Environment: Oxygen-depleted dead zones caused by reservoirs killing endangered fish embryos

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Pallid sturgeon embryos are dying in the dead zones of the Missouri River. Photo via Nebraska Games and Parks Commission.

‘We’re basically talking about a living dinosaur that takes 20 years to reach sexual maturity and can live as long as the average human in the U.S.’

Staff Report

FRISCO — A river fish whose genetic lineage goes back ten of million years has survived dramatic climate shifts and other earth-changing events, but may not be able to persist through the age of dam-building.

Oxygen-depleted dead zones between dams in the upper Missouri River Missouri River are preventing pallid sturgeon from reproducing, and there’s no sign things will get better, at least not without a little help from humans.

Pallid sturgeon, native to the Missouri and Mississippi rivers, were listed as an endangered species in 1990. The species has a lifespan of more than a century. According to the U.S. Fish and Wildlife Service, fewer than 175 wild-spawned pallid sturgeon – all adults – live in the free-flowing Missouri River above Lake Sakakawea. Since 1990, not a single wild-spawned pallid sturgeon is known to have survived to a juvenile, despite intensive searching.

“We certainly think this is a significant finding in the story of why pallid sturgeon are failing to recruit in the upper Missouri River,” said U.S. Geological Survey researcher and Montana State University professor Christopher Guy. “We’re basically talking about a living dinosaur that takes 20 years to reach sexual maturity and can live as long as the average human in the U.S.,” Guy said. “After millions of years of success, the pallid sturgeon population stumbled and now we know why. From a conservation perspective, this is a major breakthrough.”

“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. “By pinpointing the biological mechanism responsible for the species’ decline, resource managers have vital information they can use as a focus of pallid sturgeon conservation.”

The study is the first to make a direct link among dam-induced changes in riverine sediment transport, the subsequent effects of those changes on reduced oxygen levels and the survival of an endangered species, in this case the pallid sturgeon.

“This research shows that the transition zone between the freely flowing river and reservoirs is an ecological sink – a dead zone – for pallid sturgeon,” Guy said. “Essentially, hatched sturgeon embryos die in the oxygen-depleted sediments in the transition zones.”

Guy said fisheries biologists long suspected that the Missouri River’s massive reservoirs were preventing hatched embryonic pallid sturgeon from surviving to the juvenile stage. But early attempts to tie the problem to low levels of dissolved oxygen were unsuccessful.

“The reason for that is we hadn’t sampled deep enough,” Guy said. “It wasn’t until we sampled water down at the bottom, where those sediments are being deposited, that we found there was no dissolved oxygen. Because hatched pallid sturgeon embryos are negatively buoyant, they tend to sink into that hostile environment.”

“The lack of oxygen is a function of high microbial activity in the sediment laden area,” said co-author Eric Scholl, a Ph.D. student at Montana State University and a co-author on the study.

Hilary Treanor, an MSU research associate working with Guy, said they were able to show just how hostile these transition zones between riverine environment and reservoir could be to hatched sturgeon embryos.

In experiments at the U.S. Fish and Wildlife Fish Technology Center in Bozeman, scientists tried to recreate the river-bottom conditions in their lab, with oxygen levels as low as  1.5 milligrams of oxygen per liter of water. At those depleted levels, the hatched sturgeon embryos suffered almost immediately.

“We saw changes in their behavior fairly quickly. 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.”

In the past 5 years, researchers identified the most important reason for pallid sturgeon population declines in the Upper Missouri River: the lack of survival of naturally produced hatched sturgeon embryos.

Guy said this most recent study of sturgeon built on research conducted by USGS fisheries biologist Patrick Braaten, which demonstrated not enough available drift distance exists between the reservoirs for hatched pallid sturgeon embryos before entering the reservoirs in the upper Missouri River.

Before dams, hatched pallid sturgeon embryos would drift for hundreds of miles, eventually settling out of the river’s current in areas with low flow where they matured enough to negotiate the river’s flow.

Given what the new research shows about how no oxygen is available to hatched pallid sturgeon embryos, the authors of the paper propose that officials will need to consider innovative approaches to managing Missouri River reservoirs for pallid sturgeon conservation to have a chance. It also could provide some guiding principles for the construction of new dams around the world, Guy said.

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