One of the enclosure pools was drained during a break-in at the Vancouver Aquarium's off-site rescue, which comes just days after the sudden death of one of its beluga whales.

The off-site rescue facility, run by the Vancouver Aquarium, is currently housing 42 seal pups that are nearing the end of their rehabilitation and will soon be released back into the wild.

No animals were hurt during the Thursday night incident at the Marine Mammal Rescue Centre, and each one has been checked over by veterinarians.

The intruders made a mess and drained water from some of the enclosure pools.

"Way to kick us while we're down. Who breaks into an animal rescue centre?" Vancouver Aquarium staffer Meighan Makarchuk wrote on Twitter.

The manager of the non-profit facility, Lindsaye Akhurst, was at the Vancouver Aquarium checking in on the ailing beluga whale Aurora at the time of the break-in.

Several items of clothes were left behind in the enclosures, and Akhurst said police are now going through video footage to see if they can identify the people involved.

"They knew they were coming in. Thankfully no harm was done to animals but this is very disturbing," she said.

Aurora, the mother of Qila, the cetacean that died earlier this week, is now suffering the same symptoms her daughter showed right before her death.

It's been a bit of a guessing game as to how to treat Aurora, who is now the Vancouver Aquarium's sole remaining beluga whale.

Scientists say they are cautiously optimistic about Aurora's progress, and she is being monitored around the clock.

CEO John Nightingale said Friday that the 29-year-old cetacean had stabilized and appears calmer and more alert since being transferred to an isolated medical pool.

Aurora is the last beluga acquired by the Vancouver Aquarium before it instituted its policy of only accepting cetaceans that were born in captivity.

Some activists and scientists have argued that no whales should be kept in captivity, ever.

Dr. Lori Marino, an expert in neuroscience and behavioural biology, said she was not shocked to learn of Qila's death.

"This is the same thing that's been going on with belugas and other dolphins and whales in concrete tanks for decades. They just can't thrive in these situations," she said.

The aquarium is planning to expand the beluga tanks by 2018, and Nightingale says at this point the plan has not changed.

"That's a question down the road. The focus today is really on the staff and on Aurora," he said.

With many staff and volunteers at the Vancouver Aquarium, Akhurst said she hoped no one would maliciously want to harm them intentionally.

"All the animals are coming in are all rescued animals that have been given a second chance and we're prepping them to go back into the wild," she said.

"We're really concerned about our safety and the safety of these animals at this point."