Bottlenose dolphins have been spotted off Rodeo Beach in Southern Marin as they make their way in and out of San Francisco Bay in increasing numbers.
The dolphins were first photographed in San Francisco Bay in 2007 and have been steadily expanding their range north from Southern California in recent years, although the reasons are not completely understood.
Initially the dolphins — which have been seen alone and in groups as big as 40 — were only going as far north as Bodega Bay, but have since been spotted farther north, at Point Arena in Mendocino County.
“These animals are not just coming up here and staying here,” said Isidore Szczepaniak, who has studied their behavior for the Corte Madera-based Golden Gate Cetacean Research.
This year dolphins were seen in San Francisco Bay in January and by April they were off of Mendocino before turning up in Bodega Bay in May and then “just on the other side of Marin” in June, Szczepaniak reported at a symposium this month at the Romberg Center for Environmental Studies in Tiburon.
The dolphins can grow up to 11 feet long, weigh up to 500 pounds, live for 50 years and feast on fish, squid and crustaceans.
By comparing fins, Golden Gate Cetacean Research has been able to identify almost 100 dolphins that come to the region. The research team noted one dolphin that had gone as far south as Ensenada, Mexico and as far north as Bodega Bay, more than 600 miles.
“That is the longest known movement of a bottlenose dolphin,” Szczepaniak said.
Lauri Duke, a citizen scientist helping document the dolphins, said the work is rewarding.
“It is very exciting when you finally see a fin pop up above the water after watching from the beach for maybe a couple of hours,” she said. “My first thought is to … start taking as many photos as possible.”
Researchers are also noting dolphins are starting to make a harmful mark on the marine environment locally.
“They have started to feed on a resource they do not have in Southern California, and that is salmon, an already threatened population,” Szczepaniak noted.
The territorial dolphins are also taking aim at harbor porpoises, a smaller species.
“We started seeing evidence of blunt force trauma, broken ribs and lesions, in harbor porpoises,” Szczepaniak said. “In June I witnessed an attack.”
Such cases of “porpicide” have been recorded with increasing frequency, according to Golden Gate Cetacean Research.
Harbor porpoise necropsies conducted by the Marine Mammal Center in the Marin Headlands were analyzed and of 240 cases, 56 indicated trauma consistent with lethal injuries caused by bottlenose dolphin attacks.
From 2008 through this year, the highest frequency of porpicide strandings occurred in the months of August and September, which coincided with periods of frequent dolphin sightings. If the dolphins continue to expand their range, the porpoise population could be thinned, the group concluded in its research.
“Bottlenose dolphins being here affects everything,” Szczepaniak said.
