Newborn dolphins are gray in color and gradually lighten as they get older, their darker parts becoming distinct spots. Some become completely unspotted. They stay with their mothers for three to four years, but sometimes as long as eight or nine years, and typically live into their 30s.

Soon after the team spots another adult, WL168, identified by a large scar on its back. This one has also been seen near Macau, another Chinese territory 15 miles to the southwest, an indication of how local populations are not bound by political boundaries.

The dolphins eat a variety of fish, including gray mullet and lion head fish, the same sort of food, notes Mr. Ho, that appears in markets around Hong Kong. The overfishing of such species adds to the threats to dolphins, as does pollution from various sources including agricultural and industrial waste, urban runoff, discharge from ships and marine plastics.

Researchers also worry that dolphin viewing boats further stress the mammals, particularly those that race out from Tai O for a 20-minute, $25 trip.

Conservations groups say they hope the benefits of the ferry suspension will encourage regional governments and ferry companies to reconsider routes across the Pearl River. By traveling somewhat farther south, they could bypass key areas of dolphin habitat along Lantau, Hong Kong’s largest island. Such a move would only add a few minutes to the trip, they say.

It would, of course, ease just one of the many threats the dolphins are facing.

“Rerouting the ferries is not a magic cure-all,” Mr. McCook said. “But we think that can help us catalyze other actions and demonstrate it’s not a fait accompli that we lose the dolphins.”

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