The Question of Natural Mortality

With modern methods that can get more than 80% of the juveniles migrating downstream alive, you would think that we would have some idea whether we had mitigated adverse effects of the dams. One of the most amazing facts about salmon management is that federal, state and tribal fisheries managers refuse even to consider whether current rates of survival are above or below survival rates in rivers without any dams.

Before the dams were built, juvenile salmon had to navigate miles of violent rapids that would destroy man-made rafts, including Cascade Rapids, the rapids that stopped Lewis and Clark in their tracks. Early accounts note that fish arriving below these and other rapids were bruised and battered. No one knows whether waterfalls and rapids are more harmful to salmon than modern hydroelectric projects.

Until very recently, none of the fishery agencies would even acknowledge the concept of natural mortality. We went so far as to hire a retired professor from Oregon State University, Dr. William McNeil, to investigate it. Dr. McNeil was initially skeptical of our claim that conventional wisdom concerning the dams was wrong, but he had what all government fisheries bureaucrats seemed to lack: an open mind. After conducting a number of scientific studies on the subject for us, he finally became convinced that conventional wisdom attributing significant mortality to the dams was off base.

He wrote Will Stelle on December 15, 1994, advising him that NMFS

“can make meaningful estimates of natural mortality, that being mortality which occurs in the pre-project river. My review of the scientific evidence . . . comparing mortality in dammed and undammed rivers leads me to conclude that most of the juvenile passage mortality occurring as juvenile salmon migrate through the dams and reservoirs comprising the [Federal Columbia River Power System] is mortality that would have occurred without the projects.

A lot of data supports Dr. McNeil’s observations. For example, from 1964 to 1967, in-river survival of spring/summer chinook salmon from the Clearwater River confluence with the Snake to below Bonneville Dam, was estimated at 42 to 67 percent.80 Now, after the construction of four more dams, in-river survival is generally estimated at 40 to 50 percent.81

Until 1993, there were no really accurate measurements of in-river survival. PIT-tag technology allowed such measurements, by providing for the first time a way reliably to count individual juvenile salmon as they migrated downstream. The most recent PIT-tag data analysis by the National Marine Fisheries Service measured survival of yearling chinook salmon migrating from the uppermost and eighth dam, Lower Granite, to the third dam, John Day, at 56% during 1996—and that reflected a probable reduction in survival resulting, in part, from ill-conceived spill programs.82 This translates to about 89% survival per dam and reservoir, including turbine mortality (for fish passing through turbines) and reservoir mortality.

A lot of reservoir mortality from predation would probably happen in a natural river. Yet anti-dam tracts bearing catchy titles like River of Life, Channel of Death ignore such natural mortality, complaining that even the spill promoted by harvest managers is “a relatively ineffective measure; thousands of fish are killed or eaten by predators here because there is no safe way through the dam”.83 But no one bothers to think how many thousands of fish would die when passing through any particular undammed section of the river. Or how any equilibrium in salmon populations requires that the vast majority of juvenile salmon die.

Late in 1996, nearly four years after we began calling for an assessment of natural mortality, an inter-agency group dominated by the states and tribes has produced an estimate that only 10-20% of the smolts would have been lost during migration before the dams were built.84 They provided no supporting data; the estimate was essentially pulled out of a hat. If it were true, the pre-dam river would have to have become dramatically safer for smolts once they reached the site of Lower Granite Dam, because the death rate per mile would have to fall drastically from the levels measured in the natural river above Lower Granite Reservoir. The group did not review or address Dr. McNeil’s work finding that survival per mile does not differ greatly between dammed and undammed reaches.85

Again, it is entirely possible that when the positive effects of transportation are included, passage downriver is now safer for salmon than it was before the dams were built. The Great Salmon Hoax is great indeed, because it pushes us to spend hundreds of millions of dollars to get even smaller increments in survival when from a common sense perspective, "if it ain't broke, don't fix it." There are still a few problem areas, like the Ice Harbor to McNary reach where the Columbia joins the Snake, but maybe natural survival was lower there too. A fisherman’s perspective suggests that maybe predators feed more there.

80 U.S. Army Corps of Engineers, "Interim Status Report", at  2-3.

81 Id.

82 Memo, M. Schiewe to W. Stelle, Jr., July 12, 1996, at 3 (the memo notes that the survival estimates “should be considered minimal estimates that will increase slightly”; I was later told that survival to John Day was 56.4%).

83 K. Petersen, River of Life, Channel of Death 21.

84 Marmorek et al., Plan for Analyzing and Testing Hypotheses (PATH): Final Report on Retrospective Analyses for Fiscal Year 1996 (ESSA Technologies 1996).

85 W. McNeil, “Survival of Marked Juvenile Chinook and Steelhead Migrants in the Columbia Basin”, Dec. 10, 1994.

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