SEATTLE - Forty years after hunters lassoed a young killer whale off Whidbey Island, Wash., and sold it to a Florida theme park, whale advocates are turning to an unusual tactic to try to force the orca's release: the Endangered Species Act.

In a move legal experts said could have significant implications for other zoos and aquariums, animal-rights activists recently sued the federal government, arguing that the law may require Lolita, the killer whale who still performs at the Miami Seaquarium, to be reunited with pod members in the Northwest because Puget Sound's southern resident orcas were listed as endangered in 2005.

They contend that keeping a highly social animal like an orca in a tank on the far side of the country should be viewed as harassment, and the Endangered Species Act makes it illegal to "harass, harm, pursue, shoot, wound, kill, trap, capture, or collect" an animal under its protection.

The suit is part of an emerging trend as experts and lawyers debate the conditions under which animals in captivity should be subject to the Endangered Species Act.

In October, after a 10-year legal battle, activists lost an appeal of a high-profile federal court case against Ringling Bros. and Barnum & Bailey Circus over whether its treatment of elephants - prodding them with bullhooks and shackling them between shows - violated the law. The court never ruled on the central question, deciding instead that the plaintiff, a former circus employee, didn't have proper legal standing to bring the suit.

Meanwhile, the U.S. Fish and Wildlife Service is formally considering reclassifying chimpanzees held in captivity in the United States as endangered, which could severely limit their use in research.

"It's an interesting idea and poses some very interesting questions," said Dan Rohlf, a professor of environmental law at Lewis and Clark in Portland. "The question in this case may end up being whether or not anything the aquarium is doing could constitute harassment or harm of that whale.

"It would be important to look at what sort of minimum federal standards would apply to those sorts of aquariums, and whether or not this aquarium meets those standards."

The Miami Seaquarium declined to answer questions about the lawsuit, filed in U.S. District Court in Seattle, but maintained in a statement that Lolita is active, healthy, and well cared for and that her tank and other conditions far exceed the minimum requirements of federal law.

Whale activists, including Karen Munro of Olympia, a plaintiff in the suit who said an orca capture she witnessed while sailing in Puget Sound in 1976 changed her life, have long been skeptical of that claim because the marine park won't share Lolita's medical records. "I think people are starting to realize some animals don't belong in captivity," Munro said.

The lawsuit over Lolita is just the latest front in a battle that dates to her capture in 1970. That year, entrepreneurs herded more than 60 southern resident killer whales from Puget Sound's J, K, and L pods into a three-acre net pen in Penn Cove off Whidbey Island. A 1-ton, 6-year-old female from L pod, later nicknamed Lolita, eventually wound up in Florida.

Cetacean lovers for decades campaigned for her release. In the 1990s, Washington Gov. Mike Lowry demanded that the marine park return the whale to Puget Sound. The marine park wasn't interested, and maintained that Lolita - like Keiko, the killer whale made famous by the film Free Willy who died in 2003 after being returned to sea - wasn't equipped to survive in the wild.

Then, in 2005, Puget Sound orcas were listed under the Endangered Species Act. The National Marine Fisheries Service determined that whale-capture operations in the 1970s had contributed to their decline. But the agency exempted whales currently in captivity from the listing. Lolita, now about 47 years old, is the only whale still alive in a U.S. marine park from the August 1970 capture in Puget Sound. Orcas in the wild can live 80 years or more.

Carter Dillard, chief counsel for the Animal Legal Defense Fund, said the law didn't allow the use of exemptions for wildlife held for a "commercial activity."

Legal experts are split on the case's merits. Dillard, with the animal defense group, said Lolita ultimately should be transferred to a larger holding system in Puget Sound, where she could swim longer distances and communicate with other killer whales.

The Miami Seaquarium, on the other hand, argued that the very conditions that led orcas to need the Endangered Species Act would prove perilous to Lolita in Puget Sound. She "has learned to trust humans completely, and this long-standing behavioral trust would be dangerous for her if she were returned to Puget Sound," the park statement said.