The popular “Seasons in the Sanctuary” poster illustrated by Doug Ross and produced by Crystal Birns is a beautiful rendering of birds, mammals, reptiles and fish that migrate through the Monterey Bay National Marine Sanctuary.

One of the animals described there which you”ve probably never seen is the Pacific leatherback sea turtle, which just recently was named California”s official marine reptile.

Weighing 1,000 pounds and stretching to a length of 6 feet at maturity, it is the largest turtle species and is thought to live to be 30 years old or more. They”re called leatherbacks because instead of a hard shell, the animal”s top side is a network of small bones covered by a layer of oily skin.

Their bodies have evolved to efficiently move long distances in often rough waters and dive as deep as 4,000 feet. But their numbers are declining so much that the International Union for Conservation of Nature, which tracks the vulnerability of species, has listed them as “critically endangered.”

There are two populations in the Pacific that forage on a diet consisting mostly of jellies. The group that feeds off the coast of California from June to September will swim west — mostly to Indonesia — where they nest on sandy beaches and lay their eggs.

Another population consists of turtles that swim off South America and nest on shores in Mexico and Central America.

Instead of using one familiar route, they swim from the tropics to high latitudes following their food, winding up off California to gorge themselves on our abundant jellies in the autumn months. The typical one-way trip is believed to be about 6,000 miles, although there are reports of one having made a meandering voyage of more than 20,000 miles.

Wallace J Nichols, of Davenport, has been researching Pacific sea turtles for 25 years from Mexico to Indonesia and has been a Research Associate at the California Academy of Sciences since 1999. He says the Pacific leatherback population has declined 95 percent in the past three decades. Just a few thousand nesting females remain, raising concern about their ability to grow their numbers back where they should be.

“Twenty-five years ago I could walk a leatherback nesting beach in Mexico and encounter one nesting turtle after another,” Nichols said. “Now you can walk all night searching for one and get skunked.”

Because they have such a wide-geographic range and need both land and sea to survive, Nichols said he believes that the state of leatherbacks and other sea turtles is an indicator of coastal and ocean health.

“Sea turtles interact with our pollution, our plastic, our fishing gear, our boats and our coastal development,” he said. “They”re impacted in a variety of ways by climate change and our changing, warming, more acidic ocean.” Another threat to Pacific leatherbacks is single-use plastic bags, which, when in the water, resemble jellies.

“Not to oversimplify, but as go the sea turtles so go the oceans. And as go the oceans, so go we humans,” Nichols said. “Although leatherbacks are rarely seen near shore California, they are residents. But as their populations drop, they are harder to find, even with dedicated spotting planes and ships.”

Nichols and a group of his turtle colleagues have created SEEturtles.org to help people who want to travel to see sea turtles find the best places to do it. He puts it this way: “Spending time with sea turtles changes one”s perspective on our planet and makes people happy.”

Dan Haifley is executive director of O”Neill Sea Odyssey. He can be reached at dhaifley@oneillseaodyssey.org.