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Fish Researchers Still Looking
for Answers to Flow-Survival Conunudrum

by Bill Rudolph
NW Fishletter, November 18, 2004

Federal scientists reported last month that 2003 research on hatchery fall chinook in the Snake River shows that 20 to 30 percent of the juvenile fish die before they reach the first dam, but they admitted they're no closer to finding the hydro system's Holy Grail--unraveling the confounding effects of river flows, temperature, turbidity and travel time on salmon survival.

The results were announced in a draft paper submitted to the Bonneville Power Administration last month that also found mortality between each successive dam on the lower Snake was about 12 percent once the fish were in the hydro system, according to the report from the Fish Ecology Division of the Northwest Fisheries Science Center in Seattle.

But are the dams killing the fish, or could it be a large population of hungry predators that get ever hungrier as waters warm in the summer?

To get a better handle on these questions, researchers released Pit-tagged fish from upstream locations where acclimation ponds above the dams on the lower Snake are the last stop for hundreds of thousands of hatchery chinook trucked from Lyons Ferry Hatchery in Southeast Washington to begin their migration. The investigators said the hatchery fish showed higher survivals than wild fish during 2003, likely due to their much larger size (100 mm long vs. 70 mm).

Though wild fish took about two more weeks to get to the dam, the scientists said the hatchery juveniles were still reasonable surrogates for wild fish because they also spent extended time rearing upstream of Lower Granite before they began their migration. And like their wild brethren, headed for sea primarily in the summer, increasing their rate of movement as they moved downstream. But are the dams killing the fish, or could it be a large population of hungry predators?

The study found very few of the tagged hatchery fish over-wintered to migrate to sea the following spring, only about one-tenth of a percent. Fish reared in the nearby and colder Clearwater River seem to show a much greater propensity to migrate as yearlings, which has severely complicated survival analyses based on adult returns. Scale sampling of returning adults from other research has shown about half of the Clearwater fish seem to over-winter in the river and migrate before detection systems are in operation.

These problems are addressed in the latest version of a NOAA Fisheries technical memo on dam effects released Nov. 5, which points out the difficulties associated with measuring the survival of the Snake fall chinook. The memo notes that survival of both wild and hatchery fish has "varied widely among years, and within years, with survival declining as the season progressed, flows decreased and temperature increased."

The NOAA scientists concluded that current data couldn't be used to estimate hydro-related mortality through the reservoir above Lower Granite Dam since the fish were both rearing and migrating in that part of the river.

However, the water temperature factor seems to be getting more attention these days as an important element in the juvenile fish survival equation.

Another part of the BPA-sponsored study reported on a tagging effort from 1999 to 2002 that measured fall chinook survival between McNary and John Day dams in the mainstem Columbia. For two of the three years, it found that greater flows were related to lower fish survival, but in 2002 when the range of summer flows was larger, the scientists found a positive, "significant" correlation with flow. When they examined the years together, they said, on average, fish survival increased about 1.5 percent for each increase of 10 kcfs in flow.

But they also found that fish survivals were nearly constant when water temperatures were below 19.3 degrees C (about 80 percent), while above 20.6 degrees C fish survival decreased to the 60-percent range.

The researchers said they found similar correlations between flow, temperature, travel time, and survival as they had already seen in the Snake River, but were unable to unscramble effects of flow and temperature. But they did note that other research in the Sacramento River has found that when the water gets above 20 degrees C, fall chinook fry didn't grow as fast and were more likely to be eaten by predators.

Eat Or Be Eaten

The predator question was one topic in the flow/survival rubric that was mentioned at a regional symposium last week in Portland where University of Washington scientists from Columbia Basin Research explained their new model which attempts to unravel the interactions of fish and flows.

In a paper now being reviewed for publication that was distributed at the gathering, CBR head James Anderson and associate Chris Van Holmes say their analysis, which was supported with funding from BPA, presents a consistent picture that contradicts the prevailing beliefs in the benefits of flow that have held sway for the last 30 years.

Using thousands of survival estimates generated from millions of tagged fish, they tested the current survival models and concluded "that the benefits of natural flow variations on fish is five to 20 times less than previously believed."

The U.W. researchers maintain that the missing ingredient in other models used by federal scientists is water temperature and the impact it has on the eating habits of salmon predators like northern pikeminnow, who consume a lot more small salmon and steelhead when waters get warmer. They cite a 1991 study that showed pikeminnow were satiated by a third of a smolt at 10 degrees C, but at 20 degrees C the predators had to consume nearly three smolts to get full. Other research showed that the metabolism of smallmouth bass, another common predator in the hydro system, triples when temperatures go from 10 to 20 degrees C.

Anderson says the impacts of flow augmentation and water withdrawal (irrigation) on water velocity are insignificant compared to the effects of temperatures and predators. Major flow augmentation might reduce the travel time of smolts through the hydro system less than 5 percent, but adding water to the lower Snake River from Idaho reservoirs could actually decrease fish survival by boosting temperatures. On the other hand, adding cooler water from Dworshak Reservoir might improve juvenile survival of fall chinook in the Snake, but may slow rearing of smolts and contribute to the over-wintering of many fall chinook from the Clearwater River, as suspected by USFWS researcher Billy Conner.

The U.W. modelers have already published a paper that describes their theory of predator-prey interactions, which is actually based on another theory "originally developed to predict collision rates of molecules."

Bill Rudolph
Fish Researchers Still Looking for Answers to Flow-Survival Conunudrum
NW Fishletter, November 18, 2004

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