Chemical Blamed for Fish Deaths : Sepulveda Basin: The unknown substance has killed more than 1,000. Officials blame illegal dumping at a Granada Hills shopping center.

An unknown chemical has killed more than 1,000 fish in the Sepulveda Basin, perhaps coming from an illicit dump site miles upstream in Granada Hills, officials said Thursday.

Dead fish were strewn over a quarter-mile stretch of Bull Creek south of Victory Boulevard following the episode Wednesday, and more fish carcasses littered the Los Angeles River as far as a mile below the confluence with the creek.

“It was an extensive fish kill,” said Stephen C. Moe, San Fernando Valley water manager for the Los Angeles Department of Recreation and Parks, which cleaned up much of the mess Thursday.

Although mostly minnows or inch-long mosquito fish that feast on mosquito larvae, the dead fish included many six-inch carp and catfish up to 14 inches long, Moe said.

The creek water had no odor or discoloration, but high alkalinity readings revealed the presence of contaminants, investigators said. They since have theorized that the material was flushed into the creek from a parking lot at North Hills Shopping Center, where they found signs of illegal dumping.

Late Wednesday, state Department of Fish and Game and Los Angeles County hazardous materials officers walked and drove the concrete-lined portion of the creek north of Sepulveda Basin, seeking the source of the pollution. Stopping periodically to take pH readings, they followed a trail of high alkalinity to the vicinity of the shopping center, about 4 1/2 miles upstream.

In the back of the parking lot overlooking the channel, they saw toilet paper and human waste--signs that a vacuum septic pumper had been emptied at the site.

Although human waste would not be responsible for the high pH, officials said other material might have been mixed with the waste, or that other midnight dumpers might have used the site.

“The conjecture is that . . . it was material dumped clandestinely into the flood-control channel . . . late at night,” said James Odling, a hazardous materials specialist with the Los Angeles County Fire Department who took part in the search.

Police investigators are expected to follow up, but officials said the culprit is unlikely to be found without a witness and a license plate number.

Officials said they did not know if there will be lasting effects, partly because they don’t know what was dumped.

“We have seen signs of live fish, so . . . it did not kill them all,” Moe said.

Moe said there was no visible harm to aquatic vegetation, though such damage typically would not show up right away.