It lives at the bottom of the river.

It's ancient and ugly - often described as a dinosaur with fins.

And although it once made the region the caviar capital of the world, the Atlantic sturgeon is being declared an endangered species, a decision that could affect the Delaware River deepening project.

No one is saying the sturgeon will become the snail darter of the Delaware. Officials from the National Oceanic and Atmospheric Administration's Fisheries Service, which is making final the endangered listing, and the Army Corps of Engineers say the fish does not have the power to scuttle the project.

But they concede that schedules might have to change or other alterations be made to accommodate the spectacularly picky sturgeon, which will spawn only in certain areas.

"We are concerned about dredging and the impacts on the species," said NOAA's Kim Damon-Randall, supervisory fishery biologist. One of the agency's documents said the scope of the project, plus the ample unknowns about the fish's preferred spawning habitat, "indicate that the project could be very harmful" to the sturgeon.

In anticipation of the listing, the Army Corps has already been working with federal fisheries officials, said spokesman Ed Voigt.

"As we move forward, we're coordinating . . . to ensure that with each step of the project, we avoid impacts to the Atlantic sturgeon and other species."

But, he said, "there's no reason to expect some kind of change is going to transform the cost and scope of the project."

Maya van Rossum, the Delaware Riverkeeper, disagreed. She said the listing should "have significant ramifications" for the deepening project. If it does not, the Delaware Riverkeeper Network is "well positioned for a very strong legal case."

Scientists are still trying to figure out what portions of the river the fish prefers and why, and preliminary results indicate that a favored habitat is a tanker anchorage near Marcus Hook - which is slated for deepening.

"There are usually several big tankers anchored there, and right below them, baby sturgeon," said Delaware fisheries biologist Matt Fisher, who has caught young sturgeon and implanted acoustic equipment so they could be tracked.

'Critical habitat'

Once the listing is final - it is being filed Friday and will take effect in April - scientists and other officials will be required to come up with a restoration plan for the sturgeon, and that would include identifying its "critical habitat," which might need to be protected.

No one is speculating what could happen if the anchorage is identified as vital to the fish.

Overall, "I do think it's going to complicate their situation a little bit," Dewayne Fox, associate professor of natural resources at Delaware State University, said of the listing.

Restoring the fish will likely require changes to commercial fisheries on the Delaware, whose gill nets often ensnare sturgeon as well as the target fish.

Officials will also be addressing ship strikes that kill the fish. Sturgeon often feed in the deepest portions of the river - the ship channel. For whatever reason, they either don't sense when a large ship is approaching or don't get out of the way.

Last year, Delaware officials received reports of 22 sturgeon that had washed ashore, sometimes with fatal gashes, other times cut in half.

A final issue in managing the species involves the river's flow itself. Right now, freshwater releases from three Upstate New York reservoirs are regulated to maintain specific flows downstream.

Prime territory

Scientists say that releases eventually might have to be increased to rinse any prime spawning habitat because the eggs cannot survive in salty water. But those reservoirs also supply much of the drinking water of New York City.

"So it's a water-rights issue, too," said Delaware's Fox.

While few outside fisheries biologists know - or likely care - about the Atlantic sturgeon today, the species was celebrated in the late 1800s, when its eggs were highly sought for caviar.

Although the sturgeon lives in other rivers, the Delaware was its prime territory.

In one year - 1898 - New Jersey fishermen alone hauled in 5,060 sturgeons, quite a feat given that the adult fish can reach 14 feet and weight up to 800 pounds. The fishermen filled 1,067 kegs of caviar - valued then at more than $76,000.

And then the boom was over. The species collapsed because of overfishing. The river became too polluted. Dredging changed the river bottom and the tidal flows.

A Delaware River population that once boasted 180,000 females is now believed to be as little as 300.

The fish's biology has not helped it.

Although the species has survived about 75 million years and individuals can live for 50 years, females don't reach sexual maturity until they are 18 to 20 years old.

The fish spends most of its time in the ocean, but then comes into rivers to spawn. Specific conditions must be met: The water must be fresh, yet tidal, which occurs in the Delaware roughly between the Commodore Barry Bridge and Trenton.

The eggs are sticky when they are released, and they remain so for only half an hour. They sink to the bottom and need to settle onto a rocky substrate to survive.

The young stay in the river a few years, and then they follow their elders out to sea. Sturgeon tagged in the Delaware have been found from Florida to Canada.

As researchers began to pay more attention to the plight of the sturgeon, they worried that the fish were no longer breeding in the Delaware. This would all but surely spell their doom.

Then, in 2009, Delaware's Fisher was clearing a net of what he thought were white perch when he saw something odd. It was a sturgeon hatched that summer. That year, he and Kennett Square fisheries researcher Hal Brundage caught about 55.

The next year, nothing.

But in 2011 - a wet year, like 2009, with high river flows - they caught more.

Fisher has been fitting juveniles with ultrasonic transmitters - each about as big as a large vitamin pill - and receivers have been mounted on Coast Guard buoys from above Trenton through the estuary and out into the ocean.

The catch of young gives scientists hope.

"I think we have to be optimistic," said Brundage, although he conceded that "it's really hard for me to make a reasoned comment on the prospects for the recovery of the species."

There are just too many unknowns, and the fish faces significant problems.

Scientists are hoping the listing will prompt the dedication of funds to research.

Not much is known about what the fish eat. Or where the young go and why.

"Sturgeon are tricky - they live on the bottom of a very murky estuary," said Danielle Kreeger, science director of the nonprofit Partnership for the Delaware Estuary, who called the fish "a signature species" for the river. "It's just difficult to put divers down there or do anything where you can study them.

"That's one of the reasons I'm excited about the potential listing," she said. "If nothing more, it should bring more attention to an understudied animal."