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Snake Fall Chinook Studies
Get More Complicated

by Bill Rudolph
NW Fishletter, December 20, 2006

Scientists studying fall chinook from the Snake River have uncovered another new wrinkle in the various life-styles of that particular population, one that promises to make them even harder to study. In the past few years, researchers have found that many of the juvenile fall chinook overwinter in reservoirs, rather than migrate downstream by either barge or under their own power as subyearlings a few months after they hatch. But now scientists have found out that even barging subyearlings past the dams doesn't guarantee that the fish will make it to the ocean that same year.

By sampling scales and reading growth patterns like tree rings, they have found that a good portion of returning adults hung out in the Columbia River below Bonneville Dam over the winter after being barged below the dams.

That was one of the messages delivered last week at the December meeting of the Northwest Power and Conservation Council, when federal researchers presented their latest findings. USFWS biologist Billy Connor was there again to explain the growing complexity, with some fish migrating as subyearlings, others later in the fall after spill at dams is stopped, and still others (yearlings) holding over in one reservoir or another for the entire winter, and heading to sea the following spring before April when PIT tag detection systems are turned on. Conner's data showed that yearling fall chinook migrants are making up a growing percentage of overall returns. From 2002 to 2004, it doubled from 30 percent to 60 percent.

It's a phenomenon that seems to have grown as a consequence of effort to boost the ESA-listed fall chinook numbers by a supplementation program using fall chinook from Lyon's Ferry Hatchery, with another effort to develop a spawning population in the Clearwater River, which now accounts for about 30 percent of the fall chinook redds for the entire Snake population.

Scientists think the colder water in the Clearwater keeps many of the juvenile chinook there from reaching a size large enough to migrate as subyearlings, unlike most fish from the Snake. So they hold back and migrate later in the fall or overwinter, and head downstream in early spring, much larger than the typical subyearling.

Connor presented data that showed fall chinook transported in the summer from 2001-2004 made up about 25 percent of returning adults, but more than a third of those fish held over in the estuary and entered the ocean as yearlings. Nearly half of the fall chinook transported later in the year stayed in the estuary and made up about 30 percent of the adults sampled.

More than 40 percent of the sampled returns were made up of inriver migrants, but about three-quarters of that group didn't enter the ocean until they were yearlings.

Overall, Connor said yearling migrants made up 73 percent of the adults sampled from those three years. So where do they hang out before they finally head off to sea?

Just about everywhere in the hydro system, the scientists said, who had a hard time pinpointing their whereabouts because PIT-tag detection systems are turned off at dams in the wintertime because pipes might freeze. Researchers have had to use everything from pole and line to miniscule radio tags to track them down.

With the high-tech radio tags in Clearwater fall chinook, they found most of those fish don't travel very far during the summer when spill is added at dams on the lower Snake. In fact, most don't get as far as the first dam. About 80 percent of the Clearwater fish aren't even detected at the mouth of that river before fall sets in, still miles from the dam. About a third of the yearlings sampled as adults were never detected at all when they migrated through the hydro system.

NOAA Fisheries researcher Bill Muir explained why he thinks the larger, later-migrating fish have a survival advantage.

He said over 90 percent of the mainstem pikeminnow, a main predator of young salmon, were large enough to eat natural subyearling Snake fall chinook that had been transported, while only about 30 percent of the pikeminnow were big enough to eat late-migrating smolts from the Clearwater, and only about 20 percent of the pikeminnow were big enough to consume fall chinook that migrated as yearlings.

Once in the ocean, Muir said all of the transported subyearlings were still small enough to be vulnerable to predation by Pacific hake, which school in large masses off the West Coast during summer months, in the millions of tons. But, according to Muir, inriver migrating subyearlings grow large enough as they pass down the Columbia so that most hake have to pass them by.

By the time they make it to the ocean, both the late-migrating Clearwater fish and yearling migrators are too big for the hake to take. "I think this goes towards explaining why we can load fish on a barge," said Muir, "and get around the substantial mortality that occurs for dam passage by going through the entire hydropower system, which is very substantial for fall chinook salmon compared to spring migrants, but yet we don't necessarily see a great benefit from transport when adults return."

Muir said return rates for transported and inriver-bypassed fall chinook were nearly identical, while the smolt-to-adult return rate for the yearlings was nearly three times better (1.35 percent compared to transported at .51 percent, or bypassed at .56 percent).

The complicated life histories of the Snake fall chinook has raised questions about the value of court-ordered summer spill at lower Snake and McNary dams and whether it is improving survival. Before Judge Redden's 2005 order, spill was stopped altogether at four of the dams to maximize barging smolts.

However, scientists like Muir say that to estimate the real value of barging fall chinook, any new study must account for the life history diversity of the population, or management decisions could be made counter to recovery. The researchers said that they are designing more new research to study the effects of spill. For now, they said the spill probably has a positive effect on Snake-origin fish that migrate in the summer, but not for Clearwater fish that generally migrate later in the fall or hold over in one of the reservoirs until spring.

However, it may be tough to get a handle on the situation anytime soon. Muir said returns to Lyons Ferry Hatchery were less than anticipated, which may make it difficult to provide enough smolts for next year's research needs.

Meanwhile, annual returns of wild fall chinook have grown into the thousands from the low of 79 fish in 1990, when only 78 wild fall chinook were estimated to have returned to Idaho. Their numbers had declined steadily since the 1960s, after three-fourths of their habitat was blocked by construction of Idaho Power's Hells Canyon Complex.

Connor told NW Fishletter that members of the technical recovery team dealing with interior Columbia ESA stocks, told him that the Snake fall chinook had the best shot at recovery of any ESUs they are dealing with.

Bill Rudolph
Snake Fall Chinook Studies Get More Complicated
NW Fishletter, December 20, 2006

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