This is how the pallid sturgeon, the so-called “dinosaur of the Missouri River,” an animal that has survived for more than 70 million years, winds up on the federal endangered species list in Montana.
At the Fort Peck Dam, the river slows into a lake full of sediment and microorganisms. The microorganisms there act the same as they do in the Chesapeake Bay and most waterways with pollution — they suck up all the oxygen. The pallid sturgeon larvae, still eating through their birth sacks, fall to a silty bottom where they can’t breathe.
“That dead zone is right there at the bottom,” said Christopher Guy, the study’s lead author. Guy, a Montana State professor and one of the leaders of the Montana Cooperative Fishery Research Unit, set out in the midst of the study to find out how much oxygen the larvae needed to survive. He plunged a tube into the waters near the dam and pulled it back up for an examination.
“The dissolved oxygen was zero,” Guy said, and clearly, he added, the sturgeon need more than that.
As far back as the late Cretaceous period, when Nanuqsaurus, a cousin of the Tyrannosaurus, roamed Alaska, pallid sturgeon larvae floated down the river as they developed into big, funny looking prehistoric fish. But now they meet their end in waters “that no longer look like a river,” Guy said. “It starts to look like a lake.”
The study is the first to make a direct link between changes caused by dams that cause sediment build up and reduced oxygen levels that affect the survival of an endangered species, the USGS said in a statement.
“This research is a notable breakthrough in identifying the reason why pallid sturgeon in the Missouri River have been declining for so many decades,” said Suzette Kimball, acting director of the USGS. She said it will provide “vital information they can use as a focus of pallid sturgeon conservation.”
“After millions of years of success, the pallid sturgeon population stumbled and now we know why,” Guy said. “From a conservation perspective, this is a major breakthrough.”
The research paper was published last week in the journal Fisheries. Physiologists Molly a.H. Webb and Kevin M. Kappenman of the U.S. Fish and Wildlife Service were co-authors, along with Montana State University ecologists Hilary Treanor and Eric A. Scholl.
Fewer than 200 sturgeon exist in a part of the river that once swelled with them. Fish and Wildlife placed the species on the endangered list in 1990, and various government agencies have spent close to $250 million on restoration research for all marine species in the area. Yet the pallid sturgeon’s problem, according to the study, is as big as day — dams built by the federal government to manage water and supply energy to humans.
Guy said it’s not USGS’ role to recommend ways to fix the problem. That falls to Fish and Wildlife and the Army Corps.
It’s the sturgeon’s sad fate that its babies, unlike those of other fish species, bump along a current on the bottom of rivers because they lack buoyancy. A description by Fish and Wildlife put it this way: They “adapted to living close to the bottom of large, silty rivers with a natural hydrograph. Their preferred habitat has a diversity of depths and velocities formed by braided channels, sand bars, sand flats and gravel bars.”
Other river fish larvae float near the surface or between the surface and the bottom, where there’s plenty of oxygen near the dam.
Federal officials are studying at least one potential fix, creating a bypass at a dam near Glendive, Mont., that would allow embryos to float for a longer distance and develop on the Yellowstone River, which runs into the Missouri, said Tyler Johnson, a spokesman for the Interior Department’s Bureau of Reclamation. That solution is still far away.
Pallid sturgeon are a big, bossy fish that live nearly as long as humans, up to 50 years. Males reach sexual maturity at age 7, and some females don’t reach that level of maturity until they’re 15. They spawn every two to three years. In other words, it would be tough for a fish with that reproduction cycle to recover even under good conditions.
In experiments at a U.S. Fish and Wildlife lab, newly hatched pallid sturgeon embryos were placed in water with various levels of dissolved oxygen, all higher than the level of zero that Guy found. The negative impact was immediate.
“They became disoriented and weren’t able to move the way they should have,” Treanor said. “Within an hour we started to see mortality. By the end of the experiment they were all dead.”