GETAWAYS : Overseas to a Small Island : The mainland becomes a distant memory as you cross from Ventura Harbor to Anacapa, part of America’s only oceanic-island national park.

You almost expect to hear a Cousteau film voice-over. The location is Anacapa Island, 12 miles off the coast in Channel Islands National Park. The scene is a gently sloped, treeless plateau of 100 acres with 120- to 200-foot-high cliffs dropping away all around. The action: You are walking through a sea gull rookery.

Spaced at discreet intervals from one another, white-and-gray Western gulls dot the landscape.

Most sit on nests in the low ground cover, some right beside the trail. On other islands, birds must nest on cliffs or offshore rocks to escape land predators. Here, predators are just the rats whose ancestors came ashore after shipwrecks--rarely a threat to eggs and chicks.

Gulls squawk warnings as you approach. Some stand on the ground, guarding nests. Others swoop about, repelling raids by ravens or other gulls or bringing food home to the young. Now and then, you see a nest’s two or three eggs of olive speckled with black. Or fluffy hatchlings in the same camouflage, motionless under a feathered breast or wing. Linger too long and a mother may make you feel guilty by waddling away, exposing her eggs until you move on. After eggs hatch, gulls get aggressive and one might divebomb you. (Hold a cap above your head, since they nip at the high point.) Nesting activity peaks in May, but lasts into summer while chicks grow.

The rookery is the seasonal highlight on Anacapa, a five-mile-long chain of three islets with an area of only 1.1 square miles. The nearest in America’s only oceanic-island national park, it is an hour and a half or so by boat from Ventura Harbor.

There the excursion departs every day from a landing next to the park visitor center. It has four parts: the channel crossing, a close sail-by, a walk on the island and the return. En route over a sometimes heavy sea, you watch marine birds, dolphins, sea lions and flying fish. When two dozen dolphins surface in a lively water ballet, no one objects if the vessel changes course for awhile to keep them company.

Closing in, you observe spectacular Arch Rock at the island’s eastern tip. And vertical cliffs as riddled at the bottom with caves and crannies as an English muffin. Higher up, patches of snowy white mark pelican and cormorant colonies. A few harbor seals loll on rocks at seaside. So do crowds of jovial, clamorous sea lions--appearing blond to white when dry in the sun but brown or metallic gray when wet.

Built by the Coast Guard for its former lighthouse station, East Anacapa’s landing is in the only deep-water, sheltered cove. From the boat, you step up onto a platform, then climb 153 steps to the island top. There, neat white buildings house a park ranger’s residence, a tiny museum, a workshop and water tanks. There are picnic tables and restrooms, but no water for visitors, so bring your own.

A broad concrete rainwater-catchment apron nearby, now used as a helicopter landing pad, acts as a noisy, crowded singles club for gulls. Off to the east, a white lighthouse crowns a high point and a foghorn booms, rain or shine, at 14-second intervals. You are warned to stay away or risk hearing damage.

With the sea in sight all around, you get a real sense of being on an island. Three hours ashore is ample time for a picnic lunch and a stroll on two miles of trail. That takes you through fields of native and alien plants, some flowering, most turning dry by now, to four dramatic overlook points. The one at Landing Cove has protective railings but the others do not, so keep away from cliff edges.

Midway on the north side, pelicans and gulls sedately soar the wind’s updraft at cliff-top as foreground to Cathedral Cove. You look down on its dark kelp beds in clear water, sea caves, offshore rocks and barking sea lions on cobblestone beaches. Out at the west end, you look over an impassable knife-edge ridge down to the break between East and Middle Anacapa. Beyond lie West Anacapa and Santa Cruz Island. Midway on the south side, you peer down the island’s highest, steepest cliffs, carved by violent South Pacific storm waves.

Seen back across the channel, mainland America seems distant from this island California. Then too soon, the time comes to return.

WHERE AND WHEN

* Getting there: Boats leave at 9 a.m. from Ventura Harbor for six- to eight-hour East Anacapa trips.

* Fare: $37 adults, $20 children 12 years and younger.

* Information: For information and reservations, write or call the national park boat concessionaire: Island Packers, 1867 Spinnaker Drive, Ventura 93001, (805) 642-1393.