(Reuters) - Baltimore's National Aquarium will create the first U.S. dolphin sanctuary and transfer its eight Atlantic bottlenose dolphins there, the aquarium said on Tuesday.

The move comes after five years of study and as the U.S. public has grown increasingly uneasy about keeping dolphins and whales in captivity, National Aquarium Chief Executive Officer John Racanelli said in a statement.

"The way a society treats its animals with whom it shares the planet speaks volumes about us," he said.

Creating the outdoor, seawater sanctuary poses a challenge since all but one of the colony of six female and two male dolphins was born in an aquarium or a zoo, Racanelli said. One female, Nani, was born in the wild in 1972.

The dolphins will have to learn how to be ocean-dwelling creatures. The site will be much larger than the dolphins' current habitat, and will have more fish and marine plants.

Bottlenose dolphins are the one of the most common species of marine mammals, with colors ranging from light gray to black. They range in size from 6 feet (1.8 meters) to more than 12 feet (3.6 meters) in length, and adults can weigh up to 1,400 pounds (635 kg).

Humans will care for the dolphins throughout their lives, Racanelli said. A team is weighing potential sites in Florida and the Caribbean. Racanelli did not give an estimate on the cost.

The nonprofit National Aquarium resolved not to collect dolphins from the wild in 1993. Its board voted in 2011 never to collect cetaceans, such as whales and dolphins, from the wild and pledged it would oppose those who did.

Creation of a dolphin sanctuary comes three months after SeaWorld Entertainment Inc said it would halt breeding killer whales in its parks, bowing to pressure from animal rights activists.

Animal welfare groups including the Humane Society of the United States and People for the Ethical Treatment of Animals (PETA) praised the National Aquarium's move.

"This spells the beginning of the end for dolphin captivity," PETA Executive Vice President Tracy Reiman said in a statement.

Humane Society President Wayne Pacelle said there were an estimated 300 dolphins in captivity in the United States, including 100 used by the U.S. Navy.

The costs of creating an environment for wildlife were high, he said. For example, a sanctuary for hundreds of chimpanzees coming out of private laboratories, Project Chimps, has a cost of almost $100 million over the animals' lives, Pacelle said.

The National Aquarium is one of Baltimore's major tourism draws, attracting 1.3 million visitors last year, according to its website.

(Reporting by Ian Simpson; Editing by G Crosse and JS Benkoe)