Ecological relation of sympatric steelhead and resident rainbow trout in the Deschutes River, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z316q516n

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  • Sympatric steelhead and resident rainbow trout (Oncorhynchus mykiss) are known to exist in many rivers throughout the Pacific Rim. Whether sympatric steelhead and resident rainbow trout are polymorphisms within a single gene pool or two reproductively isolated populations has significant implications concerning the study and conservation of this polytypic species. I examined population structure and use of spawning and rearing habitats by steelhead and resident rainbow trout in the Deschutes River, Oregon. I used otolith microchemistry to determine the maternal origin (steelhead verses resident) of adult steelhead and resident rainbow trout based on Sr/Ca ratios in primordia and freshwater growth regions of otoliths. Only steelhead of steelhead origin and resident rainbow trout of resident origin were encountered. In the Babine River of British Columbia, however, steelhead of resident origin and resident rainbow trout of steelhead origin were present. Temporal and spatial segregation of spawning habitat served to limit breeding between steelhead and resident rainbow trout in the mainstem Deschutes River. The timing of 50% spawning by steelhead occurred 9 to 10 weeks earlier than by resident rainbow trout. Steelhead spawning sites were deeper and of larger substrate than those used by resident rainbow trout. There was an overlap in the timing of emergence by steelhead and resident rainbow trout fry in the mainstem Deschutes River, but higher levels of aggression and territoriality characterize newly emerged steelhead fry. Determination of maternal origin based on otolith microchemistry indicated that young-of- year 0. mykiss in small hydrologically unstable tributaries to the Deschutes River were exclusively the progeny of steelhead. Progeny of resident rainbow trout numerically dominated mainstem-rearing habitats. Based on the degree of segregation between steelhead and resident rainbow trout, the two life history forms act as two separate species in the Deschutes River. This relationship has significant implications concerning the restoration and conservation of steelhead and resident rainbow trout. Where steelhead and resident rainbow trout constitute reproductively isolated populations, conservation of both life history forms within the species cannot be achieved unless both populations are maintained.
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