|A breaching humpback whale. Photo by Sophie Webb.|
Marine mammals have evolved morphological, physiological, and behavioral adaptations in order to thrive in a viscous, three-dimensional medium. Most species are streamlined to reduce drag as they swim and dive. Some of the fastest swimmers in the Gulf of the Farallones are predators such as killer whales that surge to 30 mph or Dall’s porpoises that leave rooster-tail wakes at 35 mph.
Species that dive deep have evolved ways to cope with darkness, water pressure, and predators. The sperm and beaked whales can propel themselves more than a mile deep, to the base of seamounts and along the face of the continental shelf. To navigate in waters of limited or no light, marine mammals may depend on sound and biosonar. Baleen whales use low-frequency moans, thumps, chirps, and whistles to communicate (across thousands of miles, in the case of blue whales). Sound conveys subtle information about the ocean environment, which many cetacean species need and can sense.
|A lunge-feeding humpback whale has pink krill inside its stretched ventral folds. Photo by Sophie Webb.|
Today the proliferation of human activities on the ocean produces a cacophony of noise underwater, from ship propellers to human sonar to fishing gear. This can pose big challenges for marine mammals trying to feed, communicate with others of their kind, breathe or rest at the surface—or potentially dodge a ship. Ship strikes are one of the major causes of mortality of large whales, and it’s not hard to imagine how this might occur. When focused on lunging for a mouthful of krill, whales may not detect the sounds of ships transiting overhead at speeds exceeding 20 knots.
Human knowledge of the realm that marine mammals inhabit is growing. Along with surveys from vessels, to determine whales’ abundance and distribution, we are gathering masses of data on oceanography to understand the context for such sightings. Not surprisingly, marine mammals congregate where prey are concentrated.
PRBO’s work at sea with NOAA marine sanctuaries in the ACCESS program has helped scientists determine that marine mammals feed throughout the region off our coast. There are also “hot spots” where certain species predictably congregate to forage, including areas between the Farallon Islands and Point Reyes and the mouth of San Francisco Bay. Findings like this help us delineate where and when there are concentrations of both ships and whales—and develop new levels of protection for whales from ship strikes.
Aboard an ACCESS research vessel, observers can only imagine a sperm whale scanning with its biosonar for giant Humboldt squid along Cordell Bank. A hydrophone might detect the whale’s presence below the sea’s surface, but on the bridge of the ship, the observers are unaware of its existence. Several humpback whales’ tails that just disappeared below the surface may mean a group diving together to concentrate balls of krill by blowing blasts of bubbles. As they rise through the swarm of prey, each whale lunges forward, oblivious to much else, to gulp down a mouthful of water and krill. In the distance a container ship cruises toward the San Francisco harbor. The whales may pause their feeding activity until the sound recedes, and soon the ship’s pilot may hear a Coast Guard alert about the presence of whales nearby.