NARRAGANSETT, R.I. — Before dawn at a small salt marsh on Great Island, a short distance from where the Point Judith fishing fleet was preparing to depart for the day, Josh Beuth triggered a rocket-propelled net over a gathering of ducks. The 30-by-50-foot net ensnared nearly two dozen birds that had unwittingly been attracted to the area by a buffet of corn that Beuth had delivered to the site every day for about a week.

As soon as the sound of the rocket blast subsided, a group of University of Rhode Island students and Rhode Island Department of Environmental Management (DEM) biologists emerged from their vehicles and leapt into action. With temperatures below 10 degrees, they carefully removed each duck from the net, placed them in several large plastic crates, and prepared to place bands around the birds’ legs before releasing them.

“Black duck populations have been relatively steady in recent years, but they haven’t gone up when it would be expected they would go up, like when breeding and habitat conditions in eastern North America have been in good shape,” said Beuth, the DEM biologist responsible for monitoring waterfowl in the state. “We’ve had good water in the spring wetlands, so we would expect populations to go up, unless there’s something keeping them down.”

To learn what factors may be negatively affecting the birds, Beuth and state and federal biologists from the Maine to North Carolina, as well as some in Canada, have been banding black ducks every year since 2009. The data they collect when some of the bands are returned after the hunting season is slowly revealing a complicated picture of the uncertain health of the formerly common bird.

Between 1,500 and 5,000 black ducks annually winter in ponds, salt marshes and protected coves in Rhode Island, and a couple hundred linger in the state for the summer, though only a few are believed to breed here.

“Our numbers fluctuate in winter based on ice conditions,” Beuth said. “The icier it is in the Northeast from November to February, the more black ducks we get. Inland birds push south to southern New England when ponds and streams freeze. They get pressured to the coast where the open water remains.”

About four days each week, beginning after the hunting season ends and running through mid-March, Beuth and his team trap ducks for banding. They catch an average of about 300 black ducks each year and a lot of mallards, which often rest and feed with the black ducks. At Great Island recently, however, every bird captured was a mallard.

“Some days when we shoot the net, we get 20 birds and they’re all black ducks,” Beuth said. “But not today.”

One thing he has learned so far from the banding study is that hunting doesn’t appear to have a significant impact on the black duck population.

“We’ve ruled out hunting as the factor that’s limiting the population,” Beuth said. “In years when the harvest goes up, the population actually goes up a little. In years when the harvest is down, the population doesn’t necessarily jump up. It’s counterintuitive.”

If hunting isn’t keeping the black duck population from increasing, then what is? No one knows just yet, but they’re getting closer to the answer.

“After five years of the project, we found that the annual adult survival rate for females was 10 percent lower than for males,” Beuth said. “That would explain why we aren’t seeing them going on a steady upward trajectory.”

But why female black ducks are dying at a higher rate than males, and when, is still unknown.

“We’re most likely losing the females during the nesting period,” Beuth said. “But whether it’s due to predation or they’re too emaciated to survive, at this point we don’t know. We really need to figure out what’s driving it and can we do something about it.”

Beuth suspects that climate change is going to make the situation even more precarious.

“My gut feeling is that we’re not shooting too many black ducks presently, and the population is stable, so I’m not concerned about the current conditions,” he said. “However, I am concerned that black duck wintering habitat is going to change because of climate change and sea-level rise. There has already been a deterioration of our salt marshes, which is our primary black duck habitat.”

DEM, Save The Bay, The Nature Conservancy and other groups are already working to raise the level of local salt marshes to ensure that they don’t become inundated as sea level rises, which would cause their value to wildlife to decrease. Most salt marshes in Rhode Island will be unable to migrate inland in coming decades because of development that abuts the marsh, meaning that salt-marsh acreage in the state is almost certain to decline in coming years.

“Rhode Island’s biggest contribution to black ducks is wintering habitat,” Beuth said. “But in every aerial image of salt marshes I’ve looked at, there’s less marsh — it’s breaking off and deteriorating from wave action, it’s sinking in the middle from higher tides, it’s not draining, which ultimately kills the marsh. There is certainly cause for concern.”

Despite the decline in salt-marsh habitat and the uncertainty about what is causing higher mortality among female black ducks, the daily limit for hunters shooting black ducks in the Atlantic flyway will double to two birds a day next fall, the first change to the limit in several decades.

“In the last 30 years, hunter numbers have dropped significantly, especially in the Northeast, so the harvest has gone down considerably,” Beuth said. “And the populations aren’t responding to the harvest. Our updated models put the allowable harvest at a much higher rate. The new predicted harvest is still going to be considerably lower than what is needed to maintain the population.”

When the state waterfowl biologists in the Atlantic coastal states initially saw the data justifying the change, Beuth said they “had a hard time wrapping our heads around it.” But now he thinks it makes sense.

“Despite all the challenges black ducks face, we’re going to a limit of two per day and we’ll see where that brings us,” he said. “At a minimum, we’ll assess the data after the hunting season and we’ll know immediately if it’s a mistake. But I don’t think it will be.”

Rhode Island resident and author Todd McLeish runs a wildlife blog.