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Flow/Survival Link Confounds Researchers

by Barry Espenson
Columbia Basin Bulletin - February 9, 2001

Increased flows correlate highly with increased survival rates of migrating Snake River subyearling fall chinook salmon, National Marine Fisheries studies continue to show, but only drastic tests would settle debates about the importance of flows augmented by releases from upstream reservoirs.

"The real good experiment that would get a lot of answers would kill a lot of fish," NMFS researcher Bill Muir told the Northwest Power Planning Council Wednesday.

That experiment would be to end flow augmentation, he said. The extra flows are called for in NMFS' hydrosystem biological opinion because the agency believes it provides direct benefits, especially to summer migrants. The BiOp outlines measures NMFS feels necessary to ensure the survival of Endangered Species Act listed salmon and steelhead.

The value of flow augmentation in improving salmon survival has been questioned, particularly by states such as Idaho and Montana that must provide pulses of water from storage reservoirs intended for irrigation and recreational uses.

The ongoing NMFS Survival study, which has monitored spring chinook and steelhead migrations since 1993 and fall chinook migrations since 1995 is not intended to support or debunk augmentation or BiOp prescribed fish spills at dams. It was intended to monitor survival through dams and reservoirs and guide efforts to improve hydrosystem passage and survival. But the statistics produce fodder for augmentation and spill arguments.

Idaho, led by Water Resources Department chief Karl Dreher, says that establishing a correlation between increased flows and increased fall chinook survival does not establish flows as the determining factor in the salmon's fate.

NMFS agrees, saying that it has yet to disentangle confounding and related factors. The monitoring of fish migrations and survival via PIT tags also shows water temperature and turbidity to be highly correlated to subyearling survival. Higher turbidity or murkier water, which usually is associated with higher flows, helps hide the young salmon from predators in river reaches and reservoirs. Higher water temperatures, more consistent with lower flows later in the season, stall smolt growth and stir up warmwater predators' metabolisms.

"We can't distinguish between the three" variables as relates to salmon survival, said Steve Smith, NMFS research statistician. "We're faced with a very difficult problem trying to sort out these variables."

Since flow augmentation has been employed each year throughout the course of the test, there is no baseline comparison for survival in its absence. The cost of such a test, curtailing flow augmentation, would likely be high subyearling mortality with a change in all three variables that correlate to decreased survival, Muir and Smith say.

An analysis of the 2000 migration data could be revealing, Smith said. Lower than normal early summer flows and clear water prevailed above the Snake River's Lower Granite Dam while the water remained cool -- a slight separation of the normally inseparable variables.

"That may give us more insight about the interaction between the variables," Smith said.

Releases of PIT-tagged Lyons Ferry hatchery subyearlings above Lower Granite at Pittsburg Landing and Billy Creek in 2000 showed the lowest survival yet measured during the six-year study.

The NMFS data shows a strong, consistent relationships between flow and travel time for spring migrants such as spring chinook and steelhead. But the relationship between flow and survival is described as weak and inconsistent.

"It's (flow augmentation in the spring) probably not going to make a great difference in that Lower Granite to McNary stretch. We only evaluated this in the upper end of the system," Muir said of the ongoing spring chinook PIT tag survival study. That represents 32 percent of the spring chinook's freshwater migration distance.

"That doesn't mean it doesn't help in other parts of their life cycle," Muir said. For example, increased river flows can expand the freshwater-saltwater plume off the river's mouth, an area where the fish habituate before launching their ocean sojourn. It can also speed their journey.

"Getting them there faster can make a huge difference in adult returns," Muir said.

Spill can help improve the survival of in-river migrants, according to the study data. It has been the safest dam passage route with survival ranging from 100 percent to 88.5 percent, depending on the project and species. Survival though manmade bypass systems has ranged from 99 to 93 percent while turbine passage data shows survival of between 93 and 86 percent, Muir said.

He told the Council that he believed survival was 3 to 4 percent higher as a result of the spill regime's implementation with the 1995 BiOp.

The main arguing point against spill as a salmon recovery remains its cost, Muir said, in terms of hydrogeneration lost when water is spilled instead of flushed through turbines.

Improved dam passage, reduced predation and multiple other measures have helped improved survival to the point that it is "as high or higher" through the current eight-dam system as when only four dams were in place on the Columbia-Snake mainstem, Muir said. Survival took a precipitous decline in the late 1960s and early 1970s, the same period when the four Snake River hydro projects came on line.

"During the 2000 spring migration, survival was similar to past years with high spill levels," according to a summary of the survival study updates. "Per-project (one reservoir and dam) survival averaged 93.4 percent for yearling chinook salmon and 90.8 percent for steelhead."

"They have been in that range since about 1995," Muir said.

"Expansion of per-project estimates through the entire hydrosystem (head of Lower Granite Dam reservoir to Bonneville Dam tailrace) resulted in estimates of 45.4 percent for yearling chinook salmon and 35.7 percent for steelhead," according to the summary.

The average survival from Snake river Basin hatcheries to Lower Granite Dam was the highest yet estimated since the monitoring of spring chinook began in 1993. That included a 76 percent survival from Dworshak and a 36 percent survival from Sawtooth hatcheries. The average survival from upstream hatcheries to Lower Granite was 60 percent.

Muir said that survival through those unimpeded (by dams) stretches of river was likely similar for wild and hatchery fish.

Link information:

Barry Espenson
Flow/Survival Link Confounds Researchers
Columbia Basin Bulletin, February 9, 2001

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