in tributary streams from one to four Predenberg 1982, Shepard and Graham l9B3). years before migrating to the lakes. In this paper we summarize our findings on Much information has been published con- the life history, ecology, and population status cerning the life history of coastal Dolly Varden of adfluvial bull trout in the Flathead Lake (e.g.,Blackett 1968,Armstrong and Morton 1969, and inlet river system and compare our information Armstrong and Morrow 1980,Balon 1980).Pub- to the results of other investigators. lished information on the bull trout is limited. McPhail and Murray (1979), Leggett (1969), and Allan (1980) studied various aspects of the life StudyArea history of bull trout in British Columbia and The Flathead Lake and River system is a head- Alberta. Gould (1987) described the character- water drainage of the Columbia River Basin (Fig- istics of larval bull trout. ure l). Flathead Lake is a large oligomesotrophic The Montana Department of Fish, Wildlife lake with a surface area of 476 km2 and a mean and Parks has studied the bull trout population depth of 32.5 m (Potter l97B). The upper 3 m in the Flatheaddrainage since 1953(Block 1955, of Flathead Lake is regulated by Kerr Dam, con- Hanzel 1976).More intensive work was under- structed on the outlet in 1938. The Flathead taken from 1979-1984during the EPA-sponsored River enters the north end of the lake. This study Northwest Science,Vol. 63, No. 4, l9B9 133 UPPERFLATHEAD RIVER BASIN CANAOA USA STUOY A REA + I N HUNGRY HORSE I DAM O 16km BIG FORK DAM FLATHEAD LAKE Figure 1. The upper Flathead River Basin. The 28 tributaries shownwere used by spawningbull trout. 134 Fraley and Shepard was conducted in the upper Flathead Basin which age of maturity was reported for bull trout in includes Flathead Lake and the river systemup- Arrow Lakes, British Columbia (McPhail and stream from Flathead Lake. Murray 1979). The South, Middle and North forks drain The diet of bull trout in the lake consisted areasof approximately equal size in portions of almost exclusively of fish. Whitefish speciesand the Great Bear and Bob Marshall wildernesses, yellow perch (PercaJlaoescans) were the most im- Glacier National Park and the Flathead National portant food items, followed by kokanee (Onco- Forest. The upper North Fork drains southern rhynchus nerka) and,nongame species(Table l). British Columbia. The South Fork is regulated Small bull trout have been found to feed inci- by Hungry Horse Dam, located 8 km above its dentally on Mysis in Flathead Lake. Mysis relicta mouth. The Swan River enters Flathead Lake was discoveredin Flathead Lake in l98l and den- near the mouth of the Flathead River. Bull trout sities increased dramatically through 1986. coexist with 23 other species of fish in the Kokanee were the major food item for bull trout Flathead Lake and River svstem (Leathe and in Pend Oreille Lake, Idaho (Jeppsonand Platts Graham 1982). 1959),while whitefish were the major food in Up- Most bull trout that spawn in the North and per Priest Lake (Bjornn l96l). Middle Fork drainages mature in Flathead Lake, The annual growth increment for bull trout but fish maturing in large lakes of Glacier Na- in Flathead Lake, based on analysis of scales, tional Park may spawn in some tributaries. There ranged from 60-132mm (Table 2). Back calcula- are a few populations of bull trout in tributaries tions of length at annulus formation were made of the North Fork that spend their entire lives from 1,813 scale samples.Aging was checked in the streams. with otoliths from 451 of the fish. Agreement of Bull trout originally used the tributaries of aging between otoliths and scalesranged from all forks of the Flathead and the Swan rivers. The 100 percent for fish zero to three years of age, construction of Bigfork Dam in 1902 blocked bull to 52 percent for older, mature fish. Growth of trout migrations into the Swan River. Limited lake-residentfish was relatively constant after age numbers of bull trout move downstream from the IV. Growth rates of bull trout in Flathead Lake Swan drainage via a marginal fish ladder, as were similar to those reported for Priest and Up- evidenced by tag returns. Hungry Horse Dam, per Priest Lakes, Idaho (Bjornn l96l). a 164.6-mstructure which was closed in 1951, Not all mature bull trout spawnedannually. blocked all movements of bull trout into the Adult-size fish were relatively less abundant in South Fork drainage and probably resulted in a the lake during the summer and fall, as compared substantial reduction of the population in to the spring. It appearedthat 38 to 69 percent Flathead Lake. (average57To)left the lake eachspring and sum- The 28 tributaries used by spawning bull mer to spawn.The frequency of successiveyear trout in the North and Middle Fork drainages spawning varied by age and sex (Leathe and (Figure l) are characterized by gravel-rubble Graham 1982).Alternate year spawning has been substrate, low flows of 0.057-1.70m3isec, and reported for inland Dolly Varden char (Arm- maximum summer water temperaturesless than strong and Morrow 1980). 150c. Upstream Migration Resultsand Discussion Bull trout maturing in Flathead Lake began their LifeHistory spawning migration into the river system during April and moved slowly upstream, arriving in the Lake Residence North and Middle forks during late June and Ju- Bull trout populations residing in Flathead Lake ly. They traveled more than 250 km to spawn in were found to include recently arrived juveniles some North Fork tributaries in British Colum- from the Flathead River system,subadult fish less bia. The shortest distance traveled from Flathead than about 450 mm in length, and mature fish Lake was 88 km to the mouth of Canyon Creek five to six years or more in age. Most bull trout in the North Fork drainage. Observationsand tag in Flathead Lake matured at age VL A similar returns from 1974-1982indicated that adult bull Life History and Ecologyof Migratory Bull Trout r35 TABLE l. Composition by number, weight, and frequency of occurrence and calculated index of relative importance (lRI, George and Hadley 1979) for major food items in the stomachs of 95 bull trout collected between November and January, 1979, 1980 and l98l in Flathead Lake. Wet Index of Relative Item Number (%) weight-g. (%) Importance (lRI) Pygmy whitefish (2.4) 37.0 (4.0) J-Z Lake whitefish l (0.s) 104.1 (r r.2) 4.3 Mountain whitefish I (0.5) 24.3 (2.6) 4.4 Unidentified whitefish l1 (s.3) 28t.2 (30.3) 15.0 Total whitefish l8 (8.7) 446.6 (48.l ) 23.5 Kokanee 2 (1.0) 82.8 (8.e) 4.0 Unidentified trout/salmon 2 (l.0) t3.2 (1.4) 1.5 Total trout/salmon I (l.e) 96.0 (10.3) 5.r Sculpin 3 (1.s) 7.6 (0.8) l.B Redside shiner (2.4) 15.0 (1.6) 2.Q Peamouth I (0.5) 3.6 (0.4) 0.7 Sucker 2 (1.0) 74.4 (8.0) Yellow perch 83 (40.3) 105.1 (11.3) 24.6 Total nongame 94 (4s.6) 205.7 (22.r) 31.0 Unidentified fish 90 (43.7) t8l.l (le.s) 4t.4 TABLE 2.