Climate Change May Make Humboldt Squid Easy Prey

One of the most formidable predators in the Pacific ocean, the Humboldt squid, may become more vulnerable to attacks from other marine beasts as changing water conditions make them more sluggish swimmers, a study has found.

As human activities increase the level of carbon dioxide in the atmosphere, the level in the oceans also rise. Scientists believe this will make the squid lethargic, and so less able to out swim their own predators, including sperm whales that feed heavily on the creatures.

Humboldt squid (Dosidicus gigas) can grow up to 2m long and hunt in shoals of more than a thousand. Scientists have recorded the creatures, which are found in waters from Alaska to Patagonia, swimming at more than 20kmh (13mph).

In the daytime, the squid are forced to dive deep to prey on lantern fish, but because deeper waters are starved of oxygen, the squid must return to the surface at night to recover.

Rui Rosa at the University of Lisbon netted Humboldt squid off the coast of California and transferred them to water tanks aboard the team's research vessel to examine how they coped with different levels of carbon dioxide in water.

Rosa found that when the squid swim in oxygen-starved waters, they survive by slowing down their metabolism by up to 80%.

When Rosa simulated carbon dioxide levels predicted for oceans at the end of the century, he found the squid slowed their metabolism again, by around 30%, and became almost half as active.

"Their metabolism will drop with ocean acidification because there is more carbon dioxide in the water," Rosa said.

"The squid will be more lethargic and so more vulnerable to their predators because they won't be able to escape them any more." The study appears in Proceedings of the National Academy of Sciences.

Atmospheric carbon dioxide levels have risen from pre-industrial levels of 280 parts per million to more than 380ppm today, much of which is absorbed by the oceans.

Rosa said more acidic waters will also constrict the habitat of the Humboldt squid, by making them less able to hunt at depth, or in surface waters, which could have serious knock-on effects for the wider marine ecosystem.

"These squid will probably have to migrate to find more suitable waters, and since they are the main prey for sperm whales, that could significantly alter the marine foodweb," Rosa said.