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New Report Analyzes
Shad, Predator Impacts on Salmon

by Barry Espenson
Columbia Basin Bulletin - August 27, 2004

Large numbers of non-native American shad appear to be filling a food niche in the Lower Columbia River that allows aquatic predators to grow faster and, ultimately, take a bigger bite out of salmon populations that fish managers and others are trying to rebuild.

That's one of the conclusion of a recently released report, "The impact of prey and predators on juvenile salmonids," authored by Sally T. Sauter of the U.S. Geological Survey. Participating researchers were Robin M. Schrock, James H. Petersen and Alec G. Maule, all based at the agency's Columbia River Research Laboratory in Cook, Wash.

The study was funded by the Bonneville Power Administration through the Northwest Power and Conservation Council's Columbia River Fish and Wildlife Program. The report explains the results of biogenetic modeling carried out to "investigate predation on juvenile salmonids by northern pikeminnow, smallmouth bass, and walleye in the lower Columbia River reservoirs" from the Bonneville Dam forebay to the McNary Dam tailrace.

The information could eventually be used as managers and policy makers plan and implement strategies to reduce the impact of predators on juvenile salmon. The pikeminnnow, a native species, are already the target of a sport reward program funded by BPA that is intended to reduce the number of large, and most voracious, fish from the system.

The walleye and smallmouth bass, both introduced or "invasive" species, are both popular targets of sport fishers who have protested in the past suggestions that numbers of the warmwater fish be reduced.

The modeling effort indicates that the presence of juvenile shad contributes to the growth of northern pikeminnow, a species that was estimated to consume 16.4 million salmon and steelhead juveniles annually in the Columbia and Snake river reservoirs before implementation of the management program.

"Bioenergetics modeling suggests that American shad are also potentially important prey species contributing to the growth and salmonid consumption of invasive predators, particularly smallmouth bass, but better information on the feeding behavior and seasonal diets of these predators is needed," according the report.

The report authors, and BPA are seeking technical peer review of the document and accepting comments through Sept. 17. The report is posted at: Anyone unable to access the drafts via the Internet can obtain a hard copy calling Nancy Webster, BPA, at (503)-230-4511

The modeling done with existing diet and fish growth data shows that smallmouth bass, and to some extent walleye, get a big boost in mid- to late summer by gorging on plentiful and nutritious juvenile shad.

The ocean-going shad return to the river in late spring and summer to fertile spawning grounds above Bonneville Dam, especially in The Dalles and John Day pools. Their young conveniently, for the predators, emerge in mid to late summer to provide an "easy source of high-quality food for aquatic predators," the report says. The food source is provided at a time when there are very few young salmon available as prey.

"The abundance of juvenile American shad and the warmer fall water temperatures of the LCRR creates opportunities for feeding and growth of predator populations that otherwise would not exist," the report said.

Previous studies have shown that warmer water temperatures increase the metabolic demand and foraging activities of aquatic predators such as the smallmouth bass and walleye. Studies have also show that the water impounded by the Columbia/Snake hydrosystem stays warmer longer into the fall than when the rivers were free-flowing.

That means the predators become larger and their populations more robust. And when the water begins to warm again in early summer they are ready, willing and able to prey on juvenile fall chinook migrating toward the ocean.

"Our results support earlier research and reinforces concern expressed by numerous authors that smallmouth bass predation may result in heavy losses of subyearling fall Chinook (Gray and Rondorf 1986; Curet 1993; Tabor et al. 1993; Poe et al. 1994; Zimmerman 1999; Duran et al. 2003; Petersen et al. 2003)," according to the report's executive summary.

"Because of their smaller size and later migration period, wild fall Chinook from the Hanford Reach and Snake River may be more vulnerable to smallmouth bass predation than hatchery fish," the report says. The Hanford chinook are the basin's most robust naturally producing stock; the Snake River stock is listed under the Endangered Species Act.

"Juvenile salmonids may also be vulnerable to smallmouth bass predation whenever foraging or flow patterns bring migrants near shorelines, or migrants are delayed in dam forebays, particularly later in the spring once water temperatures have warmed," according to the report.

"This is a long-term, indirect effect," Sauter said of the food chain that sees plentiful shad foodstuffs fortifying pikeminnow, bass and walleye so that they have greater capability to prey on salmon. Pikeminnows' feeding is actually reduced during times of warmer water temperatures so their salmon predation is more focused on the spring migration. Because of their sheer numbers pikeminnow have the biggest impact overall on salmon of the aquatic predators in the system.

That shad are a growing factor in the Columbia River ecosystem is apparent in their huge numbers. The species, native to North America's east coast, were introduced into the Sacramento River in 1871 and in the Columbia 1885, though landings in the Columbia document that shad had made their way north before their introduction.

American shad have blossomed in recent decades. A record run size of 4 million was established in 1990, according to the "Status Report" on Columbia River fish runs published by the Oregon and Washington departments of fish and wildlife.

The new report notes that that record was raised in 2003 with 4.7 million American shad estimated passing through Bonneville Dam's fish ladders. That record was shattered this year with more than 5.5 million shad passing Bonneville this year, according to Corps of Engineer counts.

"It seems like they are steadily growing now," the Fish Passage Center's Larry Basham said of the shad population.

It is those fish ladders, at Bonneville and other lower Columbia/Snake dams, that have facilitated the rise in shad populations. The structures were clogging with shad so in the early 1970s the facilities were improved to assure passage of the species they were intended to help -- the salmon. The improvements did, however, also pave the way upstream for the shad.

The shad are now far more numerous than the salmon, and much less desirable to sport and commercial fishers. The available supply of shad far outweighs the demand.

Two seemingly obvious solutions to reverse the trend towards increased predation on salmon would be to attempt to control the number of predators in the reservoirs, or to stem the flow of shad upstream. Both represent vexing problems for policymakers.

"The smallmouth bass and walleye fisheries are important to people," Sauter said. Likewise, limiting upstream access for shad, but not for salmon, is a difficult task.

And while it is the smallmouth bass and walleye that actually consume the salmon (modeling done for the study indicates that a small measure of bass growth boosts salmonid consumption by 16 percent), it is the shad that are at the root of the problem, and perhaps other problems.

"The larger point is that we have this invasive species that potentially is having a very large impact" on the river environment, Sauter said. The shad themselves because of their numbers could compete against other native species for zooplankton and other nutrients in the reservoirs. And their role in beefing up predators could result in a greater toll on other native prey species besides the salmon.

"The impact of shad on these systems is not understood," Sauter said.

Barry Espenson
New Report Analyzes Shad, Predator Impacts on Salmon
Columbia Basin Bulletin, August 27, 2004

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