Run timing, spawn timing, and spawning distribution of hatchery- and natural-origin spring Chinook salmon in the Imnaha River, Oregon

We evaluated 16 years (1990-2005) of weir collection and spawning ground survey data to examine differences in run timing, spawn timing, and spawning distribution between naturally and hatchery-reared Chinook salmon Oncorhynchus tshamytscha in the Imnaha River, Oregon, and to look for changes in these variables over time. We compared run timing based on arrival time at the weir and found that naturally reared Chinook salmon arrived earlier than hatchery-reared fish in all 16 years. Carcasses of naturally reared fish of either or both sexes were recovered on the spawning grounds earlier than carcasses of hatchery-reared fish for all years combined and during 5 of 16 years, indicating earlier spawning by naturally reared fish. We compared spawning distribution using the percent of female carcasses recovered in each spawning ground survey reach. The percent recovery of the carcasses of naturally reared females was greater in reaches above the weir than below the weir and these carcasses were farther upstream than those of hatchery-reared females. For females of hatchery origin, carcasses were more commonly found in reaches closer to and below the weir (the smolt release location). Alternate management strategies designed to improve the maintenance of genetic integrity and life history characteristics of the Imnaha River Chinook salmon population should be considered, implemented, and evaluated. Collection strategies should be modified to ensure that the hatchery broodstock is representative of the entire natural run, and release strategies should be developed to promote a spawning distribution of hatchery adults that mimics the distribution of natural Imnaba River Chinook salmon.