A US judge threw out an animal rights group's lawsuit accusing SeaWorld of enslaving captive killer whales, ruling yesterday that orcas had no standing to seek the same constitutional rights as people.

People for the Ethical Treatment of Animalsaccused the chain of aquatic theme parks of violating the rights of whales under the 13th amendment of the US constitution, which abolished slavery.

The lawsuit, filed in the US District Court in San Diego, listed five performing orcas at SeaWorld's parks in California and Florida - Tilikum, Katina, Corky, Kasatka and Ulises - as plaintiffs.

"The only reasonable interpretation of the 13th amendment's plain language is that it applies to people," US District Judge Jeffrey Miller wrote in his ruling.

"Both historic and contemporary sources reveal that the terms 'slavery' and 'involuntary servitude' refer only to people," he added.

Legal experts previously denounced as frivolous the Peta lawsuit, which had sought a court order requiring SeaWorld to release the whales to a "suitable habitat".

David Steinberg, a professor at the Thomas Jefferson School of Law, in San Diego, said when the lawsuit was filed in October that it was "demeaning to people who were slaves".

Virginia-based Peta has staked out a number of controversial positions in the past in seeking to advance animal rights.

In 2003, the Anti-Defamation League accused Peta of trivialising the deaths of Jews in the Second World War with a campaign that compared the meat industry to the Holocaust.

Peta spokesman Colleen O'Brien said her organisation would regroup as a result of the dismissal of what it called "this historic first case" on behalf of orcas.

"Today's decision does not change the fact that the orcas, which once lived naturally wild and free, are today kept as slaves by SeaWorld," O'Brien said.

Miller said animals did have legal rights under state and federal statutes, including criminal laws.

He said the goal of Peta "to protect the orcas was laudable" but citing the 13th amendment was not the way to win the case.