Lower Granite, Hells Canyon Sturgeon Struggling

Sturgeon numbers are declining in the Snake River between Lower Granite Dam and Hells Canyon Dam, where recruitment is not high enough to overcome predation and sustain the population, and the rise of several new species in the system is being eyed for blame.

No, not walleye and smallmouth bass, though they’re also likely benefitting from the biotic changes that are impacting sturgeon.

Rather, tiny opossum shrimp, Siberian shrimp and sand rollers – three slower-water, bottom-dwelling species you’ve probably never heard of but whose appearance on the scene fits tidily with recent decades’ struggles of the Snake system’s mightiest fish.

That was the nut of an intriguing presentation by the Idaho Department of Fish and Game’s Clearwater Region fisheries manager Joe DuPont at the Northwest Power and Conservation Council’s mid-October meeting.

Drawing on data from federal, state, tribal, university and utility sources, he detailed how now-spotty sturgeon spawning recruitment appears to correlate with the explosive growth of shrimp and sand rollers, compounding a long-term decrease in sturgeon abundance since the dams went into the system. True, his argument might not be immediately accepted into and published by a scientific journal without further refinements proving causation, but it is a compelling case with plot twists and a little something something for just about everyone.

DuPont, who is retiring from IDFG soon, explained to NWPCC members during their Idaho retreat earlier this month that opossum shrimp and sand rollers are both native to the Columbia system (the former are from the estuary), while Siberian prawns probably came in in a ship’s ballast water.

Opossum shrimp first showed up at Lower Granite in 1994 and now represent 90 percent of the epibenthic biomass in the Southeast Washington reservoir, he said. They’re fed on by Siberian prawns, which themselves arrived in 1998 and whose numbers exploded beginning in 2018. Nearly 400,000 were counted last year at the dam’s juvenile fish collection facility, with others likely going through the dam and over the spillway, he said. They probably go through the shipping locks as well.

“What so what does that tell me? There are millions and millions of these things in Lower Granite Pool, and they like this very deep water, right in the same place where those very small sturgeon are showing up in the spring,” DuPont said.

To underline his suspicion that just-hatched sturgeon in spring are perfectly snack sized for Siberian prawns, he told a story about how biologists had collected some prawns from the reservoir and decided to feed them to some bass in an aquarium, but the bass didn’t eat many. So the bios collected several different minnow species and put them in the tank with the bass, figuring the predatory fish would gobble them up.

“And to their surprise, the Siberian prawns ate all the minnows before the bass did. And so, yes, they will eat minnows, and I have no doubt if they come across one of these small sturgeon, they will catch it and eat it as well,” DuPont said.

Along with shrimp appearing to affect sturgeon recruitment, he also believes sand rollers are too. It’s believed they have only been in the Lower Granite Pool since 2003.

“What USGS found is that the larger sand rollers, those kind of 3 to 5 inches, they like to use those deep-water habitats specifically during those times when the larval sturgeon are rolling into Lower Granite. And not too surprising, their primary food is opossum shrimp and is why they have, we think they have, proliferated in the reservoir,” said DuPont.

He added that the federal biologists also found larval fish in sand rollers’ diet. (Sand rollers also “were the primary food source for smallmouth bass” during a period of high abundance between 2012 and 2015, they discovered.)

To further convince his audience, DuPont overlapped baby sturgeon and sand roller counts from Lower Granite to show that as sturgeon decline, sand rollers rise. The trends reverse in one relatively high-flow year, 2017, when sand roller numbers crashed and sturgeon shot up from almost nothing.

“So this relationship is just uncanny and it makes me believe that, yes, not only are those Siberian prawns having an effect, but these sand rollers,” he said.

But as sturgeon grow bigger, the dinner table soon gets turned on the new predators.

“These very species then that are a detriment to this population can then turn around and be a benefit. And so do we think that sturgeon are eating these, especially these smallest sturgeon? Absolutely. There are so many Siberian prawns now that not only do I think the sturgeon are taking advantage of it, we see smallmouth bass are just gorged with them, walleye are just gorged with them. They’re probably providing a food source for a whole lot of things,” said DuPont.

He was speaking in response to a question from a NWPCC board member about the 20 to 40 percent higher growth rates seen in sturgeon after being translocated from the heart of Hells Canyon and near the mouth of the Grande Ronde to the Lower Granite Pool.

Some tagged sturgeon in Hells Canyon only grew 4 inches over a 20-year period, he said.

“At those kind of growth rates, they’ll never reach adulthood,” DuPont said.

He explained that completion of the Hells Canyon complex of dams reduced transport of the fine sediments that are important for growing the organisms that become food for baby sturgeon, adding that declines in salmon, lamprey and mussel populations are also emblematic of the reduced productivity of the free-flowing Snake.

DuPont noted that surveys performed by Idaho Power in the Lower Granite Pool from 2013 through 2023 showed there had only been one single year of “meaningful sturgeon recruitment,” 2017, when flows out of Hells Canyon averaged over 80,000 cubic feet per second during the spring runoff period, well above the longterm average of 55,000 cfs.

Those high flows likely suppressed predation on young sturgeon, but it also represents an apparent new paradigm for sturgeon spawning success. DuPont highlighted the work of a biologist who looked at recruitment in the late 1980s and early 1990s during a period when flows averaged less than 40,000 cfs but which still produced “meaningful recruitment.”

“You can tell that something has changed. Now we’re only seeing recruitment in those higher flow years, where historically we believe that we once had recruitment in all years, even if there were low flows,” DuPont said.

As a result, the sturgeon population in the entire reach – the Lower Granite Pool, Hells Canyon and the Salmon River – is “starting to tip over,” with a 33 percent overall decline in the last 10 years, 51 percent drop in the reservoir, 25 percent decline in the free-flowing Snake and 43 percent in the Salmon River, the main Idaho tributary.

He said the main reason for this long-term recruitment decline is “entrainment,” when fish and other aquatic life gets sucked through dams. PIT tags show that 50 to 80 percent of 2017’s otherwise good year-class of young sturgeon entrained over Lower Granite – and DuPont is willing to bet his house that it was closer to the high than low side.

He said that recent years’ spill – sending water over the top of dams to aid downstream migration of salmon and steelhead smolts – has “probably increased entrainment,” but also that probably around 50 percent of young sturgeon have been entrained through the dams since the 1970s.

“My thoughts are that [entraining] is very significant, and my thoughts are that ever since the lower Snake dams went in, that the Hells Canyon population (of sturgeon) has never been sustainable. And add that on top of those very slow growth rates that we’re seeing for these sturgeon and the fact that it doesn’t appear that they can survive anymore in the river, at least early in life, and that explains why we’re seeing this long-term decline in juvenile abundance,” he said.

DuPont said that trapping and transporting older fish between different portions of the reservoir-river system would benefit the population. Studies are ongoing to evaluate whether this could improve growth rates and boost spawning abundance, according to a briefing sheet attached to his presentation.

“Flows that now result in good recruitment have only occurred twice in the last 25 years. And so if this continues, some sort of hatchery program may be the only way to sustain the population,” he added.

There are sturgeon hatchery programs on the Upper Columbia that have resulted in a harvestable surplus of fish for recreational and tribal fishermen.

The briefing sheet also stated that the University of Idaho is “developing a population model to assess long-term trends and evaluate the potential benefits of different management actions.”