The largest creature in the history of Planet Earth, the blue whale, shows us only segments of a body that, in many cases, measures more than 90 feet in length. It announces itself with a spout of water that reaches up to 30 feet into the sky above Monterey Bay.

The enormous mammal breaches the surface head-first, then does a graceful, round, slow-motion dive, as if tanning itself one vertebra at a time. The tail briefly appears, like the caboose of a long, pokey train, providing a fitting climax to a brief but remarkable show.

“That whale’s tail is at least as wide as our boat is long — and the boat is a 65-footer,” marvels Kate Spencer, a naturalist and sea-life illustrator who mans the microphone and acts as a tour guide for Nancy Black, skipper of Sea Wolf II.

The bay sparkles on a sun-splashed Saturday morning, and the folks who paid $40 apiece have gotten every penny of their money’s worth from Monterey Bay Whale Watch, a charter service that cruises daily from Monterey’s Fisherman’s Wharf. Before the five-hour tour concludes, 50 passengers will see 18 blue whales, two fin whales (the second-largest animal in the world), and six humpbacks, along with 40 Risso’s dolphins, otters, seals and sea lions, and a low-flying albatross.

The trip, skippered by Black, a marine biologist and local whale authority, was a fundraiser for the Monterey Bay chapter of the American Cetacean Society (ACS), a nonprofit organization dedicated to educational, conservational and scientific pursuits for the purpose of expanding scientific knowledge of whales, dolphins, porpoises and related creatures.

“I’ve been on more than 100 of these trips in my lifetime, and I’d give this day an ‘A’ because of the good weather and the fact that we’ve seen a lot of whales, says Jerry Loomis, president of the Monterey chapter of the ACS. “I used to be a ranger at Point Lobos and I worked on all the whale-watching boats for years, so I’ve been out a lot, and this is a really good day.”

Blue whales are as long, front to back, as the largest dinosaur that ever walked the earth, and weigh much more — between 100 and 150 tons. They plunge to feed on four tons of krill per day, staying submerged for minutes at a time. If they’re napping, their underwater slumber can extend to 30 to 40 minutes.

And although they look like a long, slow-moving train as they come up for air, then dive again for food, blue whales are remarkably fast.

“The blue whale can travel at up to 20 knots (23 mph),” Spencer tells her whale-watchers. “A humpback whale can swim at up to 10 knots, and the boat we’re on is capable of 12 knots. So we can keep up with a humpback, but we’ve got no chance to chase down a blue whale that’s swimming at top speed.

The blue whale parade to which the passengers of Sea Wolf II were treated on Saturday was unusually fruitful — it’s not unusual for watchers to see no blue whales at all — and exhilarating for the naturalists, as well as the patrons.

Blue whales were hunted almost to extinction at the end of the 19th century and beginning of the 20th, and are still an endangered species, even though they’re apparently making a comeback. An estimated 10,000 blues are in our oceans today, and about 2,000 are summering along the California coast, fattening up on an abundance of krill before heading south to Costa Rica for winter mating.

Although collisions with ships and whale poaching continue to dent the population, whales are rallying. The Orca whale — also known as the killer whale — is the only natural predator of the blue whale. One spotted on Saturday had numerous scars on its tail and torso.

“The data on blue whales is that the population hasn’t really changed a whole lot in the last 10 to 15 years,” says Spencer, who has spent the last 13 summers working for Black. “There have been a couple of different studies recently, and those studies showed different results. Right now we aren’t really sure which is more accurate, but I can tell you that we’re seeing a lot of blue whales right now in Monterey Bay.”

Humpback whales, meanwhile, are believed to be increasing in number by about 7 percent per year.

Dennis Taylor can be reached at dtaylor@montereyherald.com or 646-4344.