Avoiding Reservoir Mortality By Reducing Travel Time May Not Help Salmon at All

While the most fundamental dispute among Northwest fishery biologists has been whether and to what extent reservoir releases can speed up salmon, few have bothered to stop and ask whether speeding up salmon does much good. The biologists instead assume the mainstem reservoirs are "lethal slackwater pools" where juvenile salmon are exterminated by hordes of predators and lose their way because there is no swift guiding current, so that speeding salmon through the reservoirs is unquestionably a good thing.

But there are large numbers of predators in undammed reaches too, both above and below the dams. For example, in 1996 the National Biological Service produced data that were “the first to document rapid switching by northern squawfish from a mostly nonfish diet to one of primarily juvenile salmonids in a location away from a hatchery release or hydroelectric dam.”70 After a hatchery release many miles upriver, squawfish in the Clearwater River in Idaho relied on juvenile salmon for 80% of their food. What happens in this undammed reach is downplayed by the harvest agencies: lots of juvenile salmon die, in ways that cannot reasonably be attributed to the effects of dams.

Each and every flow proponent has failed to recognize the simple fact that getting juvenile salmon quickly out of the reservoirs to avoid reservoir predators is only helpful if the net effect is to reduce exposure to predators. If the density of predators is lower in the reservoirs than in the lower river and estuary, the survival per day may be higher in the reservoirs. Thus, as Dr. James Anderson and Richard Hinrichsen have pointed out, it is conceivable that getting juveniles out of the reservoirs faster could even reduce overall salmon populations, especially if they wind up hitting the estuary at a time of high predation.71

I have seen no evidence that predator densities are higher in the reservoirs than in a free-flowing river. I have heard biologists acknowledge that the density of predators appears to be higher below Bonneville Dam than in the reservoirs above it. The National Research Council recently concluded that “[t]here is some evidence that predators, such as northern squawfish, have increased in abundance in the lower Columbia River”.72 The Northwest Power Planning Council’s Independent Science Group notes that some data indicates “a 54.5% loss of smolts from the tailrace at Bonneville Dam to Rainier Beach, Oregon”.73 This is consistent with European studies of the survival of Baltic salmon in estuarine conditions, which show mortalities as high as 50% for the last 50 kilometers of salmon migration before the ocean. Survival though the hundreds of miles of Columbia and Snake River reservoirs is far higher everywhere we have measured it.

Biologists have also theorized that as dams raised the water level and slowed river velocities, more silt washed over the rocks at the bottom of the river, leaving the juveniles no space to hide from predators.74 But no one has ever done experiments to test this theory either, beyond measuring the proportion of juvenile salmon migrating downstream consumed by predators. The Snake River is already so turbid that in 1995, scientists trying to find salmon redds with underwater video cameras could not do so because the water was not clear enough.75

The data offer every reason to believe that juvenile salmon are safer from predators in the reservoirs in terms of mortality per mile than they are both above and below the reservoirs. Yet the most fundamental axiom of fishery agency management is to try and rush them through the reservoirs, as if the reservoirs were an especially dangerous place.

70 R. Shively, T. Poe, & S. Sauter, “Feeding Response by Northern Squawfish to a Hatchery Release of Juvenile Salmonids in the Clearwater River, Idaho”, Trans. Am. Fish. Soc. 125:3230-236 (1996).

71 See J. Anderson & R. Hinrichson, “A Life History Approach to Managing the Columbia River Hydrosystem for the Benefit of Salmon Populations”, Oct. 28, 1994, at 1-2.

72 NRC, Upstream at 199 (Prepub. ed.).

73 ISG, Return to the River 280.

74 O. Bullard, Crisis on the Columbia 111 (Touchstone Press 1968) (reporting comments of unidentified Oregon State Fish Commission biologists).

75 U.S. Army Corps of Engineers, “Interim Status Report”, at 9-38.

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