When Europeans retreated into their homes to observe strict stay-at-home rules to contain the coronavirus, dolphins and whales on the Mediterranean coast basked and thrived in a hitherto unknown calm.

But the return of tourists, noisy boats and heavy sea transport with the end of lockdowns in France and other Mediterranean littoral countries has signalled the return of danger and harm caused by human activity for underwater creatures.

Nowhere is this more true then in the crystalline waters outside France's second biggest city of Marseille, a nature reserve important for wildlife but also thronged with day-trippers in the summer season.

"As soon as the pleasure boaters came back, we saw footage that really annoyed us," said Marion Leclerc from the conservationist organisation Souffleurs d'Ecume (Sea Foam Blowers).

In one video, three teenagers jump from a boat close to a finback whale while wearing snorkelling masks, which is dangerous for both animal and human, said Leclerc. 

"We're speaking of an animal that weighs 70 tons," she lamented.

"Many forget that the Mediterranean is also a home, where animals rest, feed and reproduce," Leclerc said.

The Mediterranean Sea is home to more than 10,000 species, despite only amounting to 1 percent of the Earth's oceans.

But the sea which separates Africa from Europe draws 25 percent of marine traffic.

Heavy traffic increases the risk of fatal collision with the sea mammals.

"It's the first cause of non-natural mortality for big cetaceans," said Leclerc.

- Reduce speed -
Out of the 87 marine mammals in the world registered by the UN, 21 have been spotted in the Mediterranean. Most of them are considered at risk of extinction.

On a bright summer's morning, a group of fifty striped dolphins splash around an inflatable blue speedboat off La Ciotat bay a short boat ride from Marseille.

"We need to reduce our speed and place ourselves parallel to their trajectory to avoid cutting their path. They come and play if they want to," said Laurene Trudelle, at the helm of the boat belonging to the scientific research group GIS3M.

The lockdown brought maritime traffic to an almost complete standstill, giving dolphins and whales the opportunity to explore areas from which they are normally kept at bay by tourists. 

All scientific studies were put on hold in the Mediterranean during lockdown, but marine drone manufacturer Sea Proven got the necessary authorisation and funds from Prince Albert II of Monaco to continue observations in the Pelagos Sanctuary, a marine area protected by Italy, Monaco and France.

- 'Reversible pollution' -
Bioacoustics researchers from a Toulon University team who analysed Sea Proven's data, observed a 30 decibel decrease in noise on the coastal areas as a result of the total lack of pleasure boaters.

And the silence allowed the aquatic creatures to interact in areas between two and six times as large, said researcher Herve Glotin.

"The lockdown period showed that we really are responsible for the noise in the bays and that this pollution is completely reversible," Glotin said.

"When you think that reducing boats' speed by 10 percent in areas highly populated with marine mammals would be enough to significantly decrease sound pollution and the risk of collision" Glotin added.

The Quiet Sea research project also saw the amount of hydrocarbon -– the principal component of petrol –- halve during lockdown. 

"It's really good for biodiversity, so indirectly for all of the food chain," said Glotin.

No binding international law obliges ship-owners to preserve marine mammals' natural habitat. 

But since 2017, France requires boats that are over 24 metres (78 feet) in the Pelagos Sanctuary to have onboard equipment which detects the animals.