Seasonal Use of Small Tributary and Main-Stem Habitats by Juvenile Steelhead, Coho Salmon, and Dolly Varden in a Southeastern Alaska Drainage Basin

-The movement of juvenile salmonids between small tributaries and main-stem habitats in southeast Alaska watersheds is poorly understood. We observed movements of steelhead Oncorhynchus mykiss, coho salmon O. kisutch, and Dolly Varden Salvelinus malma between mainstem and tributary habitats at weirs located on tributaries in the Staney Creek watershed in southeast Alaska. We used seasonal relative abundance (catch per unit effort) in eight main-stem reaches and eight tributaries to corroborate observed movement in the two streams with weirs. We observed juvenile steelhead and coho salmon moving through the weirs into tributaries during the fall as flows increased and temperatures decreased. The relative abundance of steelhead was greater in main-stem sites than in tributaries during the summer, whereas during spring and fall relative abundance in the tributaries was similar to that in the main stem. Juvenile coho salmon were abundant in tributaries during all seasons. The relative abundance of Dolly Varden was greater in the tributaries than in the main-stem during all seasons. These results underscore the significance of links between main-stem habitats and small tributaries for production of juvenile salmonids. Movements into off-channel refuge habitats by juvenile coho salmon Oncorhynchus kisutch in the fall and winter and emigration of coho salmon fry from spawning tributaries in spring or summer have been well documented, whereas seasonal movements between main-stem and tributary habitats by juvenile steelhead O. mykiss are less well known (Skeesick 1970; Bjornn 1971; Bustard and Narver 1975; Cederholm and Scarlett 1981; Peterson 1982; Tschaplinski and Hartman 1983; Swales et al. 1986; Hartman and Brown 1987; Brown and Hartman 1988; Nickleson et al. 1992). In Washington and Oregon, some presmolt steelhead move downstream out of natal tributaries into main-stem habitats to complete their freshwater residence (Everest 1973; Leider et al. 1986), and in Idaho, Bjornn (1971) described downstream movements of presmolt steelhead in fall, winter, and spring from a tributary of the Lemhi River. Cederholm and Scarlett (1981) observed fall immigration and spring emigration of juvenile steelhead into two small tributaries of the Clearwater River in Washington State. Seasonal movements of juvenile Dolly Varden Salvelinus malma between main-stem and tributary habitats also are not well known, but fall immigration into warmer spring-fed tributaries and spring out-migration of smolts from streams and lakes have been reported (Armstrong 1974; Armstrong and Morrow 1980). * Corresponding author: [email protected] 1 The Unit is jointly supported by Montana State University-Bowman; the Montana Department of Fish, Wildlife, and Parks; and the U.S. Geological Survey, Biological Resources Division. Received January 17, 2001; accepted November 9, 2001 498 Transactions of the American Fisheries Society 131:498-506, 2002 American Fisheries Society 2002 498 BRAMBLETT ET AL. MIGRATORY HABITATS 499 In southeast Alaska, juvenile coho salmon and Dolly Varden are found in tributaries and mainstem reaches throughout the year (Bryant 1984; Heifetz et al. 1986; Dolloff 1987), but little information on seasonal movements between mainstem and small tributary habitats is available. Dolloff (1987) observed little movement of transplanted juvenile coho salmon among stream reaches between surveys conducted in August, November, and the following April in a southeast Alaska stream. However, our observations of marked juvenile coho salmon indicate that a portion of the population enters beaver ponds and other off-channel habitats during the fall in southeast Alaska (M. D. Bryant, unpublished data). Previous studies in southeast Alaska suggest that juvenile steelhead are absent or rare in small tributaries (Bryant 1984; Dolloff 1986; Bjornn et al. 1991). However, all of these studies were conducted during the summer months and therefore shed no light on whether juvenile steelhead utilize small tributaries during other seasons. Johnson et al. (1986) observed large variations in steelhead abundance between summer and winter samples in three southeast Alaska streams and attributed the differences to movement within the main stem, although they did not mark fish and did not sample small tributaries. Our objectives were to describe the magnitude and timing of juvenile salmonid movements between the main stem and small tributaries and to compare the seasonal relative abundance of juvenile salmonids in main stem and tributaries.