'W e have been lucky enough to see mothers who have delivered their calves early swimming along the coastline," says Crystal De Soto, leader of the whale-watching program at Birch Aquarium. "The calves are born 15 feet long and weigh a ton, but that's very tiny compared to Mom, who's 45 feet long [and can weigh up to 70,000 pounds]. You can almost see the folds in their bodies. The fluke is still floppy, and they haven't gotten the breathing and swimming thing down, so they keep bobbing at the surface." From December 26 to March 31, 2007, the Birch Aquarium at Scripps and the San Diego Harbor Excursion offer daily whale-watching tours, coinciding with the gray whale migratory season.

"They only eat up in the Arctic," explains De Soto. "They eat as much as they can [around 660 pounds a day], build up their blubber, and use that to fuel their whole migration."

Gray whales are bottom feeders, unlike most other baleen whales that feed on organisms like phytoplankton that float in the water. "Their major prey is amphipods, or tube-dwelling crustaceans -- they look like a little shrimp in a kind of sandy parchment tube," says Bruce Stewart, director of programs and exhibits for Birch Aquarium. "[The whales] go down to the bottom, lay on their sides, and suck. They take a big slurp of this sandy milkshake with these shrimp-like creatures in there and use their massive tongues to force out water and sand and keep the food."

The whales travel from the Arctic (from the Bering, Chukchi, or Beaufort Sea) to their breeding grounds near Baja California in "what is arguably the longest mammal migration," Stewart says of the near 10,000-mile-long trip. The breeding grounds -- shallow lagoons known as "calving lagoons" -- are located off the coast of Baja California. The shallow water of the lagoons protects the whales from predators and currents, and the warmth of the area is conducive for birthing.

Stewart remembers dolphins riding the bow, or front, of the boat on at least one excursion last season. "As the boat travels through the water, it builds up a crest of wave in front that pushes off to both sides. This is called the bow wake. Dolphins love riding right off the bow; they weave back and forth and zip past each other. What you'll sometimes see is, if a whale is moving, really heading somewhere and not meandering around, a whale creates a bow wake like a boat does, and dolphins will sometimes ride the bow wakes of whales. This suggests dolphins have been riding bow wakes longer than there have been boats."

Whale watchers are likely to see whales exhaling, or blowing. The National Park Service explains the phenomenon best: "When warm, moist air exhaled from the animal's lungs meets the cool air at the ocean surface, it creates the bushy column we call a blow, or spout." Blows can be up to 15 feet high and remain visible for up to 5 seconds. "If it happens to be foggy, sometimes the skipper will shut down the engines and listen, because you can sometimes hear the blow," says Stewart. "It sounds like an outward whoosh, then almost a whistling gasp, like a forceful sigh and quick breath in, but echoey, like the sound of wind coming through a tunnel." While traveling, the whales will blow several times while staying close to the surface. But, says Stewart, "when you see them arch their back and you see the bumps, or knuckles, which is the vertebrae beneath the skin, and they roll up out of the water and lift their flukes, you'll see the big whale tail come up out of the water to help drive the whale down on a deeper dive."

Last season, De Soto witnessed a rare breaching while out on the boat. "The whales are typically swimming and breathing. Anything beyond that regular behavior is fabulous. Breaching is one we don't see very often -- that's when they leap out of the water and splash down."

On their return trip to the Arctic in the latter part of the season, some gray whales will hug the shore for safety. "Some pods of orcas will attack and kill -- especially the baby gray whales," says Stewart. "When whales go near the kelp beds [about 100 yards from shore], the air and waves breaking on the beach help disguise their sonar echoes [from the orcas]. If you see whales very close to shore, chances are you're seeing a mother and a calf."

The most common misconception De Soto has encountered on whale-watching excursions is the belief that whales are fish. "A lot of people are kind of surprised that these animals are coming to the surface," she says. "They don't know that whales are mammals." -- Barbarella

Daily Whale-Watching Expeditions Tuesday, December 26, through Saturday, March 31 9:45 a.m. to 12:45 p.m. or 1:30 p.m. to 4:30 p.m. San Diego Harbor Excursion 1050 North Harbor Drive San Diego Bay Info: 619-234-4111 or sdhe.com/ san-diego-whale-watching.html