After botulism and a virus killed at least 700 ducks and cormorants at Poplar Island this summer, wildlife managers and scientists are assessing practices to prevent similar deaths in the future.

More than half of the deaths were mallards, which died from avian botulism, a nerve toxin created by the bacteria Clostridium botulinum and New Duck Disease, Riemerella anatipestifer.

Avian botulism is common all over the country because the bacteria live in shallow lakes and waterways and in sediments. Because it is anaerobic, meaning it doesn’t need air, the bacteria spores can be dormant for a long time. When water and sediments become anaerobic, the first ducks die from ingesting the toxin from the spores as they feed; others can die if they eat the maggots that the infected carcasses attract, unless they can quickly become hydrated and pass the toxins.

Most of the remaining deaths were cormorants, which crowd the Chesapeake Bay island every summer in huge numbers. Biologists suspected the cormorant die-off was due to Newcastle Disease, a paramyxovirus.

Poplar’s team of wildlife experts have been reviewing the deaths and have identified strategies that they hope will diminish them.

They suspect the botulism came from standing water in a pit in one of the man-made island’s cells. They are certain it did not come from open waters in the Chesapeake Bay. Poplar is a perpetual construction site — the U.S. Army Corps of Engineers and the Maryland Port Administration have taken a five-acre remnant island and, in 15 years, using soil dredged from shipping channels, turned it into a 1,140-acre wildlife sanctuary filled with plants, fish and birds.

But while the birds enjoy Poplar’s paradise, the dredging must go on. When the project is completed, Poplar Island will have received 68 million cubic yards of dredged material. It has gotten 25 million cubic yards so far. The material is pumped into the island, and the contractors are occasionally digging pits to acquire sand to construct dikes that will hold the dredged material.

Michelle Osborn, an environmental specialist at Poplar, said that a contractor left a pit open in one of the island’s cells. The pit filled with rainwater, and the birds made it their home. They then began to fall ill.

One obvious solution, said longtime Poplar manager Justin Callahan, is to avoid having open pits. When managers can’t avoid that, he said, they should make sure that water levels are high enough and don’t fluctuate. It is in the fluctuations that problems occur. The Corps can keep water levels stable, he said, by pumping water from one cell to another.

It may be an extra step, Callahan said, but it’s worth doing.

“It’s a fine balance between the functionality of the site and the health of the bird population,” he said. “When we get recommendations to limit a mortality problem, we take all the action that we can.”

In the case of the cormorants, Osborn said, the problem is clearly overcrowding. As a result, wildlife officials have oiled some of the cormorant eggs to kill the embryos and give the remaining population the best chance of survival.

The 2014 deaths were surprising, as the initial suspicion focused on New Castle disease, which biologists hadn’t seen on Poplar since 2012. But the bird deaths were not as high as they were in 2012, when nearly 800 birds died from a combination of botulism in the open-pit ponds and a harmful algae bloom in the water around the island. It was an unfamiliar species of algae that had only previously been seen in Brazil and Canada.

“It was like green paint,” recalled Peter McGowan of the U.S. Fish and Wildlife Service.

Poplar’s staff began putting straw bales in the water to reduce the growth of algae. They have 1,700 bales in the water now and believe it is helping.

In some ways, the die-offs indicate that Poplar Island is a victim of its own success. It has attracted so many birds to its shores that they are crowding each other, at least in the case of the cormorants. The island managers hoped it would become a sanctuary for wildlife, and it has. Once, volunteers counted 15,000 birds in a single day. There are nesting pairs of dozens of different species, including egrets and herons, and lots of loons, mallards and ruddy ducks.

It’s also possible birds are dying of these diseases in other places. But if they are in isolated areas, they’re likely to go unreported. Several staff members report to Poplar daily, and Corps managers come often. The U.S Fish and Wildlife Service is at the island several times a week. Many volunteers help out as well. With so many eyes on the island, the wildlife activity is well-documented.

Officials with Tri-State Bird and Rescue, which nursed many of the botulism-infected birds back to health, toured Poplar recently to hear about the staff’s efforts to reduce mortality.

Cristin Kelley, a wildlife veterinarian at Tri-State, said she was glad that Poplar Island’s managers had identified some solutions and were implementing them to stop mortalities like the ones seen last summer.

“It seemed to me that everyone was not wanting it to happen again,” she said.