Natural La Jolla: Risso's dolphins are playful pests to other cetaceans

NATURAL LA JOLLA:

The Risso's dolphin (Grampus griseus) is a species that is fairly commonly seen off La Jolla, and they are widely distributed throughout the world's oceans. However, they are inconsistent in their distribution or reliability of being spotted.

Risso's like deep water, where they eat squid mainly at night. La Jolla is a great place to spot them, because we have deep water close by in the submarine canyons right off Scripps and La Jolla Cove.

Risso's dolphins are about 9-feet long and gray all over, but they are often completely covered with white scars, which gives them a pale gray-white appearance. The scars may be from interacting with each other, with other species or from encounters with sharp squid beaks.

Their dorsal fin is tall and curved, and they are sometimes mistaken for other species, like pilot or killer whales. They have markings on their dorsal fins that allow for individual identification. Risso's have a very distinct head, with a crease between their head and body. It is blunt and lacking a beak. With their upward curving mouth, they clearly have a dolphin smile.

Indeed they seem to be a very playful species, interacting often with other dolphins and whales. Either they are playing, or pestering! I've seen them bothering migrating groups of gray whales off our coast.

Generally, the dolphins initiate the contact with the whales, surrounding them tightly in a circle, and then brushing past. They may swim directly toward the whales until veering off at the last moment. It may be that the whales are passing through Risso's dolphin habitat, over deepwater canyons and the dolphins are reacting. They may be protecting their herd by checking out the intruder in their habitat or it could be that the younger dolphins are simply playing.

Gray whales usually respond by taking up defensive postures or fleeing and changing course. Other times the whales may react by breaching or slapping flukes, but after a time, the two groups separate peacefully.

Risso's dolphins hang out in small groups (although large groups are possible) and they are very active at the surface of the water. While you are at the beach, you may even see them leaping, slapping their flukes and spyhopping.

— Kelly Stewart is a marine biologist with The Ocean Foundation. Reach her by e-mail at NaturalLaJolla@gmail.com

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