BioEd Online

Mexican fishermen call it diablo rojo — the red devil. A film-maker wears armour when he goes near it. And the Humboldt, or jumbo, squid (Dosidicus gigas) is on the move, swimming north from the tropics and eating its way into commercial fish stocks.

Changing ocean conditions and fewer predators — coupled with the squid's ecological flexibility — are the likeliest reason for the species' expansion, says Bruce Robison, a marine biologist at Monterey Bay Aquarium Research Institute in California, who has led a survey of the animal's movements.

Humboldt squid can grow to 50 kilograms and 2 metres. They have more than a thousand barbed suckers to catch their prey and can swim at 43 kilometres per hour.

"They'll eat anything they can catch," Robison says — including each other. "When one member of a group is injured they become lunch for the others."

North to Alaska

Originally, the squid lived only off the Pacific coast of South America. But in the 1970s, Mexican fisherman began to notice the animals further north in the Gulf of California. In the 1990s, California anglers started catching them and the squid has been spotted as far north as Alaska.

To map the animal's migration, Robison and his colleague Lou Zeidberg analysed 16 years of deep-sea video footage captured by unmanned submersibles off the coast of central California.

The videos showed a surge in squid numbers in 1997, an El Niño year when oceans warmed and currents reversed — flowing from south to north. The squid then vanished until 2002, another El Niño year. But this time, they stayed for good.

Each time the Humboldts showed up, hake, the most abundant commercial fish on the West Coast, disappeared from the video footage. Squid stomachs contain lanternfish — their food of choice in Mexico — but hake as well, says Zeidberg. "They can alter their diets based on what's around," he says.

Not picky about its food

The march north can't be explained by warmer water alone, the team concludes. When the Pacific cooled after the 2002 El Niño, the animals stayed and reproduced. And even in the tropics, Humboldts live in cold water 1,000 metres down.

Meanwhile, the squid's predators, such as tuna and shark, have declined.

Bill Gilly, a marine biologist at Stanford University who is leading an effort to tag and track Humboldts, says their flexibility has allowed them to thrive while other more specialist animals fade. "It's likely that this is a common theme over all the world's oceans," he says.

Diver and film-maker Scott Cassell, of Escondido, California, says he has had hundreds of run-ins with the jumbo squid, from curious taps to charging attacks. Once, a squid yanked his arm out of its socket. "My shoulder's never been the same," he says.