More than 400 people gathered at a Hampton Bays beach on a frosty Sunday morning to see two seals return to the ocean that had washed them up last year.

The yearling harbor seals, Hermes in Southampton and Ares in Quogue, had arrived — Ares in October, Hermes in November — underweight, dehydrated and suffering from respiratory conditions.

After being nursed back to health by the Riverhead Foundation for Marine Research and Preservation, they shimmied into the water Sunday, healthy and equipped with satellite tags. Robert DiGiovanni, the foundation’s senior biologist and executive director, said he hoped to collect 30 to 90 days’ worth of data from the tags. The information tracked includes the time, depth of dives, movement and location, which will be used to assess the success of the rehabilitation, DiGiovanni said.

The seals were released at a beach area under Ponquogue Bridge, an area that was a “nice transition” back into the wild because of its proximity to the Shinnecock inlet, said Samantha Rosen, an education coordinator at the foundation.

DiGiovanni said it was possible that Hermes and Ares might remain in nearby waters, or swim to New England. Long Island has many coastal habitats ideal for seals, DiGiovanni added, such as Fire Island and Montauk.

Fans of Hermes and Ares can stay up to date on their movements by visiting the foundation’s Facebook and Twitter accounts, and detailed satellite maps will eventually be posted on its website.

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Hermes and Ares are the first seals to be released by the foundation this year, after residents spotted them and called the 24-hour sighting hotline. Last year, the foundation released about 40 seals, DiGiovanni said.

Sunday’s crowd included parents, children and others eager to catch a glimpse of the plump seals. Each seal had 19 families who helped support their rehabilitation costs, said Rachel Bosworth, the foundation’s spokeswoman.

Hannah Bennett, 11, supported Ares and traveled from Manhattan to attend the release. “It feels good because I don’t like to see that animals are hurt,” Hannah said. “It’s great that they can be back in the wild where they belong.”

Hauppauge residents Keira Choinski and Adrianna DiChristina, both 8, are part of a Bretton Woods Elementary School third-grade class that supported one of the seals. Keira said she braved Sunday’s cold weather because she wanted to help ensure that Hermes and Ares survived.

“We want to keep more of them in the ocean,” Adrianna said. “It’s good to learn about it [environmental preservation] so we can keep doing it as we get older.”

Foundation director DiGiovanni said it was easy to forget that people care about marine life, but Sunday served as a reminder that environmental awareness is very much alive.

“Here’s a reason to go out for a walk on the beach,” DiGiovanni said. “[People] really get attached to the environment and that’s what it’s really about.”