Where Salmon Dieby John McKern, Fish and Wildlife Biologist
U.S. Army Corps of Engineers
Idaho Farm Bureau News, May/June 1999
Question: If two spring chinook salmon, a male and a female reach the Snake River spawning grounds, what survival rate will ensure that two salmon return in the next generation?
Answer:On average, Snake River spring chinook lay about 5,000 eggs -- 2/5000=0.04%. Therefore, 99.96% of the progeny will die somewhere between the laying of the eggs and the return of the two adults to the spawning ground. This is a generalized example of where that mortality occurs, given current conditions.
Studies of spring chinook spawning success in Idaho indicate that under normal circumstances well over 90% of the eggs laid by the eggs laid female will be fertilized by the male (or males). In the gravel, some eggs will perish because they were not perfectly formed. Others will fall prey to microorganisms like fungus or parasites. Developing eggs and young fish may be eaten by fish like sculpins of bull trout, or by birds like dippers or mergansers. Of the 5000 eggs laid, up to 400 may reach smolt size in one to three years. That means 92% will have died before migration to the sea begins.
Recent PIT tag studies indicate that about 50% of the juvenile fish leaving hatcheries reach Lower Granite Reservoir. With wild fish, 80% to 98% mortality occurred between marking in the summer and migration the next spring. Much of this mortality could have been due to over-wintering conditions. Assuming that wild fish survive migration about as well as hatchery fish, about 50% of the mortality would occur after the fish smolt. Of the 400 smolts that started, 200 would reach Lower Granite Reservoir. Survival through the reservoir, according to PIT tag studies, would be 96%, so 192 would reach Lower Granite Dam. According to the 1998 National Marine Fisheries Service biological Opinion, 50% of the fish should be transported and 50% should migrate in-river. Of the 96 fish transported, 2% would die in collection at the dam or in the barge or truck, so 94 fish would be released below Bonneville Dam alive. Of the in-river migrants, assuming they were not collected and transported from a dam downstream, PIT tag studies indicate that an average of 92% would survive each dam and reservoir project. Taking .92 to the eighth power, 51% would survive passage through the eight reservoirs and dams. Of the 96 fish migrating in-river, 49 would join the 94 transported, for a total of 143 below Bonneville. In migrating to the sea, about 16% would be eaten by Caspian terns, leaving 120. Another 6% would be eaten by cormorants leaving 113, and 1% by gulls leaving 112. Another 10% would be eaten by predacious fish, leaving 101. Seals and seal lions would take another 1% leaving 100 to enter the ocean.
Ocean conditions, availability of food, and an abundance of predators might reduce this numbers by 88% over the next 2 to 3 years, leaving 12 adults to start the journey back to the spawning grounds. Up to 11% would be caught in the ocean, leaving 11 fish to enter the estuary.
About 30% would be eaten by seals and sea lions, leaving 8 fish to migrate upriver. If the Indians took 50% according to their treaty rights, 4 salmon could migrate on toward the spawning grounds. Spring chinook that enter the river in April and May must survive without eating until they spawn in September. Typically, about 50% die from the rigors of the migration or from injuries from seals, sea lions or gill nets. That would leave 2 to return to spawn.
Of the 4,988 fish that died in the example above, 44 juvenile salmon and perhaps 1 adult salmon died because of the federal dams. Obviously, the few salmon caught in the ocean and in the river were man-caused mortalities. Numbers of fish-eating birds have increased dramatically in the past decade or so. Why? The use of DDT and other chlorinated hydrocarbon pesticides has been dramatically reduced. These contaminants caused eggshell thinning in birds at the top of the food web, severely depressing their reproduction. Marine mammals which were hunted to near extinction through the 1960's have been completely protected since 1972. Their numbers, too, have gone up in geometric proportions. Spawning and rearing areas have been severely reduced and impacted by grazing, mining, logging, private dams, and other urban impacts -- all man-caused. Historically, up to 16 million salmon returned to the Columbia. If Indians harvested 1 / 4 of the runs, 11 million returned to the spawning areas, most to spawn and die. A century of overharvest significantly reduced the amount of nutrients salmon brought inland from the sea. Now, less that 700,000 adults return over Bonneville Dam each year, so spawning grounds lack the fertility to produce number produced two centuries ago.
Percentages and numbers used in this example are broad-brush averages based on the extensive literature on the subject.
Currently, we transport fish from March 25 through October 31 from the Snake River dams (Lower Granite, Little Goose, and Lower Monumental). The fisheries agencies have us spilling water up to the point where gas supersaturation is 120% below the dams. That results in about half of the fish being passed over the dam and half being transported. We transport from mid-summer through December from McNary Dam, but bypass all fish in the spring.
In transport, we usually get over 99% survival from the time the fish enter the dam until they are loaded on trucks or barges. We get 99% survival in the trucks and 99.5% in the barges. Fish transported from Lower Granite Dam experiences less that 1% loss (PATH uses 2%). Because we can't collect all of the fish at Lower Granite, some get past that dam into Little Goose Reservoir, and some of them are collected at Little Goose Dam. Of those that get past Little Goose Dam, some are collected and transported from Lower Monumental Dam. Taking the reservoir and dam mortalities into consideration. we get about 74% survival of the fish from above Lower Granite to below Bonneville Dam. If we stopped spilling for fish passage and transported all the fish we could collect, we could get as high as 90% to 94% of the fish below Bonneville Dam alive.
With our current bypass systems, we get approximately 96% to 97% survival through the dams (spring / summer chinook). With 96% to 99% survival through the reservoir, we get 92% to 95% through each dam / reservoir project. That gives us 50% to 60% survival through the 8 dam systems. Thus in-river survival is 50% to 60%, current transport is 74%, and maximum transport would be 91% to 93%. Survival through the lower Columbia River projects is about 74%. If you took out the four Snake River dams, and increased survival from 74% to 99% through this reach, survival through the Clarkston to Bonneville tailrace would be .99 x .74 = .73 or 73%. Thus you can see that taking the dams out would reduce survival below what we can achieve with the existing system.
(bluefish points out that delayed effects of barging and dam migration are being ignored here.)
There is a growing body of science that says ocean conditions and climate have more effect on the return of adult fish than what happens in passing the dams. British Columbians say that many of their adult return rates have fallen from 6% to over 10% to less than 1% just because of ocean conditions. They canceled all fishing for coho off British Columbia last year. For 1999, they have canceled commercial fishing for chinook and coho, making them sport fish only, and they have bought out 35% of their commercial fishing fleet. Commercial fish will concentrate on sockeye, chum, and pink salmon although many runs of these fish are depressed because of ocean condition.
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