Twelve of 20 fur seals died during a flight carrying them to a dolphinarium in Russia's Far Eastern city of Vladivostok, a Pacific Oceanological Institute spokesman said on Tuesday.
Local prosecutors said the seals, which were being transported in the cargo bay from Sakhalin Island, could have been affected by high temperatures. The aircraft's captain regulates the heat from the cockpit and may have set the temperature too high for the animals, who are accustomed to a much lower temperature in their natural habitat in the Northern Pacific.
A probe into the animals' deaths will uncover who the guilty party is, and whether it was the Vladivostok Avia carrier or Aerogruz responsible for the seals' transportation.
Veterinarians have ruled out any diseases the seals may have had.
The carrier refused to comment on the incident.
An expert from the Oceanological Institute however said the seals could have suffocated each other as the cargo bay was too small for the 20 adult mammals.
"The average weight of an adult seal is over 300 kilograms. If there were several animals locked in a cage, they could have crushed each other with their bodies," the expert said, adding that in this case it is the carrier who is responsible for the mishandling.
Aerogruz, a company responsible for carrying animals, cited stress as the main reason for the seals' deaths.