Dall's Porpoise: Hot Rod of the Sea

     Head west from any port on the north-central California coast to the edge of the Continental Shelf, and past the Farallones Escarpment, a submerged sea cliff. Here the water depth plunges abruptly to several thousand feet. Cold, nutrient-rich waters pushed to the surface are “sparked” by sunlight into an explosion of life, the marine food web. You’ve entered the domain of giant blue whales, sperm and beaked whales; huge herds of dolphins not often seen in nearshore waters; and swirling schools of silvery fish and invertebrates. This is also home to one species best described as rollicking and roisterous, the quintessential party animal and wave rider: the Dall’s porpoise, Phocoenoides dalli. Well, hello, dalli!

     Your sighting might start with a dazzling confusion of distant splashes, indicating frenetic feeding activity ahead. Across a shoal of ripples, triangular dorsal fins and the shining black backs of several small porpoises cleave the surface. Brilliant white patches on their ebony flanks make them resemble tiny killer whales. The porpoises plunge and leap upon their prey, churning the surface and engaging in high-speed chases that would make Steve McQueen proud. Although they’re mostly nocturnal feeders, you’ve hit a handful of Dall’s pursuing an impromptu midday snack. Suddenly, several change direction and head straight for – you! Exuberant, they ride your boat’s bow wave until, seduced by some unseen distant allure, they peel off, ending the encounter. The experience is exhilarating!

     Named by naturalist William H. Dall, the Dall’s porpoise is a stubby, muscle-bound, seven-foot long marine hot rod: speed is its forte; they have been clocked at up to 30 knots (nearly 35mph) in bursts. Its hydrodynamically-designed body creates a hollow cone of air at the surface, reducing drag and allowing it to breathe while its body is still beneath the surface! The whitewater splash, or “rooster tail” that is so uniquely Dall’s, and its cool black-and-white “paint job” confirms the identification.

     Males mature at around six and one-half feet, eight years of age; females slightly earlier. Newborn calves are around 3.5 feet and 55 pounds. The calves are high-maintenance, nursing up to 2 years. Gestation lasts 10 to 12 months, and females’ give birth roughly every three years.

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     Dall’s porpoise live throughout the North Pacific from Baja California to Alaska, and west to Japan. Although distributed widely, some are year-round residents. Here in Greater Farallones National Marine Sanctuary we often see them travelling in groups of 10 to 20, but herds from 200 up to a thousand may assemble, if prey is abundant. Their small, spade-shaped teeth interspersed with horny gum projections are engineered to grasp slippery squid, sardines, capelin, herring, hake and smelt. They produce a series of low frequency clicks, presumably for echolocation.

     Sadly, many die as incidental catch in high seas fisheries, in active and “ghost” gill- and drift nets. It is notable that California is the only west coast state that still allows drift gillnets. They are harpooned for their meat in Japan. When forty thousand were killed in 1988 alone, the International Whaling Commission imposed a quota of 18,000 annually: it’s now the largest direct hunt of any cetacean species in the world.

     Despite these challenges, they seem to be abundant in our waters. Years ago I was privileged to witness a handful of Dall’s riding the pressure wave of a fast-traveling blue whale. Oblivious of us, the unlikely group disappeared into the distance, leaving us to envy and to speculate: Was the whale even aware it had hitchhikers? Was it amused by its diminutive speedster escorts? And, among our three species, which was having the most fun? It was anybody’s guess.

 

Mary Jane Schramm, NOAA Greater Farallones, National Marine Sanctuary

Maryjane.schramm@noaa.gov

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