Our concern about the detrimental impacts of excessive straying, especially the straying of stocks selected in hatchery environments, is not theoretical but based on documented studies such as:
1. C. shasta susceptible Umpqua and Siletz summer steelhead were crossed with resistant Skamania summer steelhead. The resulting offspring had a moderate susceptibility to the disease that was intermediate between their very susceptible and very resistant parents.
2. The occurrence of C. shasta in the Nehalem River was studied. Native Nehalem coho from Cronin Creek are very resistant to C. shasta which is present in the Nehalem River. Trask Hatchery coho, which are not resistant to the disease, were planted in Fishhawk Creek ( a Nehalem River tributary) over several years prior to the collection of adults from Fishhawk Creek in 1979. The offspring from Fishhawk Creek adults were found to have resistance to C. shasta that was intermediate between the Trask stock and the native Nehalem stock. The introduction of nonresistant hatchery coho into the Nehalem River apparently reduced the resistance of the native stock to the disease.
3. C. shasta susceptible Umpqua and Soleduck coho were crossed with resistant Big Creek coho. The resulting offspring had a relatively high susceptibility to the disease that was more similar to that of their very susceptible parents than that of the very resistant parents.
4. Ocean distribution studies showed: Big Creek and Umpqua stocks of coho have somewhat similar distributions of ocean catch with about 95% of the catch of Umpqua stock off Oregon and California compared to about 80% of the Big Creek stock. Distribution of ocean catch of the offspring of Big Creek/Umpqua crosses were similar to the distribution of the Big Creek stock. Soleduck stock coho have a much more northerly catch distribution than Big Creek stock with 90% being caught off Canada and Washington. The distribution of the ocean catch of the offspring of Big Creek/Soleduck crosses was 40% off Canada and Washington.
5. An evaluation of various matings of hatchery and wild steelhead in the Deschutes River showed: The progeny of hatchery by wild matings had lower survival and slower growth than the offspring of wild by wild matings, when the two groups reared together in four streams in the Deschutes basin. The survival differences that were observed between the offspring of the hatchery and wild fish suggested that the short term effect of hatchery adults spawning in the wild is the production of fewer smolts and ultimately fewer returning adults than are produced from the same number of wild spawners.
6. Wild steelhead spawners were found to contribute 2.7 times more naturally produced subyearling steelhead in the Kalama River watershed than did hatchery spawners. Information from the Kalama River study indicates the reproductive fitness of wild steelhead may exceed the reproductive fitness of hatchery steelhead by up to sixfold.
7. In 1974, native Elk and Chetco stocks of fall chinook were reared at Elk River Hatchery and liberated as smolts into their home streams. Groups of smolts from these stocks were reared from eggs transferred to Fall Creek Hatchery on the Alsea River and Klaskanine Hatchery on the lower Columbia River. Smolts from each stock were also liberated into Coos Bay. Control groups released at Elk and Chetco rivers produced larger catches in ocean fisheries then chinook transferred to Coos Bay, Alsea, and Klaskanine rivers. Results of this study show that stocks transferred into different environments are unable to cope as well as cohorts liberated into their home streams and that considerable variability exists between stocks that survive in the alien environment. This suggests that stock differences exist that are important in determining survival. The reduced survival of transplanted stocks was due to inherited stock characteristics that are improperly adapted to recipient stream, estuary, or ocean environments.
8. Preliminary results of the ODFW coho presmolt evaluation suggest that hatchery coho spawning in Oregon coastal streams have a low reproductive success in most years. The reduced survival of offspring of hatchery fish is thought to be caused by the interaction between time of spawning and the timing of winter freshets. Eggs of early spawning coho are probably adversely affected by the first series of freshets which tend to move the spawning gravel. During mild winters with few or small freshets, the offspring of hatchery spawners will probably survive at higher rates than during severe winters. The surviving hatchery fish will emerge earlier than the later spawning wild fish. Because of earlier emergence hatchery fish will be larger than wild fish and out compete them for food and space. If flow conditions are more similar to normal when these fish return to spawn, their eggs will be lost and total natural production will be severely reduced.