Jorge Bogantes was one of the first officials to come across the sight while on a bike ride near his home in Hyattsville, MD, over Memorial Day weekend — hundreds of fish floating belly-up in a pool of water near the Northeast branch of the Anacostia River.

After a week of heavy rains, water from the Anacostia had swelled about 15 feet beyond its banks, as far as the bike trail that runs alongside it. When those floodwaters began receding at the end of the week, they left pools of water — filled with fish — around the base of railroad bridge columns that were under construction.

There were so many fish trapped in the pools that they quickly depleted the water’s available oxygen, and hundreds died. Bogantes, a natural resource specialist with the Anacostia Watershed Society, began posting photos of the dead fish on the nonprofit’s social media accounts on the Sunday before Memorial Day to raise awareness, though he thinks the fish began to die as early as the preceding Thursday or Friday.

The AWS identified most of the dead fish as gizzard shad, adding that they are an important prey species for game fish and the ospreys that return to the Anacostia River each year. Other photos showed a pair of brown bullhead catfish, which are native to the Anacostia River, trapped in the pool of water but coming to its surface to gulp oxygen from the air.

Bogantes said some invasive snakeheads, which can gulp oxygen from the air, were also taking advantage of the stagnant pool of water and its captive fish. Local residents were also fishing in the pool of fish, which presents serious health hazards, Bogantes said.

He worked with local volunteers over the weekend to carry fish from the pool back to the river. “It’s a really huge fish biomass,” Bogantes said. “I took a trash bin and probably took 20 trash bins half-full of fish, transferring them from the pool to the river.”

The Maryland Department of the Environment later counted and found that about 700 fish were killed. Water samples showed that oxygen levels were below the legal minimum.

The department worked with several local agencies and the construction company working on the railroad bridge, CSX contractor Shirley Contracting, to ensure that the company came into compliance to prevent more fish dying at the site.

“Any fish kill is a cause for concern,” said Adam Ortiz, director of Prince George’s County’s Department of Environmental Resources. Ortiz lives about five blocks from the site of the incident and quickly responded, along with several residents, to rescue the surviving fish and find out who was responsible. “When there’s actual occurring [fish kills] — and man-made occurring ones — we want to get a handle on that and prevent as many of them as possible,” he said.

Ortiz said the construction company was operating under permits to conduct its work in and along the Anacostia River, but those permits weren’t being properly followed at the time.

The holiday weekend may have contributed to the company’s slow response to weather-related conditions.

Ortiz said his department and MDE maintained a strong presence at the construction site throughout the weeks that followed to ensure that issues were remedied. They found the company was not adequately controlling sediment and erosion at the site, which “is critical in sensitive areas such as wetlands and waterways, especially the Chesapeake Bay Critical Area,” Ortiz wrote in an e-mail.

Erosion also contributed to a severely damaged bike trail, which featured two sinkholes and a drop-off where the streamside had begun to crumble the same weekend that the fish were killed. Ortiz said the trail has been repaired where erosion undermined it and made it unsafe.

“In the storms that have occurred since the Memorial Day weekend incident, we have closely monitored the site and have found no more strandings or kills,” Ortiz said.

Ortiz added that the incident became a “great example of residents and activists stepping up to fix the situation as soon as possible.”

Staff and volunteers with local nonprofits like the AWS and Community Forklift helped to dig trenches to provide oxygen for the remaining fish as they worked with local residents to transfer them to the river with nets. Volunteers also helped contain dead fish and minimize the smell until the company took over cleanup.

“With all environmental issues, everybody has a role — young people, residents, environmental groups and the government — to come together,” Ortiz said. “Those kids who were saving the catfish one by one are a great example.”