Arctic Science Journeys
Radio Script
mitch tracking salmon
Fisheries biologist Mitch Osborne operates the high-tech sonar used to count fall chum salmon migrating up the Chandalar River, a tributary of the Yukon River in Interior Alaska. Biologists and technicians with the U.S. Fish and Wildlife Service have documented a decline in salmon returns to the river over the past eight years.

Counting Chandalar Salmon

INTRO: Subsistence fishing for fall chum salmon opened this week on the Chandalar River, a tributary of the Yukon River north of Fairbanks. Fisheries managers opened the season after 74,000 fall chums passed a sonar counting station operated by the U.S. Fish and Wildlife Service. The opening is sure to help the Native villages of Venetie and Fort Yukon, two rural communities that depend on subsistence harvests of fish and game. Recently, Arctic Science Journeys Radio reporter Doug Schneider visited the Chandalar River sonar station to learn more about how biologists count migrating salmon.

STORY: The Chandalar River is the color of green tea as it makes its way out of Alaska’s Brooks Range north of Fairbanks, past the Gwich’in Native village of Venetie, and into the broad 11 million acre floodplain of the Yukon Flats National Wildlife Refuge.

A few miles downstream from the village, the river narrows. Stands of birch and aspen trees line the river’s edge, their leaves yellow and gold with the approach of winter. A small oval-shaped tent with a plywood floor sits tucked into the trees on a low sandy riverbank.

A generator hums nearby, supplying power to a series of computers and lights inside the tent, and to electric cables that snake over the riverbank into the river. Next to the cables, a wire fence about 20 feet long juts into the slow-moving current.

Just downstream from the fence, an occasional chum salmon rolls on the surface, making a half-hearted splash. Clearly, these salmon are tired. Each fall, thousands of chum salmon return from the Bering Sea some 1,200 miles away. They swim up the Yukon River to tributaries like the Chandalar, Porcupine, Sheenjek, and Tanana rivers to spawn and then die. With just a few miles remaining in their journey, these salmon swim slowly around the fence, and then pass unknowingly through an invisible beam of sonar.

  measuring salmon
  Chum salmon, like this one being measured by fisheries biologists Jeff Melegari and Mitch Osborne, swim some 1,200 miles from the Bering Sea to spawn.

OSBORNE: “So we have a sounder that’s sending out a signal to the transducer.“

Mitch Osborne is a fisheries biologist with the United States Fish and Wildlife Service. Each fall, he and a team of technicians come to this pristine spot in the wilderness to set up the Chandalar River sonar station and count returning salmon.

OSBORNE: “The sounder is talking to the processor, collecting the echoes back and displaying it in real-time.”

The sonar sits on the river bottom, projecting a beam of sound across the river. We hear the sound as a series of pings. The sonar bounces off everything within a cross-section of the river, from sticks and logs to migrating salmon. A computer screen displays each ping as a distinct dot.

Technician Todd Johnson has the tough job of figuring out which dots are actually migrating salmon, and which are merely logs, leaves, and other debris.

Fall Chum Salmon Returns to the Chandalar River, Alaska 1995-2002chum salmon chart
  Source: Alaska Fisheries Technical Report Number 61: “Use of Split-beam Sonar to Enumerate Chandalar River Fall Chum Salmon, 2000,” United States Fish and Wildlife Service, April 2002; and Mitch Osborne, fisheries biologist, USFWS, Alaska Region 7. Year 2002 data are projected estimates.

JOHNSON: “After a while you can see a pattern. It’s kind of like a big connect-the-dots.”

Look at the screen long enough and patterns emerge. Horizontal lines of pings, called a trace, stand out amid the clutter. Special software highlights which ping came first and which was last--in other words, what direction the object was moving. Mitch Osborne says that if the object is moving upstream, against the current, it’s a salmon.

Of course there’s often a lot more to it, such as when a salmon swims part way through the beam and drifts back downstream, or when they swim so tightly packed through the beam that distinguishing individual salmon becomes difficult.

OSBORNE: “You’re actually looking at it in a couple of different levels, and using some detective work to define that trace as a fish.”

This year, though, counting the Chandalar’s returning salmon hasn’t involved much detective work. That’s largely because there haven’t been many salmon to count. Salmon returns to the Chandalar and other Yukon tributaries have declined significantly. From 1995 to 1997, the Chandalar’s average return was around 230,000 fall chums. In 1998 and 1999, however, the average run declined to 82,000 chum salmon. Last year, just 66,000 fall chums showed up. The low returns have meant commercial and subsistence fishing closures this summer. As recently as two weeks ago, the Chandalar run seemed destined to post yet another decline.

But just in the last few days, a flurry of fish swam past the sonar. Within days, biologists had counted enough salmon to satisfy the river’s goal of 74,000 returning chum salmon. The additional salmon has allowed managers to open subsistence fishing on the Chandalar.

Meanwhile, fall chum returns to the other major Yukon River tributaries have remained low. While there are lots of theories, there are no hard answers as to why Yukon River salmon runs have declined. Mitch Osborne says learning more about Yukon River salmon is reason enough for his annual pilgrimage to the Chandalar River.

  scuba diver
  Placing the sonar transducer on the bottom of the icy-cold Chandalar River requires a scuba diver. Fisheries biologist Mitch Osborne is also a certified diver.

OUTRO: If you’d like to learn more about Yukon River salmon research, come to our Web site at This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program at the University of Alaska Fairbanks School of Fisheries and Ocean Sciences. I’m Doug Schneider.

Audio version and related Web sites (sidebar at top right)
Thanks to the following individual for help preparing this script:

Mitch Osborne, Fishery Biologist
United States Fish and Wildlife Service
101 12th Avenue, Box 17, Room 222
Fairbanks, AK 99701
Phone: 907-456 0291

Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks. The shortcut to our ASJ news home page is

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On the Web:

U.S. Fish and Wildlife Service, Alaska Region

U.S. Fish and Wildlife Service, Fisheries Office, Fairbanks, Alaska

U.S. Fish and Wildlife Service, Chandalar River Sonar Project

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