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

Abstract

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 (±SD) Sr:Ca ratio was significantly higher in the otolith primordia of the progeny of steelhead (0.001461 ± 0.00029; n = 100) than in those of the progeny of resident rainbow trout (0.000829 ± 0.000012; n = 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 = 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.