Endangered pallid sturgeon successfully spawns during Missouri River flow test

BILLINGS – For only the second time since pallid sturgeon were listed as an endangered species in 1990, scientists have confirmed a female fish successfully spawned in the Missouri River below Fort Peck Dam.

A sturgeon captured on July 31 was assessed by biologists, via ultrasound and other methods, to determine it had spawned, the U.S. Army Corps of Engineers announced in a news release.

The sturgeon was also confirmed to be one of the hundreds planted in the river in the past two decades after being born in a hatchery and transferred to the river, according to Pat Braaten, a research fish biologist at the Columbia Environmental Research Center’s Fort Peck Field Research Station.

The last, and only, documented instance of a pallid sturgeon spawning below Fort Peck Dam was in 2011.

Based on telemetry data, it’s believed the fish may have laid its eggs while swimming near Wolf Point. That’s more than 100 miles upstream from the head of Lake Sakakawea in North Dakota. Research has shown that if the sturgeon offspring, called larvae, have not matured enough by the time they reach the reservoir they probably die.

Flow tests

Verifying the successful spawning event is significant because this spring the Corps agreed to alter water releases from Fort Peck Dam to mimic natural runoff conditions in a first-of-its-kind test. High water is what entices pallid sturgeon to swim upstream and spawn. Dams, which moderate high flows as well as block spawning migrations, have been blamed for the precipitous decline of wild sturgeon.

After pallid eggs hatch, the warmer the water, the faster the larvae mature. Once mature, they settle to the bottom of the river to feed and grow.

“The ultimate measure of success will be if pallid sturgeon spawning produced babies that drift downriver, but not into Lake Sakakawea, and survive,” said Joe Bonneau, with the U.S. Army Corps of Engineers Missouri River Recovery Program. “What the outcome of that is is still unknown.”

Late spawn

The successful pallid sturgeon spawning event came much later than what is typically seen on the nearby Yellowstone River.

The Yellowstone joins the Missouri before flowing into Lake Sakakawea. Sturgeon, as well as other fish, move up and down both rivers. But on the Yellowstone River spawning usually occurs between June 25 and sometime around the first of July, Braaten said.

“One female caught last week below the confluence of the Yellowstone River hadn’t spawned yet,” he added.

That the hatchery fish spawned so much later may be a result of colder water coming from the bottom of Fort Peck Reservoir, Bonneau speculated.

One other reason is the fish’s heritage.

“It’s not so unusual for hatchery fish to maybe not spawn at all on their first or second cycles,” Braaten said, calling it a “hormonal dry run.”

Female pallid sturgeon do not spawn until they are about 20 years old. They also spawn only once every three years. For reasons like this, wild populations of the fish have steadily dwindled after dams blocked their traditional spawning runs.

“They’re hanging on. They live a long time,” Bonneau said. “But not forever.”

Thirty-nine wild fish are still alive in this portion of the river system, most of which could be more than 70 years old. It’s estimated there are 334 adult pallid sturgeon in the river system that were born in a hatchery before being planted in the river, the oldest of which may be around 28 years old.

Water fluctuation

For this spring’s study, the Corps increased flows out of Fort Peck Dam in late April, called the attraction flow. At peak, the water was running at 18,000 cubic feet per second at Wolf Point. It worked, Braaten said, with three reproductive females – two wild fish and one hatchery fish – moving into the river. Two females were also in the river when the testing began.

The flow test then moved into the retention phase, lower flows of 10,000 cfs meant to keep fish in the river. Some fish left, others moved in, leaving three female pallid sturgeon and several males in the stream, Braaten said.

The third pulse of water was called the spawning flow, more water meant to cue the females to lay their eggs. The releases that peaked at 20,000 cfs were delayed a bit due to high water in the Yellowstone River and concerns about flooding downstream in Williston.

Again, three females remained in the Missouri River, hanging out around Wolf Point, roughly 50 miles below the dam.

The flow test then entered its final stage, called the larval drift and dispersion phase, when water flows were lowered to around 8,000 cfs.

Since no females checked by biologists had spawned, the Corps reverted to its normal dam operations and boosted flows on July 25 to 9,000 cfs. About a week later, the spawned-out female sturgeon was captured.

Larvae test

In addition to seeing if pallid sturgeon females responded to the altered water flows, the researchers also released about 350,000 one day posthatch fish, born in the hatchery, in the warmer waters of the Fort Peck Dam spillway.

The biologists then used nets to see if any of the larvae could be captured downstream, and if so, if they were still alive after drifting into the cooler waters released from the dam.

“Eight miles downstream we were still picking up lots and lots of free embryos, and they were alive,” Braaten said.

Trawling crews working farther downstream on the Missouri will be trying to net pallid sturgeon larvae and genetically testing them to see if they came from the release of hatchery embryos or from the fish that spawned.

“That question of, did they survive, will really be answered in the next couple of months,” Braaten said.

A previous study, conducted in 2019, showed some five day posthatch embryos surviving in the river.

Those hatchery fish were released near Wolf Point. None of the one day posthatch embryos released for the study survived.

Other monitoring

The more than two-month Missouri River flow test had a lot of components to it, from tracking female fish each day to measuring water temperatures and the embryo release.

“This is a learning process and we want it to be deliberate, transparent and … scientifically sound in our approach,” said John Remus, chief of the U.S. Army Corps of Engineers’ Missouri River Basin Water Management Division.

On their own, he said the test flows are not going to recover pallid sturgeon.

Concurrent with the fisheries work, the Corps was assessing the effects of the high water releases on infrastructure like irrigation systems as well as streambank erosion, Remus said.

“We’ve put as many resources into monitoring potential impacts of the stakeholder interests as we did monitoring pallid sturgeon,” he said. “And information from both will be used to make the decision about what the next test flow might look like.”

When a similar test involving fluctuations in Fort Peck Dam releases may be conducted is unknown, but it won’t be next year, Remus said.

Assessing the existing data to formulate how to conduct future studies will take months, as will coordinating the logistics, Remus said. How many more tests are done will depend on what is learned.

“But we acknowledged up front, one test by itself wasn’t likely to get us very far,” he said.