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Monday, September 8, 2014

Not so common

Another highlight from the boat trip to Bodega Canyon and Cordell Bank yesterday was a group of Common Dolphins (Delphinus sp.).  Although I'm familiar with this species from the Atlantic Ocean, this is the first time I've seen this dolphin on a trip off Bodega Head.

In contrast to Pacific White-sided Dolphins, Common Dolphins have a more pronounced beak and a darker dorsal fin (see above) and a striking hourglass-like pattern on their sides.  The front portion of this hourglass is colored with a beautiful golden hue:

I'm tempted to call these Short-beaked Common Dolphins (Delphinus delphis), but I'm not 100% certain about the identification.  If you have more experience separating Short-beaked from Long-beaked Common Dolphins, please weigh in!  

Don Roberson does a great job at describing some of the differences on his web page about Monterey Bay dolphins and porpoises.  A Short-beaked Common Dolphin has (1) a shorter beak, (2) a brighter front panel (of the hourglass), especially between the pectoral fin and the eye, (3) a dark eyeline between the beak and the eye contrasting with the bright white front panel, and (4) a dark line extending from the pectoral fin to the vent.  

Short-beaked Common Dolphins are also known for spending time in deeper, offshore waters, whereas Long-beaked Common Dolphins are more often in shallow, coastal waters.

Here are more views of some of those characteristics: the dark dorsal fin, the golden color along the side, the bright area behind the eye, and the short beaks:

While the dolphins were bowriding, we noticed a smaller animal among them — a young calf!  This young dolphin was grayer than the adults.  In the pictures below, look for the calf swimming above the adults and note the much grayer coloration overall.

Although their range includes areas farther north, Short-beaked Common Dolphins are only seen occasionally north of Santa Cruz.  Their occurrence here this year may be associated with warmer ocean water.

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