Oil Spill New Threat to Endangered Fish : Wildlife: The tiny stickleback is also jeopardized by frogs, fast-growing plants and the drought.

A ruptured pipeline three weeks ago that sent 75,000 gallons of oil oozing down the Santa Clara River from Valencia to Ventura County now threatens one of the last three habitats of a tiny endangered fish.

The unarmored threespine stickleback has found refuge in three areas of the Santa Clara River. But it is being threatened by the fifth year of drought, an exotic river-clogging plant and a voracious African frog. And now the oil spill has added another danger.

Four researchers on Thursday surveyed a pristine pool filled with sticklebacks a few miles upstream from where the oil was spilled into the river near Valencia in west Los Angeles County.

Despite the drought, the pool was lush, and the sticklebacks that they netted were said to be healthy and ready for the mating season. Thomas Haglund, a research biologist from UCLA, held up a male stickleback, its eyes bright blue, its throat a glowing red.

“The males get a very distinctive reproductive color,” Haglund said.

The Santa Clara and its tributaries provide the last remaining habitats for the fish, once found in the Los Angeles and Santa Ana rivers. The stickleback was placed on the endangered species list in the mid-1970s.

But natural and man-made calamities have continued to work against the fish. Some ponds in Soledad Canyon, where Haglund and his colleagues worked Thursday, have shrunk with the drought. The canyon also is home to African clawed frogs, which have no natural enemies in California and are decimating the stickleback population.

The frogs are descendants of escaped lab animals or failed aquarium pets released by owners, guessed one of Haglund’s associates, Jonathan Baskin, a Cal Poly Pomona biology professor.

Another stickleback habitat in San Francisquito Canyon near the headwaters of the Santa Clara is being destroyed by arundo, a non-native plant similar to bamboo that quickly clogs the shallow streams that sticklebacks prefer. The U.S. Forest Service is studying ways to eradicate the plant.

Parts of the third remaining habitat, which extends from western Los Angeles County into eastern Ventura County, were severely damaged by the oil spill.

“The probability is good that the spill has had significant adverse effects on the fish,” said Bruce Eliason, manager of environmental services for the California Department of Fish and Game.

The spill was caused when an aging underground pipeline ruptured Jan. 31, releasing black goo into the river. Almost 200 birds and animals were killed from contact with the oil, which crept more than eight miles down the river to a point south of Piru.

It is still unclear how severely the stickleback population will be affected, but the researchers examining the pool near Valencia said they are worried.

Still, they hope to relocate some young sticklebacks born this spring to cleaner waters in the Santa Ana River in San Bernardino County. Baskin and Haglund have been promised about $12,000 for the move from the state Department of Fish and Game.

Baskin said the researchers are waiting for the U.S. Fish and Wildlife Service to approve the experiment.

Times staff writer Joanna M. Miller contributed to this story.