[3] Juvenile Fish Survival Suffers Big Drop In Lower Columbia

Overall survival of PIT-tagged yearling chinook from the Snake River to below Bonneville Dam dropped to only 42 percent this year, down from 60 percent in 2007. That's in spite of the court-ordered spill regime started in 2005 that BiOp parties agreed to implement again this year.

The PIT tag survival data created another head-scratcher for researchers. Snake steelhead survival was even higher than the chinook this year, at 46 percent, something very unusual. In recent years, the steelhead survival rate has sometimes been around 50 percent below the chinooks'.

With survival past the four lower Snake projects similar to that of the past several years, it looks likely that the large losses may have occurred in the lower mainstem Columbia.

The problem seems to be in the last leg of the young fishes' trip through the hydro system, from John Day and past The Dalles and Bonneville dams.

If the preliminary numbers are correct, only 50 percent made it down the three-dam stretch alive. Last year, it was more than 82 percent, and in 2006, more than 96 percent.

The PIT tag survival data created another head-scratcher for researchers, since it doesn't show that there was much fish loss between McNary and John Day dams, when survival over the past three years has ranged from 77 to 92 percent. In fact, the data show survival was higher than 100 percent.

That's a red flag for the scientists, who say it could mean one or more of the study's assumptions have been violated in their single release-recapture model.

A NMFS Sept. 8 memo on the preliminary survival estimates speculates that debris on turbine screens may have played a role in decreased survival. Dam operators removed the screens in late May to reduce fish injuries, a time when the proportion of water passing through turbines increased due to greater flows.

The memo also pointed to anecdotal evidence that more gulls than normal were seen in the tailraces of John Day and The Dalles dams, which could mean higher predation. The new temporary spillway weirs at John Day "are suspected to have altered the hydrodynamics in the tailrace and created an upwelling in the center of the spillway downstream of the avian predation barriers. Predation by gulls was concentrated in that zone."

Also noted was a change in hydrodynamics that could have created areas of increased predation by other fish. The memo said it was possible higher predation also occurred at The Dalles Dam and in the tailrace of John Day.

But the other part of the puzzle--higher survival than possible from McNary to John Day--may not be as hard to explain as one might think. Such a state of affairs was also seen in PIT-tagged steelhead from the Snake and chinook from the upper Columbia.

The memo says the observed pattern "is consistent with the occurrence of differential mortality downstream of John Day Dam and those not detected . . . This would occur if fish leaving the juvenile bypass facility (where PIT-tag detectors are located), were more likely to pass into zones of increased predation than were non-bypassed fish."

The federal scientists say it's possible that the hydrodynamics in the dam's tailrace could cause such a differential mortality. If that's the case, they say "the result would be an underestimate of the detection probability at John Day Dam, and a resultant overestimate of the survival probability from McNary Dam to John Day Dam."

The memo also said that the survival estimate could have been further biased downward by fish losses from descaling of detected fish by debris on turbine screens at Bonneville Dam, while fish that passed the dam via the spillway (non-detected) would have fared better.

A clearer picture of the situation may emerge later this year when results of a study of fish survival from the tailrace of John Day Dam to a mile downstream will be presented at the Corps of Engineers' annual research review.

Also, examination of acoustic-tagged fish should be completed by then, which could shed further light on the lower river mortality issue. -B. R.

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