The normal range of the squid is from Mexico to Peru. But their range expands periodically to Alaska in the north and Chile in the south, depending on the population size and oceanographic conditions.
"There are enormous numbers of squid in the ocean," Gilly said. "If whatever is killing them started killing them significantly more, we'd be up to our necks in squid on these beaches."
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In 2002 thousands of squid filled the beach at La Jolla Cove north of San Diego, California. Last month at least 1,500 of the squid ended up on beaches between San Diego and Los Angeles.
"Two years ago, they were small, 2 to 3 feet [60 to 90 centimeters] long, instead of the 4-to-5-feet-long [120-to-150-centimer-long squid] that stranded this time," said Linda Blanchard, laboratory director at the Ocean Institute in Dana Point south of Los Angeles.
Scientists say this suggests that the squid have remained in the area since the 2002 event, moving farther north and reproducing in California waters.
Sarah Allen, a senior science advisor with the National Park Service at Point Reyes, California, said around 30 squid died on Drakes Beach, north of San Francisco.
The squid did not seem to wash up dead on the shores but instead appeared to swim into shallow water where waves carried them onto the beach. According to Gilly, the Hopkins Marine Station biologist, that suggests there may be something neurologically wrong with the squid.
"They appear to be mentally deranged, not physically ill. It's clearly something abnormal," he said. "They're big-brained, intelligent creatures. It's not like they're attracted to the lights of the city or anything like that."
He speculates that the squid are eating something toxic and that this leads to their aberrant behavior.
The toxin, Gilly said, could be domoic acid, which is produced by several species of microscopic algae known as diatoms.
Studies have shown that domoic acid causes convulsions and even death in marine mammals that feed on the same things that squid do. The acid is found in organisms, including sardines and krill that eat the algae and that are in turn eaten by the squid.
"In addition," Gilly said, "I suspect there is an environmental wild card that augments the effects of the toxin, most likely thermal stress to the squid from spending too much time chasing food and eating in relatively warm surface water."
Eric Hochberg, a scientist at the Santa Barbara Museum of Natural History, said bad weather may have played a part in the squid deaths.
"My sense is that the squid in this most recent stranding moved into inshore waters to feed and perhaps were affected by low-salinity and high-turbidity [sediment-rich] waters near shore, which were the result of very heavy rains immediately preceding the stranding event," Hochberg said.
Some experts have suggested that the squid, which are found in greater numbers off South America, might be following a warm California current. But that does not explain why they are moving closer to land.
Gilly's electronic tagging work shows that the species normally spends very little time right at the surface.
"We should figure out what's happening," the biologist said. "This species is an important part of the ecosystem, both as a major predator and as prey for even larger predators, such as sperm whales."
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