Much of the debate over the negative effect of the lower Snake River dams on juvenile salmon and steelhead hinges on the idea that the young fish die from injuries, stress and delays caused by dam passage even after the fish arrive in the Columbia River estuary and ocean.
The phenomenon is known as delayed mortality. Put simply, it means the fish perish because of the dams, but they don’t die immediately at the dams or even in the reservoirs they create. Instead, the young fish are so weakened from the experience of dam passage that they succumb at a time they are seemingly out of danger.
“It is very similar to deer encountering a hard winter. They are not all dead at the end of winter, but some don’t survive as well as they would in a good winter,” said Charlie Petrosky, a retired fisheries research biologist with the Idaho Department of Fish and Game in Boise. “Those are just very common things in ecology — something that happens in one life stage can carry through to the next.”
Juvenile spring chinook salmon migrate from their natal streams to the ocean riding spring freshets that push them downstream. To reach salt water, they must pass the four dams on the lower Snake River and four larger dams on the lower Columbia River. The fish can pass the dams by going through turbines that expose them to tremendous changes in pressure and risk of physical injury. They can use routes that pipe them around and through the dams, known as juvenile fish bypass systems, that expose them to physical contact with screens and other infrastructure, hard angles, crowding and dewatering — all of which can cause injury or stress.
Some of the fish that use bypass systems are then placed in trucks or barges to be shipped through or around the rest of the dams. Doing so prevents the fish from having to go through additional bypass systems or turbines. However, fish that are transported can be susceptible to stress and diseases because of high densities of fish in barges and trucks.
Lastly, the fish can pass over spillways, deemed the safest route and associated with higher life-cycle survival.
All three routes cause the fish to suffer delays in their migration, because of the slower pace of the river pooled behind the dams and difficulty in finding routes past the dams. The delays can disrupt the physical changes the fish undergo to survive in salt water. The exception, of course, is fish that are barged. They tend to make the trip in a much shorter time period — so short that they can hit salt water before their bodies are ready.
Researchers theorize the stress, injuries and delays leave the fish in poor condition when they arrive in the estuary and ocean. Those fish are less able to forage and escape predators.
“When an organism goes from fresh water to sea water, they have to modify their whole system to adapt to those changes,” said Howard Schaller, a retired fisheries research biologist who worked for the U.S. Fish and Wildlife Service. “They have to change their whole osmoregulatory system to accommodate seawater, and if you are injured or you are stressed as an organism, that transition is difficult, or if you modify your timing to make that transition to the sea, it could also make that difficult.”
For example, he said fish disrupted during that physical transition often stick to fresh water layers that float on top of salt water layers in the estuary.
“You are closer to the surface and more vulnerable to avian predation, and plus, if you are injured and having a difficult time making that transition, obviously your avoidance technique will be compromised, and you will be more vulnerable to both avian and (fish) predation.”
Petrosky and Schaller have studied delayed mortality in Snake River spring chinook for more than 20 years. In a paper they published in 2014, they found that Snake River spring chinook that survive the journey past all eight dams suffer delayed mortality rates of 61 to 76 percent in the estuary and ocean. Fish that go through turbines or bypass systems, collectively known as power houses, perform less well.
“What we have done over the years is look at empirical information that would say fish that experience going through a number of power houses and bypass systems survive at a much lower rate once they go out to sea than fish that would not have had that experience,” Schaller said.
The two researchers also documented that Snake River spring chinook began to return at much lower rates after completion of the entire Snake and Columbia river hydropower system and that spring chinook from the John Day River in Oregon that must pass three dams instead of eight return as adults at much higher rates than Snake River fish. They say that lends credence to the delayed mortality theory. They also documented that higher rates of delayed mortality for Snake River fish have been documented even in years with good ocean conditions.
Some researches postulate that dam passage is not responsible for delayed mortality, and it can instead be associated with the size of fish. This theory is built around the idea that smaller fish are predisposed to go through turbines and fish bypass systems and those fish, because of their smaller size, are much less likely to survive once they get to the ocean and estuary.
“It’s really about can you escape predators once you get to the estuary, because there is a ton of things that want to eat them there,” said Jim Faulkner, a statistician at the National Oceanic and Atmospheric Northwest Fisheries Science Center at Seattle. “And (size) is also kind of surrogate for maybe their condition, that they didn’t grow as much in natal streams, and they may have a harder time. They just didn’t have enough energy reserves.”
The idea was criticized by reviewers at the Fish Passage Center who called the work “flawed.” Schaller points out that spring chinook from the John Day River tend to be smaller than Snake River chinook and yet survive at much higher rates.
“Their sizes are actually smaller when collected at Bonneville Dam, and they had smolt-to-adult return rates that were magnitudes higher than Snake River fish, so smaller fish size couldn’t explain it,” he said.
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