Mexican fishing fleets call them diablos rojos, or "red devils" - and when Stanford University graduate student Julie Stewart wrestles the first Humboldt squid aboard our research vessel, the Fulmar, in California's Monterey Bay, it becomes obvious why.

This beast is angry, and has flashed from white to a deep maroon. It's nearly 1.5 metres long, including the tentacles, which flail in Stewart's hair until she can offload the catch into a cooler filled with seawater. That only gives the squid ammunition, as it can now fire a powerful jet of water and ink at anyone who strays into its sights.


"Ink in your eye stings," warned Stewart earlier. She is also careful to avoid the animal's sharp beak, which can deliver a nasty bite. Within a minute or so, Stewart and her colleague John Field of the US National Oceanic and Atmospheric Administration have attached an electronic device to one of its fins. It's time to return the devil to the depths.

We are out on the Fulmar on this bright December day to learn about the animals' movements. When the device detaches and surfaces, it should send a radio signal to reveal its location, and transmit data revealing how the animal has been migrating up and down in the water. This is key to understanding why the Humboldt squid (Dosidicus gigas) has invaded the waters off central California, and how it may affect the region's valuable fisheries.

While the total size of the invading horde is still unknown, observations from autonomous submersibles and sonar indicate that they often swim in dense schools of at least 100 individuals. "We've seen densities as high as eight per cubic metre," says William Gilly, Stewart's supervisor at Stanford's Hopkins Marine Station in Pacific Grove.

Humboldt squid are voracious predators that historically have inhabited the lower latitudes of the eastern Pacific, from Mexico down to Chile, apparently venturing further north only in conjunction with the disturbance of currents and sea temperatures known as El Niño. But since 2002, they have established themselves off the coast of central California. Their arrival coincided with a decline in Pacific hake, which are among their prey. "That's a huge commercial fishery," says Gilly.

So far, radio-tagging indicates that the squid spend the daylight hours near the top of an oxygen-poor layer in which their prey try to hide. Here in Monterey Bay, this layer lies some 500 metres below the surface, and its migration upwards could be one reason for the invasion. "As seas get warmer, there's a lot less mixing, so oxygen in the atmosphere has a harder time getting transported down," Gilly explains. But climate change is unlikely to be the whole story: removing rival predators such as tuna by fishing has also been implicated.

By sunset, we have only landed the one animal that Stewart tagged and released. With darkness our fortunes begin to change, just as Gilly said they would, for this is when the squids' prey leave the oxygen-poor layer and migrate to the surface.

Landing a Humboldt squid requires a line with a spiked lure called a "jig". Those we are using are fluorescent, and are charged under the Fulmar's halogen lamps before being lowered into the black waters. From here on, brute strength is needed. "When they jet, you really feel it; they really want to pull the pole right out of your hands," says Field.

Soon, my rod bends sharply, and I'm doing battle with the red devil. But I lose the beast by letting the line go slack, so decide to leave it to the experts. In total, they bring 14 on board. One more is tagged and released, two are kept in a tank for study back in the lab, and others are measured, sexed and dissected so that Field can study their stomach contents.

Unfortunately, the squid don't reveal their secrets easily: one radio tag pops off after only a day, and the other never sends the anticipated signal. But previous efforts have revealed that the squid can cover huge distances: one tag surfaced off Tijuana, Mexico, after 17 days - a journey that required the animal to cover over 20 miles a day.

Still, the landed squid provided further valuable data points in the effort to monitor this marine invasion. And there was calamari steak for anyone who wanted it.