Fans who attended the Stadium Series game last weekend at Heinz Field were treated to waddling penguins during the pregame show, but organizers are getting backlash from animal welfare activists.

People for the Ethical Treatment of Animals is criticizing the Pittsburgh Penguins’ use of live penguins before the game against the Philadelphia Flyers.

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“At Saturday's Pittsburgh Penguins game, live penguins from the Pittsburgh Zoo & PPG Aquarium were paraded in front of the screaming crowd and (were) caught on camera scrambling in terror from a fireworks explosion,” PETA said in a news release.

PETA representatives sent a letter to the hockey franchise, calling on the team to never again use live animals at Heinz Field.

"It's inherently stressful for wild animals like penguins to be hauled around, used as props and exposed to noisy crowds, with or without explosives going off," PETA executive vice president Tracy Reiman said in the news release. "PETA is calling for the only Penguins on the ice at Heinz Field to be humans on skates."

PETA's letter to the Pittsburgh Penguins read, in part:

"It's inherently stressful for wild animals -- who naturally shun contact with humans and are extremely sensitive to environmental changes -- to be hauled around, used as props, and exposed to noisy crowds, with or without explosives going off. Hockey fans come to see talented athletes compete, not shy animals terrorized.   "Being held in captivity is stressful enough to make penguins susceptible to illness, and putting them in a crowded, noisy stadium only makes matters worse."

The Pittsburgh Zoo & PPG Aquarium responded, saying they worked with NHL officials to ensure the animals' safety, which is their main priority. In a statement, a spokesperson said:

"The NHL worked with us to ensure that the penguins would be safe and enjoy their visit to Heinz Field. The Pittsburgh Zoo was honored to be asked to be a part of the Winter Classic as well as celebrating the 50th anniversary of the Pittsburgh Penguins.   "The penguins who attended the Winter Classic are some of the same penguins that participate in our weekend Penguins on Parade event.  They are very comfortable around people and noises. In addition, it was a great enrichment opportunity for our penguins to be introduced to new sounds, sights, and smells."

In addition, zoo officials said, the penguins' keepers and a senior veterinarian were with the penguins at all times on the ice.

In response to PETA's allegation that the birds were "scrambling in terror," zoo officials said: "The loud pop from the pyrotechnical display temporarily startled the penguins and their first reaction, similar to a human’s when startled,  they flapped their wings. It was less than 10 seconds, and the penguins were back to normal and exploring and playing on the ice."

PETA fired back after hearing the Pittsburgh Zoo's defense. PETA Foundation Captive Wildlife Veterinarian Dr. Heather Rally issued the following response:

"These penguins were terrified into a flight response. There's simply no refuting that. While human beings were able to realize that the fireworks were merely startling and not dangerous, these penguins were obviously frightened and attempted to flee.   "True enrichment involves a safe and familiar environment, where animals have some choice and control over what they do, not booming pyrotechnics and a boisterous hockey crowd. The Pittsburgh Zoo should know this and do better by the animals in its care."