An unusual increase in dolphin deaths in the Black Sea since February may have been caused by the war in Ukraine, according to scientists.

Researchers told the UK’s Guardian newspaper that higher than normal noise pollution as a result of 20 Russian navy vessels and ongoing military activities in the north of the Black Sea may have driven the dolphins south towards Turkey and Bulgaria, where large numbers are being stranded or caught in fishing nets.

The major increase in the number of deaths off the coast of Turkey, recorded at more than 80 dolphins, was first reported at the end of March, one month after the war began, by the Turkish Marine Research Foundation (Tudav).

At the time, Tudav wrote: “It is not yet clear why they are concentrated in the region at this time of the year compared to previous years, and why incidental net catches have increased so much.”

Initial investigations by the foundation found that around half of the more than 80 dolphins had died after getting entangled in fishing nets.

The other half showed no signs of entanglement or gunshot wounds.

Dr Bayram Ozturk, Tudav’s chairperson, told the Guardian that while they were not certain, the foundation now believes acoustic trauma may have caused the deaths.

Military-trained dolphins

Since marine mammals depend on sound for communication and other functions, the sonars used to detect enemy submarines may have been fatal for the dolphins.

Dr Pavel Gol’din, a researcher at Ukraine’s National Academy of Sciences, told the newspaper that while acoustic trauma may explain the strandings, constant underwater noise caused by military activities could also explain an increase in deaths.

Gol’din explained that while permanent underwater noise may not kill the marine mammals directly, it may cause them to head for unfamiliar territory to try to avoid it.

“It might be the cause of mass migration of fish and cetacean stocks to the south,” he said.

Last month, it was reported that Russia had deployed military-trained dolphins to protect its Black Sea naval base in Sevastopol from underwater attack.

Images taken by the US satellite company Maxar showed that two dolphin pens were placed at the entrance to the port’s harbour around the beginning of Russia’s invasion of Ukraine in late February.