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

ScholarsArchive at Oregon State University

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Title Ecological relation of sympatric steelhead and resident rainbow trout in the Deschutes River, Oregon
Names Zimmerman, Christian E. (creator)
Reeves, Gordon H. (advisor)
Date Issued 2000-05-05 (iso8601)
Note Graduation date: 2000
Abstract 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.
Genre Thesis/Dissertation
Topic Steelhead (Fish) -- Variation -- Oregon -- Deschutes River
Identifier http://hdl.handle.net/1957/20450

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