The aquarium says vets are monitoring her condition.
"As with all of the belugas and animal residents under our care, this female beluga has been receiving constant, individualized attention and medical support from our best experts on a 24/7 basis since joining our habitat," the aquarium said in a social media post.
The revelation comes three weeks after a male beluga, who was also part of the group of five that arrived from Marineland in Niagara Falls, Canada, died at Mystic.
The Mystic Aquarium says the sick whale has a low white blood cell count and gastrointestinal issues.
The aquarium specializes in beluga research.
"Mystic Aquarium has additionally reached out to beluga experts from across the country who are making the journey to the facility to assist with the situation," the aquarium said.
The Animal Welfare Institute is calling for a federal investigation.
"The belugas brought to Mystic were all born under human care, and therefore would not survive if released into the wild," the aquarium included in their statement.
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