–In early September, at the south end of Ocean Beach, just a wave roll beyond the surfers, I thought I saw dorsal fins periodically rolling over the surface. After watching for about twenty minutes, confirming to myself that I was indeed seeing fins, I saw two bearers of fins breaching the water. Were they dolphins or porpoises in S.F. ocean waters and what type? What’s the difference between dolphins and porpoises?
from reader Claire De Land
Congratulations on being very observant. The small animals you saw were most likely harbor porpoise. These are among the smallest of the 78 species of cetaceans. They reach 6 feet in length and weigh about 190 lbs. Gee about like me. As the name indicates they are often found close to shore. Though common they are not particularly acrobatic and do not bow ride ships, so they are frequently overlooked. I regularly see these small porpoises in the channel of the Golden Gate and about half way out to the Farallon Islands. Found throughout the northern hemisphere in cool temperate waters; these porpoises are not picky eaters and feed on a large variety of small fish.
In the 1980’s an estimated two thousand of these diminutive, dull colored porpoises were trapped and drowned in gill nets along the central California coast! Fortunately most of these fishing strategies are currently better regulated and the population is expected to eventually recover. Nevertheless their habit of frequenting shallow water puts harbor porpoises in close contact with human activities and their numbers have been decreasing worldwide due to pollution, boat collisions, netting and general habitat degradation.
Aristotle recognized that dolphins and their kin were air breathing mammals and not fish. He called them Porcine Pisces – the pig fish – which became porpoise in English. The Greek philosopher and scientist may have actually been examining a harbor porpoise; they were common in the Mediterranean Sea. But alas, no more.
Michael Ellis – Ask the Naturalist – Bay Nature Magazine