Mystery As Harbor Porpoises Found 60 Miles Upstream in San Francisco

Two harbor porpoises that entered the San Joaquin River near San Francisco in September have now traveled even further upstream.

The mother and calf were spotted near Bethel Island in the San Joaquin River on September 25, 60 miles inland from the ocean.

"This year, we had a sighting of a mother and her dependent calf far upriver, into the lower San Joaquin River east of Bethel Island, on September 25, and three days later, a mother-calf pair was seen farther down river in the town of Pittsburg, California," Bill Keener, a researcher and member of the Cetacean Field Research Team at The Marine Mammal Center, told Newsweek.

"We assume this is the same pair of porpoises. What surprised us was that this September 25 sighting was approximately 95 kilometers (60 miles) upriver from the Golden Gate Bridge. This may be the record for a porpoise inland in California."

Harbor porpoises are a small, shy species of cetacean, only growing to around 5 feet long. Distant cousins of dolphins and whales, and one of eight species of porpoise, harbor porpoises live in small groups of two to three individuals, and usually stay in the coastal waters of the Northern Hemisphere.

"There are many reasons why cetaceans may travel up rivers in freshwater. Some like the tucuxi dolphin of South America do it as a normal part of their day. Other dolphins like the Amazonian River dolphin live exclusively in rivers and don't really swim in the ocean. So many cetaceans are quite comfortable in rivers, and by no means would this be a record freshwater journey for a cetacean as some live their whole lives in rivers," Jason Bruck, an assistant biology professor at Stephen F. Austin State University, told Newsweek.

"Saying this is a record river distance for a harbor porpoise is also difficult as these animals tend to travel either in small groups or alone and are often not conspicuous as they don't splash or act gregarious like some larger cetaceans. So, it is likely there are more excursions like this than are known to observers, especially in areas where animals are not monitored acoustically."

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According to Keener, while porpoises are usually saltwater dwelling cetaceans, they do occasionally travel into freshwater rivers.

"The harbor porpoise that we have here in California, and found across the Northern Hemisphere in Europe, as well, is a saltwater (marine) creature. But sometimes they do explore rivers. While this small porpoise was historically found in San Francisco Bay, it left for several decades starting about World War II when military activity, noise and pollution sharply increased.

"It's possible that the local fish populations were reduced, and the porpoises found the bay to be a less friendly, food-rich environment. However, around 2007, after the bay was cleaned up, they returned, and ever since they have been exploring this large complex estuary ecosystem. They have been seen well up the Napa River, and recently, the Petaluma River," he said.

According to Keener, harbor porpoises will sometimes travel upriver in other parts of the world, such as in 2009, where harbor porpoises in London made it up the Thames as far as Kew Gardens. While the San Joaquin River journey may be extreme for a harbor porpoise, it is not a record for cetaceans: Beluga whales are known to travel far upstream in the Arctic, with belugas going 60 miles upriver to the town of Bethel, Alaska last year.

"As to why they do this, the assumption is that they are looking for food. Very small cetaceans such as the harbor porpoise have high metabolisms, and must eat regularly, so they are always on the hunt for the small schooling fish that are the mainstay of their diet. In Germany, the largest numbers of porpoises seen in rivers there have at times coincided with an abundance of fish."

While the porpoises may enter the rivers willingly to hunt, staying in freshwater is not healthy for them over a long period of time.

"The freshwater is not good for the skin of marine cetaceans, and they do have to get back to saltwater to remain healthy, but for a small animal like the harbor porpoise, they can survive for many days before they have to return," Keener said.

"But they can die if they get truly lost and cannot make their way back downriver. In 2015, a local porpoise was seen in the Napa River, and after almost two months, it died there. Earlier this year, a beluga whale got lost up the Seine River in France and died. Two years ago, a minke whale went 400 kilometers up the St Lawrence River in Canada, nearly all the way to Montreal, and died."