[env-trinity] Study Finds High Rate of Juvenile Steelhead Mortality in Rivers' Estuaries

Sari Sommarstrom sari at sisqtel.net
Fri Feb 12 15:41:37 PST 2010

Does any tagging, similar to the study below, occur of steelhead 
juveniles from the Klamath and Trinity in order to track survival 
through the Klamath estuary?

Weekly Fish and Wildlife News
February 12, 2010
Issue No. 519

Study Finds High Rate of Juvenile Steelhead Mortality in Rivers' Estuaries

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 said, "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 saltwater 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 said.

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 
saltwater. 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 

"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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