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Pit-Tags Show Snake River Steelhead In-River Survival Lags

by Barry Espenson
Columbia Basin Bulletin - November 22, 2002

Survival estimates for yearling chinook salmon and juvenile steelhead migrating in-river through the Columbia-Snake River hydrosystem rebounded this year from the precipitously low levels estimated during last year's drought.

But trouble spots remain, particularly for steelhead, whose in-river survival rate has charted a steady decline, according to estimates compiled by National Marine Fisheries Service scientists in an ongoing study that began in 1993.

The survival of steelhead juveniles this year from the head of Lower Granite Dam's reservoir to the Bonneville Dam tailrace was an estimated 26.7 percent. That's much better than the 3.8 percent survival estimated for last year's in-river steelhead outmigrants. But the 2002 survival through the system was lower than in other recent years.

System survival for the PIT-tagged yearling chinook was estimated at 50.4 percent this year, as compared to 26.4 percent last year. The chinook survival was generally equal to or greater than previous years of the study, with the exception of 2001.

In 2001 the Columbia Basin experienced the effects of a prolonged drought. The second lowest runoff on record made for fatal migration conditions -- feeble, warm flows. The NMFS study charts the progress of Snake River yearling and steelhead from upriver hatcheries, and from the head of Lower Granite reservoir, to Bonneville. Lower Granite is the eighth dam in the system and the first the young Snake River fish pass on their way to the Pacific Ocean. Bonneville is the last dam the fish pass before hitting saltwater.

Generally, except for survival dips during very low water years of 1973, 1977 and 2001, the survival rates of Snake River juvenile salmonids have rebounded after falling off drastically after the last of four dams -- Lower Granite, Little Goose, Lower Monumental and Ice Harbor -- were built on the lower Snake River late 1970s.

System survival rates that had been over 50 percent for steelhead and over 45 percent for chinook in the mid-1960s plummeted to lows of less than 10 percent in the late 1970s. Now, with elaborate juvenile passage devices at work and dams operated to accommodate fish passage and migrations, survival rates through eight dams are about on par with those of the 1960s when the in-river migrants had to pass only four dams, NMFS' Bill Muir said.

"With all the improvements (in the hydrosystem), survival has improved," Muir said.

He presented the study's 2002 Columbia Basin Reach Survival estimates during this week's during the Corps of Engineers' Anadromous Fish Evaluation Program annual review. During the four-day session, researchers from across the basin briefed fellow researchers on the most recent findings from estuary, transportation and delayed mortality, bypass and fish passage studies.

The study detailed by Muir estimates survival for individual reaches -- from dam to dam -- as well as through the entire system. The research involves tracking PIT-tagging fish. Their progress downriver is charted via detectors at Lower Granite, Little Goose and Lower Monumental dams on the Snake and McNary, John Day and Bonneville dams on the Columbia and detectors on trawlers in the lower Columbia.

The in-river survival for Snake River steelhead through the entire hydrosystem, however, has fallen in each year of the study since 1995 -- from more than 50 percent that year to 2002's 26.7 percent survival. The only exception when the rate climbed from 2001's drought-caused low. The 2002 rate was, however, slightly lower than the 2000 rate.

Both yearling chinook and steelhead fared well from the tailrace of Lower Granite to the next dam downriver, Little Goose (90.1 and 90.3 percent respectively) and from Little Goose to Lower Monumental (97.4 and 91.2 percent).

And 81.6 of the yearling chinook detected at Lower Monumental survived through Ice Harbor and McNary dams to the McNary tailrace. But only 65 percent of the steelhead survived that leg of the journey. Some of the damage was done by avian predators such as cormorants, Caspian terns, gulls and pelicans.

"It certainly accounts for a big chunk of it," Muir said of the steelhead mortality in those reaches of the river. Higher than expected mortality was also experienced at Ice Harbor, with newly installed flow deflectors or "flip lips" in spill bays suspected as a cause.

Birds along the reach are estimated to have eaten at least 9.7 percent of the PIT-tagged steelhead and 1.5 percent of the yearling chinook that had been detected passing Lower Monumental Dam, Muir said. The estimates are minimums since not all of the PIT-tags from fish that have fallen prey to birds can be recovered. A Caspian tern colony on Crescent Island --which numbered 580 breeding pairs this year -- took the largest share of those juvenile fish. The island is located just below the Snake's confluence with the Columbia, between Ice Harbor and McNary.

A NMFS predation study presented by Brad Ryan says that researchers have identified a fairly consistent prey pecking order, particularly for terns.

"We have found steelhead to be more vulnerable than coho salmon, and coho to be more vulnerable than yearling chinook salmon," according to the study's abstract. "We have also found hatchery chinook salmon to be more vulnerable than wild chinook salmon. However, we have found no preference for hatchery over wild steelhead. In addition, we have found that transported salmonids are generally less or equally vulnerable to avian predation than their in-river counterparts."

This year the migrating juvenile fish survived equally well as previous years (except 2001) between the Columbia's McNary and John Day dam tailraces (90.7 percent for chinook and 84.4 percent for steelhead).

But the trip from John Day through The Dalles and Bonneville reservoirs and pools took a heavy toll on steelhead with only 61.2 percent surviving this year. That was the only segment of the trip in which steelhead survival was actually lower than last year, which was nearly 75 percent. The chinook survival was 84 percent during that leg of the trip this year -- better than 2001 and 2000 and comparable with 1999.

The impact of declining steelhead in-river survival has been counteracted by a force of nature. Those that do make it out of the freshwater are surviving in greater numbers because of favorable ocean conditions. Smolt-to-adult returns to Lower Granite have been on the rise since 1997, coincident with the shift from less favorable ocean feeding conditions.

Muir cautioned observers, however, saying that had in-river survival improved, the adult returns would have been even greater. Also, the cyclical nature of the ocean habitat will, almost certainly, take a turn for the worse.

Only an average of from 5 to 15 percent of the steelhead that reach Lower Granite attempt to complete the outmigration in-river. The balance are guided onto barges at Lower Granite, Little Goose and Lower Monumental dams on the lower Snake River and at McNary Dam on the mid-Columbia and transported for release below Bonneville dam.

Even fewer migrated in river in 2001 when transportation efforts were maximized to pull fish from the unfriendly river environment. The drought came amidst an energy crisis in which many federal hydrosystem operations were curtailed to all generation of more hydropower.

In a separate NMFS presentation at the research review, estimates have been showing that the transported Snake River fish are returning to Lower Granite at a higher rate than in-river fish. The preliminary "transportation-to-in-river" migrant return rate for hatchery chinook released as yearlings in 1999 was 1.3 while the return rate for wild chinook was 1.8 transported fish for every juvenile in-river migrant.

Statistics related by NMFS Douglas Marsh indicated that adult returns of wild yearlings released in 2000 indicated a preliminary T/I ratio of 1.2.

The return rate for steelhead seem to show a stronger benefit from barging, according to NMFS' "Evaluation of Juvenile Salmonnid Transportation, 2002." The preliminary T/I rate for hatchery fish released in 1999 was 1.4 while the preliminary transportation-to-in-river return rate for wild steelhead released that year was 2.6. The preliminary T/I rate for wild steelhead release at Lower Granite in 2000 was 1.9.

Related Sites:
U.S. Army Corps of Engineers: AFEP

Barry Espenson
Pit-Tags Show Snake River Steelhead In-River Survival Lags
Columbia Basin Bulletin, November 22, 2002

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