Researchers Surprised By High Death Rates For Steelhead
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For a young steelhead on its way to the ocean, river estuaries are the last stop – in more ways than one.
Research from Oregon State University shows that about half of migrating steelhead never make it the ocean. Instead they get as far as the estuary at the river’s end and abruptly die. That came as a surprise to OSU researchers who are trying to figure out if that’s a typical mortality rate, or if something new is killing steelhead in larger than normal numbers.
Professors David Noakes and Carl Schreck inserted small ultrasonic transmitters into 280 juvenile steelhead over a two-year period. They were released in the middle sections of the Alsea and Nehalem rivers, then tracked as they swam to the ocean.
90% of the fish made it all the way to the estuary. But once they got there, transmitter signals from 50% – 60% of the fish suddenly stopped.
One reason is predators. Earlier studies on the Nehalem River followed the signals to a cormorant rookery and to a growing population of seals. Seals, sea lions and sea birds are all major predators of juvenile steelhead.
Another thought is that the steelhead are having trouble making the adjustment from fresh to salt water. A high mortality rate might be normal, a way to weed out the weaker fish who can’t adapt.
“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.,” says Noakes. “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.”
The men are looking at possible environmental causes that could lead to higher than normal death rates. Warmer water, more parasites, chemical contaminants, or higher acidification of ocean waters coming into the estuary are possible factors.
“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,” says Noakes.
Noakes and Schreck hope their ongoing research will help fishery managers get better information. “It’s hard to predict adult returns if you don’t have good data on outgoing juveniles,” Schreck says, “and this study is an effort to make that monitoring more precise.”
The research is based at the Oregon Hatchery Research Center located on Fall Creek near the community of Alsea.
