Police Officer Anthony Kowalski of the Philadelphia Marine Unit said that a school of herring heading upriver to spawn probably attracted the dolphin, the Philadelphia Daily News reported. Police followed the dolphin upriver, but it got away from them near the Philadelphia International Airport, only to be spotted Thursday in the Schuylkill.

Kowalski predicted that the dolphin would head back to Cape May and the open Atlantic once it has spent a few days feeding up.

Ocean wildlife has become more common in the river in recent years, a sign that it has become cleaner. In April 2006, a baby gray seal swam upriver, and in 2005 a 20-foot beluga whale got as far upstream as Trenton, N.J., 30 miles from Philadelphia.

Philadelphia is about 100 miles by river from Cape May and 50 miles from the head of Delaware Bay.