The whales, which migrate along the coast of California, are congregating in feeding grounds near the port in such numbers the spot has become "the area of densest concentration close to shore in all of California," research scientist John Calambokidis told the Los Angeles Times.
The underwater buffet of krill, shrimp-like crustaceans that are the whale's preferred food, is in the path of a virtual freeway of ship traffic that puts them in danger of fatal collisions with huge cargo ships, the newspaper said.
The gathering of whales outside Los Angeles Harbor has been a huge draw for sightseers.
"While this is a unique and exciting opportunity to have these animals out here, it also puts them at great risk," said Calambokidis of the Olympia, Wash.-based Cascadia Research Collective.
Dozens of whales have been injured or killed by ships of the California coast in the last 10 years, and scientists think the slowly recovering population of about 2,500 West Coast blue whales is especially vulnerable.
The abundant food has the creatures coming back to the area of the port again and again, Calambokidis said.
"They're constantly in this mode of looking for a place to feed," he said. "So when they find a patch of prey, they stick around for a while."