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Scientists work to identify cause of fish die-off in area waters

Low oxygen levels in water, viral disease of menhaden fish considered as potential factors of increased deaths

Researchers from the University and the Rhode Island Department of Environmental Management are working to determine the causes of an increased number of dead fish in local waters this summer.

Hundreds of dead menhaden fish washed ashore in the Providence River and Narragansett Bay this summer, prompting a rise in public complaints. Menhaden are small, flat fish that were declared depleted due to overfishing by the Atlantic States Marine Fisheries Commission in 2012.

Low oxygen levels in the water caused by algal blooms may have played a role in the fish die-offs, said Chris Beacupis, supervising environmental scientist for the R.I. Department of Environmental Management. Menhaden may have travelled upstream and then suffocated when oxygen levels dropped in the evening hours, he said.

Oxygen in the Seekonk River reached low levels this summer, falling to 0.5 milligrams  per liter in comparison to the normal level of seven to 10 milligrams per liter, Beacupis said. Menhaden can tolerate oxygen levels down to about two milligrams per liter, and they travel up the river when other fish remain in the bay, he said.

Yet these oxygen levels have not been significantly worse than in previous summers, said David Murray, senior research associate and facilities manager in the department of earth, environmental and planetary sciences, who collaborates with other faculty members and community leaders to study oxygen levels in Narragansett Bay.“Anecdotally, people are saying things look a lot cleaner” in the Seekonk despite the fish kills, Murray said.

The algal blooms responsible for low oxygen levels are generally caused by a nitrogen surplus, said Tom Uva, director of planning policy at the Narragansett Bay Commission. But “we do not attribute (the fish kills) to nitrogen loading,” he said, adding that the Narragansett Bay Commission has spent over $130 million in recent years upgrading its sewage treatment plants to reduce the amount of nitrogen discharged into the river by 80 percent.

Providence is also beginning the third and final phase of a $1.3 billion Combined Sewer Overflow Abatement Project. The project includes a tunnel running under Providence that is 30 feet wide and intended to prevent sewage overflow from polluting the rivers during periods of heavy rain. Alternatively, the waste is to be diverted to a sewage treatment plant.

Several of Rhode Island’s beaches were also closed this summer due to high levels of bacteria in the water. Though unrelated to algal blooms, bacterial levels are an indicator of Rhode Island’s water health.

Ultimately, low oxygen levels due to algal blooms should not be viewed as the only potential cause of the fish kills, Uva said, citing a fish kill in Pawtuxet Cove that occurred despite high oxygen levels. Instead, the kills could have been the result of the “whirling disease” virus in the menhaden population, he said. “There are a lot of crazy things going on simultaneously,” he added.


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