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Study: Strays Dominate Snake River's Trapped Natural-Origin Sockeye by K.C. Mehaffey Northwest Fishletter, September 7, 2021

This year, a total of 630 sockeye passed Lower Granite Dam and about one-third were trapped
while two-thirds of them remained in the river to continue their arduous journey.

After trapping 201 sockeye at the Snake River's Lower Granite Dam and hauling them to the Eagle Fish Hatchery in July, Idaho Department of Fish and Game scientists have now genotyped the 28 fish with an intact adipose fin and discovered that 16 of them -- more than half -- were Columbia River strays.

This year was the second time the agency turned to an emergency trap-and-haul program. The first was in 2015, when a similar drought and warm-water conditions prompted the effort to help some of the fish on the last leg of their journey.

Genotyping the 28 fish with intact adipose fins is the first step in IDFG's process designed to preserve diverse genetics of the endangered Snake River sockeye, John Powell, a fisheries research scientist at the department's Eagle Fish Hatchery's genetics lab told NW Fishletter. The process will continue as scientists analyze the hatchery sockeye that were trapped and hauled, and some of the 429 sockeye that passed Lower Granite Dam that remained in the river and are able to make the 900-mile journey all the way to the Sawtooth Basin.

Just one of the 28 trapped sockeye had unidentified parentage from Redfish Lake's wild spawning population. Its parents could not be identified through IDFG's captive broodstock program. Eight others were from sockeye with known parents from prior broodstock that had been released as adults into Redfish and Pettit lakes to spawn naturally.

Three of the 28 fish were actually hatchery sockeye whose adipose fins had not gotten clipped. The rest of the 201 trapped sockeye had clipped adipose fins, identifying them as hatchery sockeye.

"Right now, we're trying to figure out which fish go where and a lot of that depends on how related they are to other fish in the program," Powell said.

Once they've figured it out, biologists will release some of those adults into the two lakes to spawn naturally, and keep some to raise in hatcheries, using their genetic information to determine how best to preserve diversity for future generations.

IDFG's captive broodstock program began after the run was listed as endangered in 1991.

From 1991 until 1998, biologists captured the 16 adults that returned those years to use as a foundation of their broodstock program. They also incorporated freshwater sockeye -- also known as kokanee -- which do not migrate out of the lake, but which spawn with the returning sockeye. The program also trapped juvenile sockeye leaving the lake and reared them to adults to develop the broodstock.

This year, a total of 630 sockeye passed Lower Granite Dam and about one-third were trapped while two-thirds of them remained in the river to continue their arduous journey. At least five sockeye have made the entire trip in the river to the Sawtooth Basin, Powell said. That's less than the mid-August return most years, he said, but returns can continue into October, when spawning begins. "Right now, we're in the middle of when we trap most of our [in-river] fish. A lot will show up in August to the end of September, and it slows down as you get later into September and October," he said.

It's hard to know what to expect from the sockeye that were left to migrate in the river, he said. In an average year, about half of the sockeye passing Lower Granite Dam make it the remainder of the way. But lower-than-average flows combined with extreme heat waves caused water temperatures to rise above 68 degrees Fahrenheit in the Snake River and its tributaries multiple times and left sockeye with harsh migrating conditions. Anything above 68 F can be lethal for fish.

"It's amazing to me that, given the conditions, the fish are still making it. It's a question of how many are going to make it, but it's really nice to see some are able to make the journey," he said.

IDFG turned to trap and haul as an emergency measure, since the in-river journey this year is so perilous. Their efforts were deemed successful. "At this point, we were able to trap and haul enough fish at Lower Granite Dam that we're above our 10 percent anadromous contribution threshold for the broodstock," Powell said.

He said one of the 16 sockeye identified as Columbia River stock perished. The other 15 were taken to the outdoor aquarium at the Morrison Knudson Nature Center in Boise so that people can see them. Unlike the Snake River sockeye, the Columbia River sockeye -- which return to lakes by way of the Wenatchee and Okanogan rivers -- are not listed under the Endangered Species Act.

Those sockeye will remain at the aquarium for a couple of months, until they attempt to spawn, and die, according to an IDFG press release.

Scientists don't want to use Columbia River sockeye in their captive broodstock program because their genetics will weaken traits specific to the Snake River population.

Powell said although more than half of the natural-origin sockeye trapped at Lower Granite Dam strayed from the Columbia River, he's not concerned about Snake River sockeye mixing with the Columbia River fish that made it past Lower Granite and were not trapped. "We do see Columbia River stocks making it up the Snake River, and have sampled them at Lower Granite, but we've never trapped a Columbia River sockeye all the way up the Sawtooth," he said.

Biologists decided not to trap sockeye for the captive broodstock program at Ice Harbor Dam -- the first dam on the Snake River -- because even more of them are likely to be strays from the Columbia River.

Powell noted that Ice Harbor and Lower Monumental dams often have similar window counts for sockeye, but there's a big drop off in those numbers by the time they get to Little Goose and Lower Granite dams. "Some of that might be that they're dipping into the Snake [River], and then realizing that's not where they need to go," he said.

Powell said it's no surprise that of the nine natural-origin Snake River sockeye that were trapped -- six from Redfish Lake and three from Pettit Lake -- it's no surprise that only one of them did not originate from IDFG's captive broodstock program. "Typically, most of our fish coming back we can assign two parents," he said, and very few had parents that can't be identified. "One out of six falls into the realm of what we normally see. It's not surprising to see that sort of ratio."

The sockeye that were trapped were in rough shape, but those that survived have bounced back, Powell said. The warm water resulted in sockeye with lesions that had secondary infections from common pathogens found in the water.

"We got them into colder water and they started doing a lot better," Powell said. Their wounds were treated and are healing, and they are now beginning to look like the classic sockeye, with that fire engine red body and a green head, he said.

Related Pages:
Count the Fish, 1977-2014, Salmon Recovery Effortsby GAO