THE SWIFT AND MYSTERIOUS DALL’S PORPOISE
by Michael Engelhard
SHIP PASSENGERS AT A RAILING in Kenai Fjords or Prince William Sound may witness a sight to gladden their seasick hearts: zigzagging, bicolor mini-subs escort their vessel, cutting through its wake, shooting ahead, or surfing the pillowing bow wave. Then, for no obvious reason, the visitors lose interest and vanish as if dissolved. Unlike some fluked kin, they are seldom airborne, and their frolicking makes them hard to photograph, or at least, photograph well.
Landlubbers mistake them for baby orcas because their pearly white belly and flanks contrast with stout, black bodies. However, their dorsal fin, “frosted,” like the indented tail, is triangular and much shorter, a clear identifier.
This fellow pilgrim, at home from Japan to Baja, California, is the Dall’s porpoise, the largest of seven porpoise species and among the swiftest of lightweight toothed cetaceans. William Healey Dall, one of the territory’s earliest scientific explorers, in 1873 sketched the small-beaked mammal, and a curator friend at the U.S. National Museum later named it after him. Traveling up to 35 miles per hour—the speed of coursing hyenas—slashing the cobalt-blue sea, it kicks up “rooster tails” with its fin. The telltale spray forms a hollow cone, which allows the animal to breathe while still submersed. Russian sailors called its kind “little sea pig,” and the generic “porpoise” derives from Medieval Latin for “pig-fish,” referring to their blunt snout and porcine figure. Regardless of these monikers, few creatures show such powerful grace; the group designation became a verb, synonym for elegant motion.
Hyperactive denizens of the high seas, Dall’s porpoises migrate locally, cruising barely below the surface. They even ride snout waves caused by bigger whales. The humpbacks’ presence may offer a measure of protection— humpbacks have interfered when orcas attack other animals. Dall’s on occasion float limply, suspended, heads elevated, or lollygag in swells, yet are never caught napping. They prefer lower ocean temperatures and waters deeper than 600 feet. Of the 11 distinct populations, several reside yearround in cooler, near-shore waters off the Aleutians, or in glacier-fed Kenai Fjords, Prince William Sound, and the Inside Passage. It remains to be seen if and how they’ll respond to the North Pacific’s recent warming.
Expert divers and muscular foragers, Dall’s porpoises descend to about 300 feet, hounding anchovies, herring, squid, and crustaceans. Gum ridges and spade-shaped, specialized teeth let them vise-grip slippery prey. Ten or more sometimes torpedo a school of fish, scattering it to smithereens. Dozens may gather in feeding frenzies that have the sea boiling.
The hunters easily become the hunted. Transient orca pods and, more rarely, great white sharks, stalk Phocoenoides dalli. So do people and have done for centuries. Historical Chugach refuse piles near Cordova contain porpoise bones, and John Muir reported Hoonah Tlingits serving the meat to the missionary Samuel Hall Young. Alaska’s circa 83,000 Dall’s porpoises now benefit from the Marine Mammal Protection Act, though in the world’s largest whale fishery, northern Japanese hand-harpoon up to 15,000 annually for food, oil from blubber, and fertilizer from ground-up bones. More die as “commercial by-catch” in salmon drift nets or from internal parasites. Pesticides, contaminants in untreated sewage, and mercury from coal-fired plants accumulate in the fish-eaters’ blubber, weakening their reproductive and immune systems. Underwater noise pollution scrambles their messages, still little understood since, unlike bottlenose dolphins, they don’t thrive in captivity. They may not be very chatty—orcas are always listening. Unbothered, sevento eight-foot-long adults as heavy as a small safe can live up to 20 years, and with just over one million Dall’s porpoises worldwide, the species seems not endangered.
Their gunboat velocity is no mere quirk of nature. It evolved in tandem with that of brainy orcas, their eightton, equally agile nemeses. Dall’s porpoises feel safer far out at sea, with room to maneuver. NOAA fisheries scientist John Moran observed southeast Alaska orcas working together to corner them by blocking their escape route to open water. The porpoises appeared to panic, and the orcas killed several. Shallow bays and estuaries beckon regardless with their irresistible smorgasbord of spawning, schooling herring. Lately, Prince William Sound Dall’s porpoises craving gilled fare have been entering those confined waters more frequently. They’ve become bolder as the number of patrolling orcas has plummeted, one backwash from the Exxon Valdez oil spill.
The late Alaskan marine biologist-essayist Eva Saulitis watched Dall’s porpoises in the sound rising, shiny as lacquer, playfully nudging orca calves. One individual spent a whole summer attached to an orca pod—a resident, fish-eating clan. Some porpoises likely distinguish between whale killers and placid piscivores.
Ironically, these conspicuous athletes rank among the most difficult marine mammals to study. Traditional surveying methods fail because often the subjects, skewing a tally, crowd around research vessels, keen to catch a party wave, to relish its kick.
Michael Engelhard, a landlubber, enjoyed the company of Dall’s porpoises in Prince William Sound during a short stint cooking on a research vessel.