CORVALLIS, Ore. - A new study by researchers at Oregon State University found that up to nearly half of the ocean-bound juvenile steelhead surveyed in two Oregon river systems appear to have died when they reached the estuaries - before they could reach the ocean.
The scientists aren't sure if such a mortality rate in the estuary is typical or elevated due to increased predation - most likely by marine mammals or seabirds. One goal of their research is to begin establishing better baseline data on juvenile salmon and steelhead mortality so resource managers can make more accurate predictions on runs of returning adult fish.
"A female steelhead may lay 2,000 to 5,000 eggs - and in rare cases, more than 10,000 eggs - and for the population to remain stable, at least 2-3 percent of the juveniles migrating to the ocean have to survive and return as adults," said Carl Schreck, a professor of fisheries and wildlife at OSU and leader of the Oregon Cooperative Fish and Wildlife Research Unit on campus. "If you get much more than that, it's a banner year.
"But it's hard to predict adult returns if you don't have good data on outgoing juveniles," Schreck added, "and this study is an effort to make that monitoring more precise."
Declining salmon and steelhead runs have been blamed on everything from habitat loss through logging to housing developments on coastal rivers, but the consensus has been that ocean conditions are perhaps the single most important element in how robust the populations may be in a given year. Yet the OSU study found that mortality is significant before the fish even make it to the Pacific Ocean, said David Noakes, a professor of fisheries and wildlife at OSU and one of the principal investigators in the study.
"Steelhead will live in the fresh water for one to two years and then migrate out to the ocean where they'll spend another two or three years," Noakes said. "If only 2-3 percent survive, it would be interesting to know what the keys to survival may be for the select few. Are the biggest juveniles more likely to survive? The fastest? Those that have the fewest parasites? Is there something in their genetics that better helps some of them adapt to the new salt water environment?
"We need to determine what the so-called 'normal' predation rates are in the estuary, and get a better handle on what is killing the fish," he added.
In their study, the OSU researchers inserted small ultrasonic transmitters into 280 juvenile steelhead over a two-year period. The dollar bill-sized fish were captured in traps at sites on the middle stretches of the Alsea and Nehalem river systems, tagged and measured, and then released back into the rivers and tracked on their way to the ocean. About nine out of 10 fish made it safely from the release point to tidewater, and then the ultrasound transmissions from 50 to 60 percent of those survivors abruptly stopped when they reached the estuary.
The scientists received enough signals from surviving fish to know that it wasn't a failure in signal transmission. And, Schreck says, during an earlier study using tags that broadcast a radio frequency, they recovered transmitters from a cormorant rookery near the mouth of the Nehalem River, and have tracked signals from the tags to a burgeoning seal population - also near the Nehalem's mouth..
"There are a lot of seals right near the mouths of both rivers and seals can eat a lot of young fish," Schreck said. "It's why the steelhead need thousands of eggs to keep the population going."
One other possible explanation for the high mortality, Noakes said, is that the young fish couldn't handle the transition from fresh to salt water. Salmon, steelhead and other "anadromous" fish have a complex life cycle and for centuries have utilized both the ocean and river systems. But a high mortality rate might be normal and a way to weed out weak fish that can't make the adaptation to a new environment.
"We know that fish need a number of things to trigger their migration to the ocean, including the amount of seasonal light, certain temperatures, enough water flow, etc.," Noakes said. "But we don't know why some fish remain in the river for one year before heading out to sea, and others stay for two years. Just preparing to go from fresh water to a salt water environment requires an enormous adjustment.
"There may be something about that adaptation that contributes to the mortality," he added.
If the mortality rate of juvenile steelhead is atypical, it could be increasing because of some environmental factor - warmer water, more parasites, chemical contaminants, or higher acidification of ocean waters coming into the estuary, for example.
Or predation may be higher because of more seals, sea lions and seabirds.
Much of the research about steelhead migration, spawning behavior and basic biology is emerging from studies done at the Oregon Hatchery Research Center, a joint venture between OSU's Department of Fisheries and Wildlife, and the Oregon Department of Fish and Wildlife. Located on Fall Creek, a tributary of the Alsea River, the research center is giving fish biologists unprecedented new looks at the physiology and behavior of steelhead.
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David Noakes, 541-737-1953