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New Paper Suggests Hatchery Fish
Can Crowd Out Wild Stock

by Bill Bakke
NW Fishletter, July 22, 2003

To enhance the sports fishery, hatchery-raised summer steelhead were first added to Oregon's Clackamas River in 1971, but it has taken over thirty years for biologists to get around to evaluating the effects of this strategy on the river's wild stock. The results were recently published in Transactions of the American Fisheries Society (vol. 132 No. 4, pp. 780-790) by Kathryn Kostow(ODFW), Anne Marshall(WDFW), and the late Stevan R. Phelps(WDFW).

The authors reported that hatchery fish made up 60-70 percent of the natural spawners and produced 36-53 percent of the naturally-produced smolts. However, the survival from smolt to adult was poor. "Very few natural-origin summer adults were observed, suggesting high mortality ... following emigration," they said. An evaluation of two brood years showed that hatchery fish produced one-third the number of smolts per parent that wild fish did and one-tenth as many adults per parent as the wild steelhead.

The paper notes that the Oregon Department of Fish and Wildlife was focused on decreasing the interbreeding between hatchery and wild fish and the "low apparent breeding success by the hatchery fish has often been assumed to indicate low risk to the wild fish." Fortunately, the 160,000 hatchery steelhead released into the river were marked so they could be identified. The ODFW "... assumed that these fish did not spawn successfully," say the authors. "It was also assumed that life history differences ... precluded interbreeding between summer and winter runs. Risks to wild winter steelhead from the hatchery summer steelhead program were assumed to be very low," said the authors.

However, the authors hypothesize that the hatchery summer steelhead were successfully spawning and producing juveniles that exhibited poor survival to adulthood. Even though summer and winter steelhead show distinct differences in adult life histories, such as time of spawning, the paper says "their juvenile life histories and habitat requirements are thought to be similar" and this would "maximize competitive interactions between them."

Because the summer steelhead adults spawn earlier than wild winter steelhead, the juveniles would emerge earlier and occupy the best feeding territories and place winter steelhead at a disadvantage.

But even with the advantage of more adult spawners, a competitive edge at the juvenile stage, and more juvenile production, the hatchery fish still produced one-third the number of smolts per female than did wild steelhead. This finding is corroborated by research on other streams such as Washington's Kalama River where wild fish outperformed hatchery steelhead at every life stage, but since hatchery steelhead adults were more plentiful, they predominated.

Based on genetic analysis, the authors found that the Clackamas wild and hatchery steelhead did not interbreed to any great extent, which means that reproductive isolation was high. So, the recorded decline in the wild steelhead was not a result of interbreeding and genetic effects on the wild fish.

"Whatever interbreeding may have occurred between hatchery and wild fish," say the authors, "it has not diminished the genetic and biological distinctiveness of the wild winter steelhead population, and the productivity of the wild population. The decline in wild winter steelhead abundance was not likely due to diminished reproductive success of a greatly hybridized population. "

However, this decline in wild steelhead was masked by the presence of summer steelhead smolts in the total counts at the dam where they are tallied as they migrate out of the upper basin. The authors concluded that "even though naturally spawning hatchery steelhead may experience poor reproductive success, they and their juvenile progeny may be abundant enough to occupy substantial portions of spawning and rearing habitat to the detriment of wild fish populations."

The authors say the "smolt offspring of hatchery fish appear to have wasted the production from natural habitat because very few survived to return as adults," and caution fish managers about drawing conclusions about the success of supplementation efforts by looking at natural spawning and smolt production by hatchery fish.

"Evidence for success," they say, "must also include adult offspring and no depression of wild fish productivity," pointing out that "supplementation programs should be attuned to basin carrying capacities so that they do not reduce wild fish productivity through competition for resources."

Related Sites:
Abstract to Naturally Spawning Hatchery Steelhead Contribute to Smolt Production but Experience Low Reproductive Success by Kathryn E. Kostow, Anne R. Marshall, Stevan R. Phelps, pages 780-790.

Bill Bakke
New Paper Suggests Hatchery Fish Can Crowd Out Wild Stock
NW Fishletter, July 22, 2003

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