Identification of Steelhead and Resident Rainbow Trout Progeny in the Deschutes River, Oregon, Revealed with Otolith Microchemistry

—Comparisons of strontium:calcium (Sr:Ca) ratios in otolith primordia and freshwater growth regions were used to identify the progeny of steelhead Oncorhynchus mykiss (anadromous rainbow trout) and resident rainbow trout in the Deschutes River, Oregon. We cultured progeny of known adult steelhead and resident rainbow trout to confirm the relationship between Sr:Ca ratios in otolith primordia and the life history of the maternal parent. The mean (6SD) Sr:Ca ratio was significantly higher in the otolith primordia of the progeny of steelhead (0.001461 6 0.00029; n 5 100) than in those of the progeny of resident rainbow trout (0.000829 6 0.000012; n 5 100). We used comparisons of Sr:Ca ratios in the primordia and first-summer growth regions of otoliths to determine the maternal origin of unknown O. mykiss juveniles (n 5 272) collected from rearing habitats within the main-stem Deschutes River and tributary rearing habitats and thus to ascertain the relative proportion of each life history morph in each rearing habitat. Resident rainbow trout fry dominated the bimonthly samples collected from main-stem rearing habitats between May and November 1995. Steelhead fry dominated samples collected from below waterfalls on two tributaries in 1996 and 1998. Rainbow trout Oncorhynchus mykiss is a polytypic species characterized by populations of resident, adfluvial, and fluvial rainbow trout and anadromous steelhead (Scott and Crossman 1973; Behnke 1992). Similar migratory polymorphism is observed in several other species of salmonids, including brown trout Salmo trutta (Skaala and Nævdal 1989), Atlantic salmon S. salar (Verspoor and Cole 1989), sockeye salmon O. nerka (Foote * Corresponding author: [email protected] 1 Present address: U.S. Geological Survey, Alaska Science Center, 1011 East Tudor Road, Anchorage, Alaska 99503, USA. Received September 8, 2000; accepted February 28, 2002 et al. 1989), coastal cutthroat trout O. clarki clarki (Trotter 1989), Arctic char Salvelinus alpinus (Nordeng 1983), and brook trout S. fontinalis (White 1940). Migratory polymorphism may result from phenotypic plasticity within a single gene pool or from fixed differences between sympatric but reproductively isolated populations. The relationships between alternative life history forms vary among species and locations. Nordeng (1983), for example, demonstrated through rearing studies that resident and migratory Arctic char were from the same gene pool and that migration was under environmental control. Reproductive isolation between life history morphs has been identified in some locations for Atlantic salmon (Verspoor and Cole 1989) and sockeye salmon and kokanee (lacustrine sockeye salmon; Taylor et al. 1996; Wood et al. 1999). Analysis of the maternal origin of adult resident rainbow trout and steelhead suggested that these life history forms are reproductively isolated in the Deschutes River, Oregon, because adult steelhead of resident origin and resident trout of steelhead origin were not observed (Zimmerman and Reeves 2000). This apparent reproductive isolation was reinforced through segregation in the timing and location of spawning between steelhead and resident rainbow trout (Zimmerman and Reeves 2000). Whether resident and anadromous forms constitute a single randomly mating gene pool or exhibit reproductive isolation between life history forms has significant implications for the study and management of