Northwest Fisheries Science Center

The case of the missing sockeye

A team of NOAA scientists is using an advanced network of receivers along the Salmon River in Central Idaho to detect where and why young sockeye salmon are disappearing on their way to the ocean. The initial results have prompted new measures to boost survival of this endangered species.

The research is led by Gordon Axel of the Northwest Fisheries Science Center's Pasco research station. Electronics technicians at the Center worked with a company that specializes in radio-frequency engineering to develop a system sensitive enough to track sockeye smolts a few inches long through the rugged Idaho mountains.

"The telemetry system provides excellent sensitivity and range while simultaneously scanning nine different frequencies for fish that are passing each fixed site," Axel says. "This is important because we're finding that juvenile sockeye waste little time in the Salmon River, averaging around seven days to travel 460 miles from Redfish Lake Creek to Lower Granite Dam." Snake River sockeye are among the rarest of Pacific salmon and were long considered to be on the threshold of extinction. But a hatchery program that began in the 1990s as a last-ditch effort to save the species carefully bred the few remaining fish to rebuild the population.

Idaho hatcheries now release hundreds of thousands of young sockeye annually to swim to the ocean. Some of those eventually return as adults to their historic spawning grounds in Redfish Lake, encircled by the peaks of Idaho's Sawtooth Range. The goal is to push returns higher.

Where did the fish go?

But biologists spotted a problem: Many young sockeye never make it through the first 460 miles down the Salmon and Snake rivers to Lower Granite Dam, the first dam they pass on their nearly 900-mile migration to the ocean. Tiny passive integrated transponder (PIT) tags implanted in the fish at the hatchery are detected at the dam, so researchers can tell that in some years more than half of the fish disappear before they get there.

Biologists want to know why. Predators? Conditions in the river? Answers might reveal ways to avoid juvenile losses and increase adult returns. The questions are especially timely because the Bonneville Power Administration is funding construction of a new sockeye hatchery in southeast Idaho, near Pocatello, that will raise as many as a million sockeye smolts for release each year.

Two major policy actions called for tracking sockeye to unravel the answers: NOAA's 2008 biological opinion outlining mitigation for the impacts of federal dams on the Columbia and Snake rivers, and the Adaptive Management Implementation Plan added to the BiOp in 2009. BPA funds the research.

Axel and his team began tracking the juvenile sockeye in 2011 in collaboration with the Idaho Department of Fish and Game. The research has continued since and the team is back in the field this year. The team uses both passive PIT tags, which emit signals only when they pass detectors such as those at dams, and small radio telemetry tags that actively broadcast coded signals identifying each fish as it swims downriver.

While the PIT tags document overall survival to the dam, the radio tags let researchers more closely follow the fish to tell how fast they travel and what happens to them. Advances in batteries and electronics have shrunk the radio tags to so they measure just 12 millimeters long and weigh only 0.7 g - about a third the weight of a dime - letting researchers more safely implant the tags in smaller fish.

Technology improves tracking

In 2012, and again this year, the NOAA team positioned radio telemetry receivers at 21 sites along the Salmon and Snake rivers, partitioning the 460 miles of river to identify areas of increased fish mortality. The sensitive receivers are finely tuned to reduce interference and can detect signals on nine different channels, instantly determining which tag and which fish they are picking up. Solar panels power the detection sites, recharging batteries during the day to keep the receivers running through the night.

The team also used mobile receivers on backpacks to track individual fish even more closely.

One of their first findings last year was that about 10 percent more fish from the Sawtooth Hatchery vanished shortly after their release compared to fish from the Oxbow Hatchery. (Fish from both hatcheries are transported to release sites below Redfish Lake.) The researchers determined based on the location that predatory birds and bull trout probably ate the sockeye just after release.

They suspected that the daytime release of the Sawtooth Hatchery fish exposed them to predators more than fish from the Oxbow Hatchery, which were released at night. The Sawtooth hatchery will adjust the timing of its releases this year to assess whether that helps cut the losses.

"We want to identify areas or hot spots where mortality might be occurring and tell whether there's anything we can do to increase survival," said Mike Peterson, the Idaho Fish and Game senior research fisheries biologist for the sockeye program. The researchers are tracking another generation of young sockeye this year to further measure how survival varies according to different river levels and conditions such as turbidity and temperature, which may yield additional clues that help further increase survival.

"What's really fascinating about the radio telemetry study is that it gives researchers information immediately on where the juvenile sockeye are in the study area, how quickly they are moving downstream and their survival throughout the various reaches," said Jonathan McCloud, BPA's project manager for the research. "What we learn can be applied to release strategies, increasing the probability of survival to aid adult returns back to the Redfish Lake."

FSV Bell M. Shimada. View slideshow A fixed telemetry site located at Chief Timothy State Park on the Snake River (L-R Scott Davidson, Jesse Lamb, and Sam Rambo).