Research Suggests In Some Cases
Removing species -- such as gulls, mergansers, terns and cormorants -- that prey on salmon and steelhead stocks may not improve the fishes' overall survival rate, according to a University of Washington research paper published last month in the Ecological Applications, a publication of the Ecological Society of America.
"Our models suggest that one consequence of removing birds from the system may be increased pikeminnow abundance, which -- even assuming 80 percent compensatory mortality in juvenile pikeminnow survival would theoretically result in an annual average savings of just over 180,000 smolts, calculated over a decade," according to the study abstract in Ecological Applications' April edition. The publication's web site is http://esapubs.org/esapubs/journals/applications.htm
The Caspian terns wing into the Columbia River estuary each spring and leave in summer. The Caspian terns' stay in the estuary roughly coincides with the period when the basin's juvenile salmon and steelhead provide a steady prey stream as they pass through on their way to the ocean.
The salmon in the mid-Columbia study area, on the other hand, are only available as prey there for a relatively short time before they pass downstream. The researchers analyzed bird diet data from an area that includes the mainstem Columbia between Grand Coulee and McNary dams. Effort was concentrated at Rock Island and Rocky Reach dams, 18 kilometers and 6.5 km south and north of Wenatchee, Wash., respectively, and their associated reaches -- 100 kilometers in all. Both hydro projects are owned by the Chelan County Public Utility District.
Juvenile salmon from the upper Columbia are among those migrating in spring and summer. And when they're gone, young northern pikeminnow get more of the birds' attention. The pikeminnow gulped by mergansers and gulls would have in 3 years' time begun eating salmon themselves.
A better way to boost salmon survivals may be to haze or otherwise deter the birds, rather than killing them, while salmon are present in the river and " allowing bird presence after the diet switch to act as a tool for salmonid-predator control, and conducting adult-pikeminnow control throughout," according to the research conducted by .
Francis K. Wiese, Julia K. Parrish, Christopher W. Thompson and Christina Maranto of the UW's School of Aquatic and Fishery Sciences and/or its biology department.
The paper is titled, "Ecosystem-based management of predator-prey relationships: Piscivorous Birds and Salmonids."
"Our analysis demonstrates that identifying the strength of ecosystem interactions represents a top priority when attempting to manage the abundance of a particular ecosystem constituent, and that the consequences of a single-species view may be counterintuitive, and potentially counterproductive," the abstract says.
"These birds eat a lot of other stuff," said Wiese, the paper's lead author, who also serves as North Pacific Research Board science director. He says fish and wildlife managers can be too myopic in their search of solutions -- in this case, tools to improve salmon survival.
The abstract notes that, "In the Columbia River basin (Pacific Northwest, USA and Canada), piscivorous predators have been implicated in contributing to a lack of recovery of several endangered anadromous salmonids (Oncorhynchus spp.), and lethal and nonlethal control programs have been instituted against both piscine and avian species."
Four species of salmonids – chinook, steelhead, sockeye and coho – frequent the mid-Columbia study area. Among them are Upper Columbia spring chinook and steelhead, both listed as endangered under the Endangered Species Act.
Mid-Columbia PUDS were in 1997 mandated by NOAA Fisheries to implement a bird control program in the vicinity of five of the mid-Columbia dams (Wells, Rocky Reach, Rock Island, Wanapum, and Priest Rapids) using nonlethal (hazing) and lethal (shooting) control, according to the paper. NOAA is charged with protecting ESA-listed salmon.
"To estimate what the total present-day impact of avian predators on salmonid smolts would be in the absence of lethal control, we added the birds that were shot by USDA Wildlife Services in 2002–2004 (total of 2,316 or 8.8 percent of live birds counted in the system, and up to 60 percent of birds present in any given week) back into the system (predator-abundance parameter in the bioenergetics model), and conservatively assumed that they stayed for the remainder of the season. Thus, we calculate a worst-case present-day smolt-mortality scenario," according to the study.
The researchers used diet data to calculate how many young salmonids the birds might have eaten had they not been killed.
Those totals were then compared to estimates of how many juvenile salmon would have been taken by a pikeminnow population increased in size because fewer juveniles were consumed by birds.
"... removal of predators from a system (in this case, the annual removal of bird predators) rarely translates into a concomitant increase in prey survival. It is likely that other predators would have consumed a fraction of the juvenile pikeminnow not eaten by the (removed) bird predators, i.e., there would be compensation. However, even with 80 percent of the juvenile pikeminnow cohort removed annually by other predators, keeping birds in the system (i.e., having no lethal control of birds) still resulted in an average net salmonid benefit" of 2 million fish over the 11-year period modeled by the researchers.
"In this ecosystem-based strategy, late-season birds actually become indirect tools of salmonid predator control instead of hyper-predators," the study says.
The recommended approach for the mid-Columbia may not work everywhere. In the Columbia estuary for example, all the salmon and steelhead from the Snake and Columbia rivers and their tributaries are funneled together.
"There may not be a compensatory effect, or it may not be as large" because the terns and other waiting predators all have enough to eat, Wiese said. The U.S. Corps of Engineers is amidst an effort to relocate a part of the tern colony -- the world's largest -- to other nesting sites in hope of reducing predation on salmon. Among the passing salmon and steelhead are 13 listed stocks.
Regardless, managers "need to look at in a broader context" before making such management decisions, Wiese said.
"Our analysis demonstrates that identifying the strength of ecosystem interactions (e.g., Essington and Hansson 2004) represents a top priority when attempting to manage the abundance of a particular ecosystem constituent ...," the paper says.
learn more on topics covered in the film
see the video
read the script
learn the songs