Natural La Jolla: Acrobatic dolphins delight by sheer number

• NATURAL LA JOLLA:

A favorite of ocean enthusiasts everywhere is the dolphin. We tend to think of them as happy creatures, because they appear to wear a permanent smile. Dolphins do have strong social structure in which they are very playful, care for their sick, and attend to family members.

The common dolphin is the most abundant small cetacean in San Diego waters, although it’s actually two species that are difficult to tell apart. Even when their bodies are half exposed porpoising through the water, it can be hard to see the differences in coloration. This is the main way to distinguish short-beaked common dolphins from long-beaked common dolphins — relying on beak length is not precise — unless you saw them side by side, and not swimming at up to 25 mph!

Common dolphins have a black or dark gray back and white belly, but the color pattern alternates mid-body, giving them a crisscross or hourglass appearance. The area behind the eye and above the pectoral fin is creamy yellow white (the thoracic patch), which sharply contrasts with the gray/black back. In general, the short-beaked common dolphin (Delphinus delphis) has more white and yellow in its thoracic patch that blends more smoothly with the belly.

Short-beaked common dolphins prey on schooling fish like anchovies, and do much of their feeding at nighttime with dives as deep as 650 feet. This behavior takes advantage of the vertically migrating fish schools that approach surface waters during the night hours. During the day, common dolphins mainly rest and socialize. This is when they probably sleep, although they sleep differently than we do. Dolphins rest half their brain at a time (unihemispheric sleep), with one eye closed. Then, they rest the other half.

Common dolphins are a staple for whale watching cruises, and a thrill to come upon while boating. Hundreds of common dolphins swimming together (a megapod) may change course to meet up with a boat so they can take turns bowriding. They put on a great show, leaping out of the water, somersaulting and breaching.

Baby dolphins (calves) are a special treat to see as they swim alongside mom, getting milk from her for about 10 months after being born. Being surrounded by a sea of dolphins exhaling loudly, putting on aerial displays and bowriding makes you marvel at their maneuverability and agility. They look like they are having the time of their lives.

Kelly Stewart is a marine biologist with The Ocean Foundation, working with NOAA’s Southwest Fisheries Science Center in La Jolla. Her column about the floral and fauna of La Jolla appears second Thursdays in La Jolla Light. Her e-mail is NaturalLaJolla@gmail.com

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