About 10 percent more sockeye salmon from the Sawtooth Hatchery vanish shortly after their release compared to fish from the Oxbow Hatchery, a new radio telemetry study by the National Oceanic and Atmospheric Administration shows.
Predatory birds and bull trout probably ate the sockeye just after release, researchers said. Fish from both hatcheries are transported to release sites below Redfish Lake but the Sawtooth fish were released during the day while the Oxbow sockeye were released at night.
Idaho Department of Fish and Game biologists at 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,” Mike Peterson, the Idaho Fish and Game senior research fisheries biologist for the sockeye program said in a press release.
Gordon Axel of the Northwest Fisheries Science Center’s Pasco research station leads the team that has installed a network to follow the 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.
The researchers found the sockeye average 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 remain endangered.
Many young sockeye never make it 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 so they might avoid juvenile losses and increase adult returns. 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.
Axel and his team began tracking the juvenile sockeye in 2011 in collaboration with Fish and Game. The research has continued since and the team is back in the field this year. The team uses both 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.
Advances in batteries and electronics have shrunk the radio tags to so they measure just 12 millimeters long and weigh the same as a dime – letting researchers more safely implant the tags in smaller fish. In 2012, and again this year, the NOAA team positioned radio telemetry receivers at 21 sites along the Salmon and Snake rivers, partitioning the river to identify areas of increased fish mortality. The team also used mobile receivers on backpacks to track individual fish even more closely.