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Sea nettles, the annoying stinging jellyfish that chase swimmers out of the waters of the Chesapeake Bay and its tributaries, are late in arriving this summer. They usually drift into the Southern Maryland region by July, but there is still no sign of them.

Ranger Christy Bright, manager of Point Lookout State Park, said there have been no reports of sea nettles so far at the park where the Potomac River meets the bay. “It’s nice for our visitors” who swim at the public beach there, she said. “It’s not been a rainy season. I’m not sure what the deal is.”

Jack Russell (D), president of the St. Mary’s County commissioners and a bay educator, said last week, “The water’s been too fresh for sea nettles this year,” he said. Sea nettles thrive in saltier water.

“The jellyfish are very late this year,” said Jacqueline Tay, graduate research assistant at the University of Maryland Center for Environmental Science. At the Horn Point Lab in Cambridge on the Choptank River, “we have only recorded one small jellyfish the entire summer at our monitoring location when we usually see many. I believe there aren’t any in the Patuxent River where other researchers are sampling as well,” she said.

The sampling station at Horn Point Lab “is unseasonably fresh this year” and sea nettles prefer higher salinity, she said, though salinity had increased slightly before a deluge of rain fell Tuesday in central and northern Maryland.

“One possibility is that the [sea nettle] populations have shifted south due to low salinities. We are not sure,” said Raleigh Hood, professor of biological oceanography at the university’s environmental science center.

“Because jellyfish are not evenly distributed throughout the bay, it makes it difficult for researchers to know how the population is doing baywide,” Tay said.

When they are mature, sea nettles are in their final medusa stage, which is what people are familiar seeing floating in the water.

While the “lack of sea nettles is very good for swimmers,” Tay said, it is not known what other ecological impact there may be. Sea nettles catch prey that they come across with their stinging tentacles. Among their diet are comb jellies, a kind of ctenophore, which eat oyster larvae. But there aren’t many comb jellies to be found either so far this summer, Tay said. “We are unsure of how the lack of both jellyfish may be affecting the bay ecosystem,” she said.

Russell said back when he caught crabs commercially, sometimes there would be so many sea nettles caught up in crab pots the pots would become too heavy to lift off the bottom. “It’s strange how things have changed,” he said.

According to the website, which the Horn Point Lab also watches, the only submitted reports of sea nettles this summer have been at the mouth of the Chesapeake around Norfolk, Va.