Seven penguins were found dead at the Calgary Zoo Thursday morning.

The animals drowned in a back holding area of the popular Penguin Plunge exhibit.

"It was very surprising for us to find this group of birds dead," said Jamie Dorgan, the zoo's director of animal care.

"Our penguin keeper went up to the building this morning to do her normal routine and unfortunately found one penguin deceased in the pool and was quite shocked to see that multiple penguins were dead in the pool."

The zoo's veterinary team has completed necropsies on all the birds, which are Humboldt penguins, and say the aquatic birds died as a result of drowning.

"Penguins are marine animals. They spend huge portions of time in the ocean — in fact, Humboldt penguins will go out fishing for weeks and just stay out in the open ocean," said Dorgan.

Mass panic possible

The zoo is launching a full investigation, including looking at surveillance video of the secured entrance and exits of the building.

"What caused these animals to get into a situation where they would have drowned is something we are going to look at," Dorgan said.

The loss of seven penguins is devastating, says Jamie Dorgan, the Calgary Zoo's director of animal care. (Evelyne Asselin/CBC)

"It seems to be some stressor led to these birds going into some kind of panic … but it's hard to say what could have triggered that."

A total of 22 birds lived within the Humboldt colony. The surviving penguins, who don't show any signs of distress, won't have access to a pool at night while they roost.

Three other species live in Penguin Plunge, including king, gentoo and rockhopper penguins. But they were not in the same enclosure as the Humboldt penguins.

'A bit of a mystery'

It's unusual to hear of so many penguins dying in a zoo, says Rob Laidlaw, executive director with Zoocheck Canada, a group that monitors zoos across the country.

"It seems like a bit of a mystery," he said. "I can't really imagine — and the colleagues that I called about this to see if they could provide any insight on this as well — they can't imagine what could have happened. We've never heard of seven aquatic birds like penguins drowning."

The Calgary Zoo has had a number of animals deaths over the years and Laidlaw hopes staff will take a close look at everything the organization is doing.

"They've had a long history of problems, and they have to be their own worst critics and continually self-evaluate and do their best to do better."