New Study Links Delayed Mortalityby Natalie M. Henry
A group of four biologists have published an independent study in the North American Journal of Fisheries Management linking delayed mortality of Snake River salmon to their having to pass around or through as many as eight hydroelectric dams during migration. While other factors such as ocean conditions can also contribute to delayed mortality, this new study represents the most comprehensive review of data yet making the link with the hydropower system, and the study could color the 2003 check-in on the federal salmon recovery program that so far has squelched dam breaching.
Delayed mortality is the term used when juvenile salmon successfully out-migrate from the hydropower system but fail to return in a few years as adult salmon. Delayed mortality could be caused by a number of things, including ocean conditions, harvesting, stock viability, habitat conditions, predation and the hydropower system, to name a few. The new study, "Evidence Linking Delayed Mortality of Snake River Salmon to Their Earlier Hydrosystem Experience," examines the latter hypothesis.
When out-migrating juvenile salmon pass the four lower Snake River dams and then the four mainstem Columbia River dams, they pass or attempt to pass in one of four ways: by falling over the dam as a result of water purposefully spilled from the top; swimming through a looped fish bypass tube that brings the salmon down near the bottom of the dam first, then up near the surface to a collection channel, then down again and finally out the bottom on the other side of the dam; travelling in a barge; or navigating a turbine. Theoretically, the fish that make it to the last dam, Bonneville, without dying inriver (which is called direct mortality) will return in several years to navigate their way up the river, back over the dams and into their natal streams to mate. But many of them do not return to their natal streams and are said to have experienced delayed mortality.
According to one of the authors, Howard Schaller of the U.S. Fish and Wildlife Service's Columbia River Program Office, the new study gathered and peer-reviewed the full weight of evidence that exists relating delayed mortality to the hydropower system. The three other authors are scientists from the Idaho Department of Fish and Game, Utah State University's Department of Fisheries and Wildlife, and Eco Logical Research, a consulting firm based in Utah.
The study is broken down into three sections. The first reviews all the existing evidence and literature showing that salmon navigating the Snake River dams incur numerous stresses. The authors begin by saying the impacts of stress on fish and fish populations have been widely studied and documented. That said, the study then details a number of acute and chronic ways that hydropower dams stress salmon, with an emphasis on chronic stress.
For one, dams create reservoirs, which slow the river and cause out-migrating salmon to expend more energy to get to the dam. The reservoirs provide opportunities for more predation, which also causes stress. And reservoirs tend to raise the water temperature, which can stress salmon as well, the study says.
Bypass systems further stress fish. The authors say spill is the preferred method of fish passage, although it is not foolproof because it creates quick changes in pressure, causes fish to fall a long distance, exposes fish to dissolved gases that can lead to disease and other health problems, and predators sometimes await at the bottom.
If fish are not spilled over a dam, they are forced down toward the bottom of it. Some end up going through a turbine, which often kills fish. If the turbine does not kill a fish, it usually causes hemorrhaging and a loss of scales, which causes visible stress and health problems.
If fish do not go through the turbine, they are shunted up and away by a fish screen, entering a tube that takes them up toward the surface into a collection channel. At the collection channel, the fish are tagged and many -- sometimes all -- are taken out and put into barges. If not, the fish continue into another tube that takes them to the bottom of the dam again and out the other side, where predators sometimes await.
The report says the waiting predators are an added stress even if they do not create direct mortality. It also says the rapid atmospheric pressure change involved in the bypass tube is a problem. Juvenile salmon go from a level of one atmospheric pressure to three and back to one in a period of about 10 seconds, the report says.
At Lower Granite Dam, the farthest dam up the Snake River, the bypass system puts fish through "high turbulence and rapid deceleration at the end." Then the juveniles pass through a device to separate fish by size, they are dewatered, and they pass through a tube that leads to a PIT-tag (passive integrated transponder) detector (the tags used to identify the fish and collect data). "Remember, this account describes the passage through only one of eight dams," the study says. Four of the dams have collection devices; all of them have means for bypass.
Another problem is the rate of migration caused by the dams, according to the study. Over many thousands of years, salmon have mastered the ability to gradually move from a freshwater environment to a saltwater environment. The study maintains that it is stressful for salmon to be held too long behind a dam, or to be transported too quickly on a barge because the salmon's body is changing at its own pace to prepare for the ocean.
The second part of the study provides indirect evidence that the hydropower system causes delayed mortality in salmon. Direct mortality from hydropower eliminates 25 to 73 percent of juveniles and adults, according to the study. After direct mortality, Snake River fish may experience 37 to 68 percent "additional mortality," i.e. delayed mortality. The authors by and large rule out harvest, habitat, hatcheries and climatic conditions as reasons for declines in Snake River stocks, which leaves hydropower. The authors state that the hydropower system is likely partially responsible for overall declines in survival and, therefore, delayed mortality.
Finally, the study offers direct evidence relating direct mortality to hydropower based on PIT-tagged fish. The PIT data shows that while direct mortality is lower for salmon transported via barge than for fish that navigate the dams, delayed mortality is higher for fish transported via barge. The authors attributed this to the stress experienced in the hydropower system and collection channels. The PIT data reflects even higher delayed mortality in fish that pass through one or more dams and then are collected and transported from a lower dam.
Environmental groups heralded the study as more evidence that partially removing the four lower Snake River dams is the best thing for salmon. And the authors of the study agree: "If delayed mortality takes place and is related to the hydrosystem, then dam breaching is likely to greatly increase the survival of Snake River stocks and ultimately may provide a high probability of recovery. ... Whether research can reduce this uncertainty is unclear; under current conditions, however, extinction is highly probable and decision makers will be faced with making a decision about dam breaching soon."
Nicole Cordan of the Save Our Wild Salmon coalition said, "There's no shortage of evidence for what's been killing Snake River fish, and all paths lead back to the dams."
Trout Unlimited's Alan Moore said, "Few folks question that the hydrosystem kills salmon and steelhead, but I think a lot of people still just have the image of a fish getting diced up in a turbine. This is the latest installment in a growing body of scientific evidence showing the broad variety of ways, shapes and forms fish are dying from the hydrosystem." Moore called for returning to a natural river system.
Rob Masonis said federal decision makers should address the issue of delayed mortality at the 2003 check-in for the biological opinion governing salmon recovery, at which point dam breaching will be up for consideration again.
The Army Corps of Engineers said it could not comment on this article by press time. Dam breaching opponents such as the Columbia River Alliance have said in the past, however, that ocean conditions and overharvesting are the primary culprits in declining salmon populations, and that the Snake River dams should not be removed on any account.
learn more on topics covered in the film
see the video
read the script
learn the songs