Mystic — A second beluga whale that arrived at the Mystic Aquarium in May has died, this one after being treated for multiple health issues over the past several months.

The female beluga had been receiving constant, intensive care over the past several months, the aquarium said in an announcement Friday on its Facebook page. Life-saving efforts made Friday morning were unsuccessful.

The cause of death will not be known until a full necropsy is completed, the aquarium said. The whale's name was not released.

Mystic Aquarium President and CEO Stephen Coan thanked the aquarium's veterinary care staff, animal care team and consulting veterinarians from around the world for their work with the animals. "The loss of an animal is devastating to our team personally, and indicative of the challenges that we face in advancing research for the well-being and survival of beluga whales in the wild," he said in the Facebook post.

“Our team provided a continuum of world-class care to this whale for many months," the aquarium's chief zoological officer, Allison Tuttle, said in the post. "When an animal passes in the wild, it is the circle of life. When it happens at an Aquarium, with an animal you have grown to know, care for 24/7 and adore, it is devastating.”

The female beluga was one of five transported from MarineLand in Niagara Falls, Canada, under an export permit as part of research to help critically endangered beluga populations in the wild and to preserve the species, according to the aquarium. The group had included four females and one male.

The approximately 6-year-old male, named Havok, died in August. He had a history of gastric ulcers and associated bleeding, anemia and low appetite and was being treated for these conditions at the time of his death, according to documents filed with the National Marine Fisheries Service.

The decision to transport the whales to Mystic from Canada was a controversial one, with many animal rights groups speaking out against the move.

One of those groups, Animal Welfare Institute, issued a statement Friday noting "the only male beluga imported from Marineland, died in August at Mystic, apparently from a 'preexisting condition' related to gastric abnormalities. This latest death, well short of a year after the import, is equally disturbing, though the cause is not yet known."

The group said it would be monitoring the fisheries service's website for the full necropsy reports for both whales, which it said "are required to be submitted and posted publicly under the permit conditions for the import."

“These five whales were supposed to be healthy when they were imported, but it is becoming increasingly clear they were not,” Dr. Naomi Rose, Animal Welfare Institute's marine mammal scientist, said in the emailed statement. “If Mystic knew this and imported them anyway, that is unacceptable. If they did not know, that is incompetence. These whales deserve better — we demand a full government investigation into this situation. What was meant to be an uneventful import for research has become a disaster.”

The aquarium said it currently has six other whales. One of the animals remains in the intensive care unit, while the other five are healthy and residing in the Arctic Coast habitat.