A longstanding debate over the benefits of spill ordered by two federal judges at eight dams on the Snake and Columbia rivers is likely to resurface with a new NOAA Fisheries study.

The peer-reviewed study questions a theory known as latent mortality, which has served as a basis for sending juvenile fish over spillways instead of through bypass systems at hydroelectric projects.

The study found that smaller juvenile salmon and steelhead are more likely to pass through a dam's bypass system, and that their smaller size—not the bypass system itself—is the main reason that fewer bypass juveniles return as adults.

The latent mortality theory—also known as delayed mortality—assumes that juvenile Columbia River salmon and steelhead suffer some kind of injury, stress or disorientation from traveling through the hydropower system that causes them to die later, in the estuary or ocean. The reason for their later death has never been determined. This research suggests that the juvenile bypass systems do not affect their survival.

The study—"Associations among Fish Length, Dam Passage History, and Survival to Adulthood in Two At-Risk Species of Pacific Salmon"—was published in August in Transactions of the American Fisheries Society, and funded by the Bonneville Power Administration through the Northwest Power and Conservation Council's Fish and Wildlife Program.

"The finding raises questions about whether spilling additional water past dams to carry more fish through spillways instead of bypass systems will substantially increase their survival in the ocean and the number that return to rivers as adults," a NOAA Fisheries news release says.

Jim Faulkner, a research statistician at NOAA's Northwest Fisheries Science Center and lead author of the study, told NW Fishletter the study doesn't disprove latent mortality, and it doesn't discount other potential benefits of spill, such as the reduced travel time to the estuary.

"We don't want to say spilling more isn't going to do anything. Spilling more may not do much more, but this study isn't addressing that," he said.

The study does show that adult returns depend more on the size of the juvenile than the way it passes through hydroelectric dams on their migration to the ocean. "It raises questions about the magnitude of benefit you'd get from higher spill," Faulkner noted.

Juvenile salmonids in the Columbia Basin travel past hydroelectric dams through three basic routes—spillways, bypass systems and turbines. The bypass systems were built to send juvenile fish away from the powerhouse turbines, using screens and pipes or channels that carry them to the tailrace. Faulkner noted that the dams are equipped only to track PIT-tagged fish going through bypass systems, so a direct comparison between fish that use bypass systems and spillways cannot be made. Instead, the comparison is between fish that use bypass systems and those that do not.

The study used PIT-tag data over multiple years to track juvenile spring/summer Chinook and steelhead from the Snake River's Lower Granite Dam or traps above it, past eight dams including Bonneville Dam and a PIT-tag array in the estuary below.

The data included the length of each fish, and results were calculated from fish that returned.

Along with other research, the results demonstrated that larger juveniles have a higher probability of surviving to adulthood; that larger juveniles were less likely to use a bypass system at six of the seven dams compared to other methods; and that, after accounting for fish length, use of a bypass at one or more dams had little to do with whether they survived in the estuary and ocean and returned as adults.

The scientists noted that spillways may favor larger fish because of the vertical depth in the water where they swim, and because their physical ability makes them more likely to escape the bypass screens.

"[B]ased on our results and those of others, it is imperative that researchers investigating return probability or bypass probability include the length of individual fish in their models," the study concludes. "If other data related to measures of individual fish health exist (e.g., condition factor, disease status, etc.), then those data should also be included. Neglecting these important sources of information could lead to spurious modeling results, which could misinform management decisions and lead to misallocation of limited resources."

Fisheries scientists have long been split over the benefits of spill for juvenile salmonids. Numerous studies have used tagged fish to estimate survival and the results show that direct survival—to a short distance below the dam—for yearling Chinook and steelhead ranges from 90 to 100 percent across the eight dams, with a mean of 97 percent, according to the study. The direct survival is slightly higher for fish going through bypass systems compared with those traveling through spillways, it says.

That debate over bypass survival is outlined in the Washington Department of Ecology's environmental impact statement proposing to increase spill levels at dams in the lower Columbia and Snake rivers to 125 percent total dissolved gas.

The environmental impact statement notes that scientific support for increased spill comes primarily from the Comparative Survival Study, a multiyear life cycle model produced by the Fish Passage Center. "The CSS model predicts steady improvements in juvenile survival and adult returns as spill increases up to at least 125 percent TDG," including a 2- to 2.5-fold increase in Snake River spring Chinook salmon abundance if spill is increased to 125 percent total dissolved gas, the environmental impact statement states.

NOAA Fisheries models are "less optimistic about the benefits of additional spill compared with the CSS model, largely because of the assumption of latent or delayed mortality due to powerhouse (i.e., non-spillway) passage routes and different conclusions about the relative benefit of fish transportation as an alternative to spill," the EIS notes. It adds that the Independent Scientific Advisory Board has not directly compared models analyzing spill, but "seems to find value in both."

Michele DeHart, manager of the Fish Passage Center, told NW Fishletter that they have seen NOAA's study, and are reviewing it. They expect to provide a detailed written review after considering the underlying data and methodology, she said.

The most recent Comparative Survival Study—a draft of which was released in December 2018—includes a chapter that reviews and summarizes studies involving delayed mortality. It notes, "With new spill agreements being negotiated in the near future, decreased water transit times, increased spill, and the associated decrease in bypass probabilities during the spring migration may provide a useful experiment to observe and evaluate delayed mortality under a new flow management regime."

DeHart said the ISAB gave the chapter an enthusiastic response, and said that they are working to answer questions raised by the ISAB.

In its review, the independent scientists said they were "pleased" by the new chapter, but cautioned against conducting any new evaluations of delayed mortality based on new spill regimes and upstream/downstream comparisons that would not meet scientific criteria.

The ISAB also noted that the review briefly mentions several studies that refute the importance or existence of delayed mortality, and suggested that, for completeness, those studies should be included.

NOAA Fisheries spokesman Michael Milstein said ISAB reviews of previous latent mortality studies prompted NOAA's study. The independent scientists noted in 2012 that "the factors responsible for latent mortality remain poorly understood and inadequately evaluated," and suggested further research.

The board noted that the latent mortality studies assumed "fish entering the bypass system are randomly selected, or at least, that the sample of fish entering the bypass system are not less fit in terms of their prospects for survival to return as adults. This assumption needs to be tested both with available data and with further experimental investigation."

K.C. Mehaffey covers fish issues for Clearing Up, and is editor of the NW Fishletter. She joined the NewsData writing team in February 2018. From lawsuits to scientific studies, she is enjoying the deep dive into the Columbia Basin's many fish topics.