Under that designation, federal agencies would be required to prevent or modify activities, such as dredging, that could alter or destroy areas necessary for the sturgeon's survival.
According to the two litigants, the Delaware Riverkeeper Network and the Natural Resources Defense Council, the declaration of the fish as an endangered species set a deadline for designating critical habitat - one that passed 17 months ago.
"It's a clear showing of disregard for a species on the brink," said Maya van Rossum, who as Delaware riverkeeper heads the Riverkeeper Network.
The National Marine Fisheries Service is "well aware of the situation," she said, "yet at every opportunity they had to do something to protect the species, they've chosen essentially not to."
Population estimates vary for the Delaware's sturgeon, which can grow to 14 feet and 800 pounds. It is a genetically distinct population from others in East Coast rivers, which also are threatened or endangered and are included in the suit. But a total of 90 to 300 adults is generally accepted as probable.
Maggie Mooney-Suess, a spokeswoman for the fisheries service, said she could not comment on the suit. But because the sturgeon is listed as endangered, she said, "federal agencies that are funding or carrying out an activity already have to consult with us . . . and we will provide recommendations for what they can do to mitigate impacts."
She added, "We do look at habitat when we're making our recommendations."
The Army Corps of Engineers is in charge of the channel-deepening project in the Delaware and monitors the material both from the dredge boat and in the deposit areas to determine if sturgeon have been killed.
Corps spokesman Richard Pearsall said the agency had an allowable "take" of 20 sturgeon over the life of the project, but "the good news is that, to date, we haven't had any problems."
The corps, he said, also has been working with fisheries biologists who have been implanting acoustic devices in sturgeon to track them.
A big question mark for potential impacts to the fish is the blasting of rock in the Marcus Hook area, planned for 2015. Pearsall said some fish there were captured by a research trawler, then implanted with acoustic devices and relocated upstream.
"The idea is to try to figure out if it would be practicable to clear the area, take them upstream, do the work, and hope they haven't found their way back down," he said.
Meanwhile, researchers are learning more about the fish, an ancient species sometimes described as "a dinosaur with fins." Sturgeon were so numerous in the late 1800s that they were the basis for thriving fisheries. Eggs were highly sought as caviar.
Then the population crashed.
Biology works against the sturgeon: Although individuals can live 50 years and females can lay up to three million eggs at a time, they do not reach sexual maturity until they are 18 to 20 years old.
Experts say too little high-quality spawning habitat remains in the river.
Matt Fisher, a biologist with Delaware's Department of Natural Resources and Environmental Control, has been studying first-year fish. He nets them, implants acoustic devices about the size of a multivitamin, then releases them. A network of buoys with acoustic receivers picks up the devices' pinging signals; triangulation shows the locations.
Areas where young fish seem to congregate include anchorages near Marcus Hook and across from Tinicum Island near Philadelphia International Airport, where dredging also is planned.
Dewayne Fox, assistant professor of fisheries at Delaware State University, has focused on adults. There are so few in the Delaware now that catching them is difficult, so his team puts out nets off Bethany Beach, Del., a coastal "highway" for sturgeon from several rivers.
So far, they have implanted acoustic receivers in 360 adults. Of those, nine moved into the Delaware last year. Evidence suggests that the most likely spawning areas are Tinicum Island's southern end and farther upstream in the Burlington City area.
"Sturgeon need very specific habitats," Fox said. Eggs will not survive in salty water, or on bottoms that are too silted.
Any removal of hard-bottom habitat in the Delaware, he said, "is cause for concern."