A mysterious force has stunted the growth of Humboldt squid in the Sea of Cortez, and marine biologists suspect a change in the weather is to blame.

The ravenous animals normally weigh up to 30 pounds when they spawn at 12 to 18 months of age, but Stanford biologists have discovered a group of the squid that weigh only a pound apiece and spawn at less than 6 months old.

The rubbery animals with their long tentacles are a precious livelihood for Mexican commercial fishermen along the Gulf of California, and they're a prized prey for gringo sportsmen.

But to William Gilly, a marine biologist at Stanford's Hopkins Marine Station in Pacific Grove, they're a scientific puzzle.

In a paper recently published in the journal Marine Ecology Progress, Gilly said he suspects the squid's shrinkage was caused by the abrupt warming of the gulf's water as a result of an El Niño that was detected during the 2009-10 winter.

The El Niño phenomenon, also known as the Southern Oscillation, occurs periodically when high surface air pressure over the Western Pacific pushes temperatures up throughout the tropical Eastern Pacific, including the Gulf of California, causing water temperatures to rise.

In September 2009, Gilly said, he and his colleagues cruised the Gulf of California, better known as the Sea of Cortez, and found abundant squid in their normal spawning grounds and their usual size.

"But in May, a year later, we couldn't find any normal-sized squid in their normal spawning grounds," he said. "Instead, the area was full of smaller squid - really small."

A month later, Gilly said, the squid were still very small and spawning in what was formerly the normal spawning area for normal-size squid, while one group of full-size ones had migrated and were thriving 100 miles north around the gulf's Midriff Islands.

"No one really understands the El Niño phenomenon," Lilly said, "but it seems to be the best explanation - a change in the temperature is enough to change the total environment for squid or any other living organisms in the gulf."

Lilly noted that a similar phenomenon seems to be occurring along the Pacific coast off Santa Barbara where smaller squid are also being found.

Gilly agreed that it is possible that small increases in sea temperatures in both the Sea of Cortez and California's coastal areas as an early result of global climate change may already be affecting the Humboldt squid population.

"That would be astounding," he said, "but it's certainly possible, that the smaller squid are just adapting more quickly to climate change - maybe even making genetic changes." But that, he agreed, would merely be speculation.

-- To see a video report, go to: links.sfgate.com/ZLET