Dense overnight fog hampered efforts to locate two people who were clinging to an overturned jon boat in Charleston Harbor early Saturday, authorities said.

A 911 caller near South Battery and Limehouse Street reported hearing a male and female yelling for help from the harbor, but the caller could not see the victims because of the heavy fog, according to a news release from the Charleston Fire Department.

Another resident reported that someone could be heard yelling for help near White Point Gardens.

Rescuers were dispatched just before midnight. When they arrived, they used a PA system to make contact with the victims.

“We could hear them but we couldn’t see them,” Assistant Fire Chief Raymond Lloyd said. “The fog was so dense.”

The city’s Fire Department and Police Department were assisted by the St. John’s Fire Department, the Coast Guard, Charleston County EMS, and the Charleston County Volunteer Fire and Rescue Squad.

The city’s new fireboat, the Louis Behrens, was able to get on the scene in less than 10 minutes, Lloyd said. The fireboat’s crew located the two victims and pulled them from the water. The victims, ages 22 and 23, were found on top of a small, overturned boat in the harbor.

“They were in a small jon boat,” Lloyd said. “They told us they had put in on James Island and they hit a clump of seaweed.” The clump of seaweed caused the small boat to capsize, he said.

They were plucked out of the harbor near the High Battery wall and East Bay Street, Lloyd said. They had been in the water for more than an hour.

Firefighters rendered first aid to the victims, who were taken ashore and turned over to paramedics. Information on the victims condition was unavailable.

The overturned boat was recovered by the Coast Guard, the release said.