Around 6 a.m. Monday a passerby noticed a sea lion in the Colma Creek Drainage Canal in the 400 block of North Canal Street, South San Francisco police Cpl. Bruce McPhillips said.
The sea lion "appeared to be in distress," McPhillips said.
Firefighters responded and observed the animal until Marine Mammal Rescue Center personnel arrived at about 1:30 p.m., he said.
Rescue center personnel located the 8-foot sea lion at the bottom of the muddy canal, about 50 feet down, and used a crane to lift the animal to street level.
The creature "had really labored breathing and was very lethargic," Marine Mammal Rescue Center spokeswoman Mieke Eerkens said.
The sea lion was suffering severe seizures and was soon diagnosed with domoic acid poisoning, caused by exposure to toxic algae. The condition affects animals' brains and can cause permanent brain damage, Eerkens said.
The sea lion most likely swallowed some fish that had eaten the toxic algae, Eerkens said.
"This is something we see quite often," she said. "When (sea lions) are acutely affected, we can sometimes treat them and release them, but when it becomes a chronic case it means their brains have been permanently damaged and it causes continuing seizures."
The sea lion's brain damage most likely caused it to swim into the canal, since the disease affects the part of the brain that controls navigation, Eerkens said.
In July, a sea lion found in a Santa Clara aqueduct was euthanized after also being diagnosed with domoic acid poisoning. Marine Mammal Center officials said that animal, nicknamed San Tomas after the expressway near which it was found, had also lost its way because of the poisoning.
The sea lion found in South San Francisco was euthanized this morning since damage caused by the poisoning was found to be irreversible, Eerkens said.