People for the Ethical Treatment of Animals, or PETA, had accused the chain of aquatic theme parks of violating the rights of whales under the 13th Amendment of the U.S. Constitution, which abolished slavery in the United States.

The lawsuit, filed in U.S. 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 in the complaint.

"The only reasonable interpretation of the 13th Amendment's plain language is that it applies to persons, and not to non-persons such as orcas," U.S. District Judge Jeffrey Miller wrote in his ruling.

"Both historic and contemporary sources reveal that the term 'slavery' and 'involuntary servitude' refer only to persons," the judge added.

The 13th Amendment was enacted in 1865, the year the U.S. Civil War ended.

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

David Steinberg, a professor at the Thomas Jefferson School of Law in San Diego, told Reuters when the lawsuit was filed in October that it was "demeaning (to) the integrity and humanity of people who were owned as slaves."

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

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

Colleen O'Brien, a spokeswoman for PETA, said in a statement that her organization 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 who once lived naturally wild and free, are today kept as slaves by SeaWorld," O'Brien said.

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

He added the "goal" of PETA attorneys who brought the lawsuit "to protect the welfare of orcas is laudable," even if the 13th Amendment was not the correct way to approach the case.

(Reporting By Alex Dobuzinskis; Editing by Cynthia Johnston and Peter Cooney)

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