The dolphin's behavior has befuddled Peter Wallerstein, program director for Southern California's Marine Animal Rescue.
"He's not trapped in this area," Wallerstein told CNN.
The dolphin, dubbed Fred by some spectators, appeared healthy and in no danger, though he showed "some signs of being disoriented," he said.
"He's eating fish, sardines," Wallerstein said. "That's a good sign."
The 6-foot common dolphin was first spotted in the Bolsa Chica wetlands Thursday. Would-be rescuers on paddle boards nearly coaxed him back into the harbor Saturday only to have him reverse course when two other dolphins showed up, Wallerstein said.
"As soon as he saw them, he changed his behavior immediately and swam back under the bridge," Wallerstein said. "I've never seen that type of interaction.
"He chose to be in here for some reason."
He said another attempt to encourage the dolphin back to sea would be made Monday.