Two dolphins at a Swiss theme park died after ingesting a heroin substitute around the same time as a weekend rave at the park last November, according to reports from Switzerland.

A toxicology report from a forensics institute in St. Gallen, Switzerland, showed tests found traces of the heroin substitute buprenorphine in the dolphins' urine, according to a report in London's Daily Mail online, citing Swiss media reports.

The dolphins died within five days of each other in November, after the area near their pool in the Connyland marine park had been rented out for a weekend rave, according to the reports.

Cornelis van Elk, a Dutch marine biologist, said the drugs turn off a part of the dolphins' brains which tells them when to surface for air.

Connyland keeper Nadja Gasser told local media how one of the dolphins, named Chelmers, died, according to the Daily Mail.

"He was drifting under the water and was clearly in trouble and so we jumped into the water.

"We tried to hold him. He was shaking all over and was foaming at the mouth.

"Eventually we got him out of the water. His tongue was hanging out. He could hardly breathe.

"He was given adrenalin, but it didn't help.

"After an hour the dolphin died."

In a report in the Daily Mail shortly after the rave, wildlife experts said noise from the event could have stressed the animals' immune systems and led to the death of the first dolphin, Shadow.