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Considering Predation Levels When Reintroducing Salmonids Above High Head Dams
Posted on Friday, May 20, 2016 (PST)

When reintroducing anadromous fish into blocked areas upstream of high head dams, such as what is being considered at Grand Coulee Dam, biologists and policymakers want to know if there is sufficient quality habitat to support salmon and steelhead before investing the considerable resources needed.


The Northwest Power and Conservation Council at its April meeting in Missoula, Montana, approved such an assessment in habitat on the Columbia River from Grand Coulee and Chief Joseph dams to the Canadian border. (See, CBB, April 15, 2016, “Council Votes To Move Forward On Salmon/Steelhead Habitat Assessment Above Grand Coulee,”


Biologists on the North Fork of the Lewis River in Washington also want to know that, if salmon and steelhead are reintroduced upstream of a series of three dams, will they be devoured by predators?


A recent study found that, yes, predators do have an impact, but not to the extent previously thought.


The Lewis River dams -- with Merwin, Yale and Swift reservoirs -- are host to kokanee and trout, as well as the predators Northern pikeminnow, an indigenous species found throughout the Columbia River basin, and Tiger Muskellunge, or Tiger muskie, a hybrid between northern pike and muskellunge that can't reproduce. About 1,400 Tiger muskie are planted annually in Lake Merwin by the Washington Department of Fish and Wildlife to control the Northern pikeminnow population and provide a sport fishery.


“We found that the intensity of predation on juvenile salmon by Northern pikeminnow and tiger muskie was less than expected,” said Mark Sorel, a graduate student at the time of the study in the Washington Cooperative Fish and Wildlife Research Unit, School of Aquatic and Fishery Sciences, University of Washington.


He said that most Northern pikeminnow ate invertebrates, such as zooplankton (i.e. water fleas) for smaller fish, and crayfish for the larger pikeminnow, as well as smaller pikeminnow (cannibalism). The main prey of tiger muskie was Northern pikeminnow and largescale suckers. 


“Most of the Northern pikeminnow were actually relatively small sized and didn't appear to eat juvenile salmon at all,” he said.


The study says that the Northern pikeminnow diet is seasonal and increasingly relies on size of the pikeminnow. The spring and summer diet for pikeminnow that are 200 millimeters (7.9 inches) to 299 mm (11.8 in.) in length is 72 to 88 percent zooplankton and smaller proportions of sculpin, signal crayfish, insects and plant matter,  but no salmonids. During the spring pikeminnow greater than 300 mm ate mostly fish and signal crayfish (45 percent) and smaller pikeminnow (43 percent). The diet for these larger fish changed in the summer to 51 percent signal crayfish, 19 percent smaller pikeminnow, 9 percent sculpins and 14 percent kokanee. That increased to 14 percent sculpin and 28 percent kokanee in the fall. Kokanee was the largest prey fish consumed by the pikeminnow and at times kokanee were as much as half the size of the pikeminnow.


The Tiger muskie diet was 65 to 80 percent pikeminnow, 10 to 25 percent Largescale suckers, while kokanee and other prey were a much smaller part of their diet.


Overall, if the fish in the reservoir were juvenile reservoir-rearing year-0 chinook salmon weighing about 18 grams (0.63 ounces) and 120 mm (4.7 in.) in length, some 39,250 of the fish would be consumed annually by l,000 pikeminnow greater than 300 mm in length. Less than half (16,022 salmon) would be consumed if they had reached 45 grams (1.9 oz.) and 150 mm (5.9 in.) in length.


That’s for reservoir-rearing salmon. If the salmon were simply migrating through the reservoir from streams, 969  smolts weighing 18 grams would be consumed by 1,000 of the predators greater than 300 mm or 388 smolts weighing 45 grams would be consumed.


However, according to the study, the vast majority of pikeminnow in Merwin reservoir were less than 300 mm in length and fed predominantly on invertebrates, sculpins and plant matter, “with seemingly no consumption of salmonids.”


This study is part of a planned reintroduction of salmon and steelhead to the upper Lewis River, Sorel said. The reintroduction is already underway above Swift Dam, which is the uppermost of the three reservoirs. This study, he said, is helping to evaluate the feasibility of reintroduction in Lake Merwin and is one of the studies that he believes should be done prior to any reintroduction in the Northwest.


