Ever since a family of 15 bottlenose dolphins surfaced in two rivers near the New Jersey shore in June, wildlife officials have urged people not to get too close to the animals.

Now they're putting some teeth into their appeal, warning that the same stretch of river where the dolphins are frolicking also has been home to some of the most dangerous sharks in the world.

Bull sharks, which can grow to between 7 and 12 feet long and are among the three most likely species to attack humans, have been known to swim in the section of the Navesink River near the Route 35 bridge, where the dolphins have most recently been staying, said Bob Schoelkopf, co-director of the Marine Mammal Stranding Center in Brigantine.

"In that area are bull sharks that go in there quite frequently," he said. "You never know what you might be looking at; it could be a dolphin or it could be a shark."

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Schoelkopf and others have been concerned about a worsening pattern of harassment by boaters and people on personal watercraft getting too close to the dolphins in the Navesink. Federal regulations require that boaters stay at least 50 yards away from the dolphins; harassing them is punishable by a $10,000 fine

He said one woman recently reported that her husband heard a loud splash in the river and saw a large fish break the surface of the water. Thinking it was a dolphin, he dove into the river to try to swim with it.

But the animal never resurfaced, Schoelkopf said. That means it probably was not a dolphin since they need to regularly come up to the surface to breathe.

Although no one has documented the presence of a bull shark in the river in recent weeks, Schoelkopf said that encounter sounds like it could have involved one.

"They are very aggressive sharks, and are known to swim into fresh or brackish water including rivers," he said.

According to The National Geographic Society, bull sharks favor shallow coastal waters and can travel far inland in rivers or even small tributaries of inland waterways.

Schoelkopf said a bull shark might have been to blame for the July 1916 deaths of two people in the Matawan Creek — four miles inland from the Raritan Bay — although other accounts blame the attacks on a great white shark.

He also said that earlier this summer, "quite a few" injured seals were found in the area showing evidence of having been attacked by sharks.

Teri Frady, a spokeswoman for the National Oceanic and Atmospheric Administration, said there are no plans to try to remove the dolphins from the river or shoo them back out to deep water.

"It all depends on conditions there, if conditions are not good for them there, or if they start to show signs of stress physically," she said.

Neither of those things has yet happened, Frady said.

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