The waters near the nation's largest port complex have become a bustling feeding ground for increasing numbers of blue whales, putting the endangered animals at greater risk of being hit and killed by enormous ships, according to researchers.

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LOS ANGELES — The waters near the nation’s largest port complex have become a bustling feeding ground for increasing numbers of blue whales, putting the endangered animals at greater risk of being hit and killed by the enormous ships moving in and out of the harbor, according to researchers who’ve been tracking them for nearly two years.

The whales, which migrate along the coast of California and are regularly seen from May to December, are congregating in such numbers in the midst of this virtual freeway of ship traffic that the spot has become “the area of densest concentration close to shore in all of California,” said research scientist John Calambokidis.

Daily appearances by the world’s largest animal feeding along an underwater drop-off outside Los Angeles Harbor have been a huge draw for sightseers. But the underwater buffet of krill, the shrimplike crustaceans the whales feast on, is in the path of a major shipping lane and puts them in danger of being hit and killed by vessels leaving the port.

“While this is a unique and exciting opportunity to have these animals out here, it also puts them at great risk,” said Calambokidis, co-founder of the Cascadia Research Collective based in Olympia, Wash.

Over the past decade, dozens of whales off the California coast have been injured or killed by ships, and scientists think the slowly recovering population of about 2,500 West Coast blue whales are especially vulnerable.

Four blue whales were struck and killed by vessels in 2007 near the Channel Islands National Marine Sanctuary off the coast of Santa Barbara, raising the alarm of federal wildlife officials, who now monitor the whales from the air and use their coordinates to issue notices asking freighters to voluntarily slow down.

With the increase in blue whales near the Los Angeles-Long Beach port complex, Calambokidis said, “now we’re worried about here.”

For two years, Calambokidis has headed the project to photograph, tag and keep tabs on whales that feed near the shipping lanes. Interns for the Aquarium of the Pacific accompany tourists aboard twice-a-day whale-watching trips by Harbor Breeze Cruises to photograph and mark coordinates of the few dozen blue whales that have taken to grazing about five miles off the Los Angeles coast.

As part of the ongoing work to track the whales’ movements through the busy waters off Long Beach, Calambokidis — who looks the part of a seasoned mariner: wind-swept gray hair, gray-blue eyes and a thick beard — searched recently for one of the bright-orange transmitters used to monitor the creatures. The transmitter had been attached with a suction cup to the back of a surfacing blue whale a day earlier.

The 80-foot whale whose tag was scooped from the ocean Tuesday as part of a demonstration for reporters is a regular visitor that has been in the area for about a month.

When researchers tagged the same whale a week before, they downloaded data that revealed a typical behavior pattern. The animal spent most of the day just outside the port, diving as deep as 1,000 feet. After dark, it stayed near the surface, perhaps to rest, and swam to Santa Monica Bay. The tag, which is designed to fall off after less than 24 hours, was eventually recovered in the South Bay.

The GPS tracking device records the whale’s coordinates each time it surfaces, measures how deep it dives and how it reacts to passing ships. Some are equipped with acoustic sensors that record the animals’ low-frequency calls and the rumble of passing freighters. Researchers approach a whale in an inflatable motorboat and use a long pole to press the shoe-size apparatus to its back as it surfaces.

The information collected so far has uncovered a disturbing pattern: At night, the whales spend twice as much time lingering near the surface, where they are most vulnerable to being hit by ships. And they show no sign of trying to avoid approaching container ships.

For reasons that are not yet understood, the whales often draw closer to the vessels, increasing the odds of a collision.

A better understanding of the whales’ behavior in busy waters could help authorities decide how to separate them from ship traffic. Ocean carriers are backing a proposal to alter shipping routes to avoid whale feeding grounds while conservation groups have petitioned the Obama administration for a speed limit through California’s national marine sanctuaries.

Later on the research trip, Calambokidis spots a group of whales surfacing to breathe as a fully loaded cargo ship cruises by. “These whales are in the outbound shipping lane,” he says. Among them is the regular — the same whale they had just retrieved the tracking device from. He can recognize the individual by its skinny dorsal fin and unique pattern of dark blotches, markings that are like fingerprints but even more detailed.

It’s not surprising the abundant food has the creature coming back again and again.

“They’re constantly in this mode of looking for a place to feed,” Calambokidis says. “So when they find a patch of prey, they stick around for a while.”