“As far as I know, this type of study should be done prior to reintroducing salmonids above Grand Coulee Dam,” Sorel said, adding that similar studies are underway as part of the reintroduction efforts in the Willamette River basin. “I think these types of reintroductions have the potential to really help recover and conserve Columbia River salmon and steelhead, but we do need to evaluate how predators in large storage reservoirs could impact their success.”


The predator most at issue in Lake Roosevelt, the reservoir formed behind Grand Coulee Dam, is the invasive northern pike. Managers of the lake’s fisheries are concerned about the quick spread of the fish throughout the lake and ultimately into the mid-Columbia River. A survey of Lake Roosevelt in 2015 found “an alarming increase in Pike abundance.”


See CBB, April 8, 2016, “With Science Review In Hand, Efforts Continue To Halt Non-Native Pike Expansion In Upper Columbia,”


The study, “Predation by Northern Pikeminnow and Tiger Muskellunge on Juvenile Salmonids in a High-Head Reservoir: Implications of Anadromous Fish Introductions,” was published online April 19 in Transactions of the American Fisheries Society


Sorel’s co-authors are Adam Hansen, Kristin Connelly and Erin Lowery, staff research scientists, and Andrew Wilson, undergraduate student, all with the School of Aquatic and Fishery Sciences at UW; and David Beauchamp, professor at UW and unit leader of the Washington Cooperative Fish and Wildlife Research Unit.


In 2004, PacifiCorp Energy and Cowlitz County PUD, which jointly operate Swift, Yale, and Merwin dams, signed a Federal Energy Regulatory Commission relicensing agreement for the next 50 years. In that agreement, the utilities agreed to reintroduce chinook and coho salmon, and steelhead. According to Sorel, the reintroduction began above Swift Dam because it directly connects to the vast majority of the 117 miles of accessible river and stream. Yale reservoir has about 17 miles of stream habitat and Merwin reservoir has about 5 miles.


Adult coho were first released above Swift Dam in 2010 to begin conditioning the habitat, and a state-of-the-art juvenile fish collector went online in 2013. Adults are trapped at the base of Merwin Dam and transported by truck upstream of Swift Dam. Juveniles are trapped at Swift Dam and transported by truck downstream of Merwin Dam.


The licensing agreement calls for downstream fish passage at Yale Dam by 2021 and Merwin Dam by 2025. However, managers have the option to forego reintroduction in Yale and Merwin and spend those mitigation funds on other projects. The decision whether to conduct reintroduction in the middle reservoir, Yale Reservoir, will be made in 2017.


In order to have a successful reintroduction, there needs to be suitable spawning habitat, quality habitat and food for juveniles to grow, and there needs to be a system to safely transport them around the dam.


“All of these factors are currently being evaluated and considered by managers, who will decide whether to move forward with reintroduction in Lake Merwin soon,” Sorel said. If managers decide to move forward with Yale reservoir, they will begin releasing adult fish above the dam and a juvenile bypass system will be installed.


There is more to consider than the abundance of predators alone when evaluating their impact on juvenile salmon. For example, temperature (especially thermal stratification) and behavior are very important factors.


“Reservoir food-webs can be complex and there is a lot that we still don't know about them,” according to Sorel. “This and other similar studies are helping us understand how different predator species may impact the success of reintroductions above high-head dams in the Pacific Northwest.”


Also see:


--CBB, March 11, 2016, “Council FW Committee Moves Forward On Salmon Reintroduction Study Above Grand Coulee,”


-- CBB, Feb. 5, 2016, “Washington Legislature Considers Memorial For Salmon Re-Introduction In Upper Columbia Blocked Areas,”


-- CBB, January 15, 2016, “Council Considers More Money For Pike Removal: ‘An Alarming Increase In Pike Abundance,”


-- CBB, December 18, 2015, “Council Moves Proposal For Evaluating Salmon Habitat Above Grand Coulee To Science Review,”


-- CBB, Nov. 19, 2015, “A Northern Pike Caught In John Day Reservoir: For Salmon, Canary In The Coal Mine?”


-- CBB, October 16, 2015, “Can Salmon, Steelhead Survive Above Grand Coulee Dam? Council Investigation May Provide Answer,”


-- CBB, September 18, 2015, “Council Moves Ahead With Plan To Assess Potential Salmon Habitat Blocked By Grand Coulee,”


-- CBB, July 17, 2015, “Invasive Northern Pike Spreading Further, Reproducing; Council Hears Information On States’ Policies,”


-- CBB, Jan. 16, 2015, “Tribes Lay Out Process For Investigating Feasibility Of Salmon Reintroduction Above Grand Coulee Dam”


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