The zoo's remaining nine stingrays are being closely monitored. (Colin Hall/CBC)

Calgary Zoo staff were in shock Monday after the sudden deaths of nearly all of the zoo's stingrays over the weekend.

The zoo's new stingray exhibit was closed Sunday afternoon after the ray keeper noticed some of the creatures were swimming erratically and seemed distressed. The stingrays began dying suddenly, and by Monday morning 34 of the cownose rays were dead.

The remaining nine rays were being closely monitored by zoo officials.

"Our staff and volunteers are shocked and deeply saddened by this tragedy," said Cathy Gaviller, director of conservation, research and education, in a release. "There is always a deep connection to the animals we care for, and those who have worked with the rays hands-on are devastated."

The stingrays were part of a hands-on educational exhibit, which opened in February. (CBC)

Zoo officials continued to investigate the deaths Monday.

"What we do know is that there was no mechanical failure in the life support system," said Gaviller. "We know that all of the numerous water tests conducted on a daily basis since the exhibit opened have been consistently within normal range. This was the case both early yesterday and even after the rays began experiencing difficulties. We will be relentless in investigating the cause."

An initial examination found that the rays had severe irritation of the gills but were otherwise in good health.

The zoo said it has sent food and water samples to an independent laboratory for testing, but it may take more than a week to determine if there were toxins present in the water. Tissue samples from the dead rays have also been sent away for further tests.

The zoo said the exhibit, which opened in mid-February, will remain closed until further notice.

The zoo has lost several animals over the past two years, including four gorillas and six-year-old Hazina the Hippo.

Stingray death timeline

Sunday

• 9:00: Rays fed, everything seemed normal.
• 10:00: Exhibit opened to public.
• 11:00: Rays ate well, behaved normally for group of people.
• 13:00: Rays appeared to have lost their appetites, staff begins monitoring them.
• 15:00: Rays show signs of stress. Water tested and appears normal.
• 16:00: Rays resting, a few swimming erratically.
• 16:20: Rays appear in extreme distress.
• 16:30: Ray keeper learns of several deaths. Exhibit closed.
• 17:00: 26 rays dead.

Monday: Eight more rays dead.

Source: Calgary Zoo