ABSTRACT
Hatchery summer steelhead were introduced into the Clackamas River, Oregon, USA in 1973. This study demonstrates impacts to the productivity of the native wild winter steelhead population caused by the passage of large numbers of hatchery adults into natural spawning habitats. The impacts are ecological since interbreeding between the hatchery and wild fish is precluded by differences in life history. Genetic admixture analysis was used on naturally-produced out-migrating steelhead smolts to demonstrate that the hatchery fish were producing a substantial portion of the smolts leaving the Clackamas River, even while these smolts had very poor survival to adulthood. The productivity of wild winter steelhead, measured as the production of both smolt and adult offspring per parent, declined substantially with the onset of the summer steelhead hatchery program. We conclude that the natural-spawning hatchery adults failed to produce adult offspring but still occupied natural spawning habitat and produced juveniles that occupied natural rearing habitat and thereby depressed the productivity of the wild population through density-dependent mortality.
QUOTES FOR THE TEXT:
Previous studies of the ecological risks and influences that hatchery fish may impose on wild fish focused on interactions between hatchery and wild juveniles immediately after the hatchery fish were released. This study demonstrates that the ecological risks caused by natural spawning by excessive numbers of hatchery adults and by the juvenile offspring they produce should also be taken into consideration.
The natural productivity of the basin, as measured by smolts produced per parent and by pre-harvest adult offspring produced per parent, promptly declined as the number of natural spawning parents abruptly increased. It appears from these relationships that the Clackamas basin has a steelhead carrying capacity of about 2,000 to 2,500 spawners. The number of adults passed into the natural spawning areas above North Fork Dam exceeded 2,500 fish 65% of the time after the start of the Skamania hatchery program, and in some years exceeded 4,500 adults, which are abundance levels not observed prior to the hatchery program.
Fewer smolts from wild parents translate into fewer adult offspring per wild parent. The number of adult offspring produced per wild winter-run parent dropped to very low levels immediately after the start of the Skamania hatchery program and remained below replacement in most years. The wild population had been able to substantially increase the number of adult offspring per parent when population sizes fell below 2,000 prior to the implementation of the hatchery program. Likely poor ocean survivals during the 1980s also influenced these results, producing a double impact of lower freshwater productivity and lower smolt to adult survivals. Meanwhile, the number of adult offspring produced per summer-run parent was very low throughout the program, consistent with our earlier observations of very few unmarked summer steelhead in the basin.
The hatchery parents were able to produce smolt offspring, although they did so with less success than wild parents. However, they were very unsuccessful at producing adult offspring. The wild Clackamas steelhead remained much more successful at natural reproduction than the hatchery fish, even when their productivity was substantially depressed. The hatchery parents produced only 66% and 51% as many smolts per parent as did the wild fish, and only 3% and 34% as many pre-harvest adult offspring per parent as did the wild fish.
It is remarkable, but evident in this study, that the hatchery fish were able to spawn and produce smolts, the two life stages that are captive in the hatchery program. The biggest die-off occurred from smolt to adult, the life stages of hatchery fish that occurs in the natural environment.
The decrease in wild fish productivity was not due to genetic effects caused by interbreeding in this case because of the different life histories of the hatchery and wild adults. We conclude that the ãSkamaniaä hatchery adults occupied spawning habitats and their juveniles occupied freshwater rearing habitats thereby impacting the productivity of wild steelhead through density-dependent mortality