Traditionally the squid have been found mostly in the warm waters of Mexico's Gulf of California (see a Mexico map).
Before 1997, the underwater videos revealed no Humboldt sightings in Monterey Bay, Robison said.
Large numbers of squid appeared in the 1990s when El Niño weather events brought warm water up from the tropics. Then the squid disappeared again, except for occasional stragglers.
In 2002 they reappeared and have been abundant ever since.
In addition to moving to California coasts, Robison said, the squid appear to be moving southward into the waters off Chile.
Robison thinks the squid are expanding from their normal range because of declining tuna populations.
Historically, he said, the squid and tuna competed for the same prey. In addition, tuna ate baby squid.
But heavy fishing for tuna and other predatory fish is allowing the squid to flourish.
When El Niño currents brought the squid north, Robison added, they found an environment where their predators, such as sharks, were also on the decline.
"They're here to stay," Robison said. "We know they're reproducing. We see little ones."
But, except for their ecological impacts, he added, the squid pose no threat to people.
"There are a couple of odd stories," he said, "but I don't think they're any threat at all."
William Hanshumaker is a researcher at the Hatfield Marine Science Center in Newport, Oregon.
He said that fishers have been finding Humboldt squid at least as far north as the Columbia River, where he believes the squid are hunting sardines. The river forms much of the border between Oregon and Washington State before emptying into the Pacific (see map).
Furthermore, he said, Oregon has seen numerous beachings of dying squid.
One possibility is that the squid are being killed by a toxin called domoic acid produced by certain marine plankton.
The toxin works its way up the food chain when sardines eat the plankton and squid eat the sardines.
The entire situation reveals how strongly different parts of the oceans are linked, said study co-author Louis Zeidberg of Stanford University.
"[You] could consider the entire Pacific as one ecosystem," he said. "Changes you cause in the tropics can ultimately have an effect all the way to the [temperate] regions."
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