From one operation to the next, it is not the people who are saved but the ones they can do nothing to help who leave the worst mental scars on migrant rescuers in the Mediterranean.

"There is always the one that could not be saved," says Marie Rajablat, a psychiatric nurse who works with the often young crews on board charity search and rescue ships.

To help prepare them for the trauma ahead, the crew of the Ocean Viking, which set off from France on Sunday, was given psychological counselling and support.

The vessel, jointly run by the humanitarian groups SOS Mediterranee and Doctors Without Borders (MSF), is on its way to international waters off Libya from where migrants set off in often dangerous, rickety boats hoping to get to Europe.

From January to July this year, nearly 700 people died while trying to cross the Mediterranean, according to the International Organization for Migration (IOM).

Rajablat recalls an incident in January 2018 when a migrant boat sank, throwing about 70 people into the sea, including several babies who were near death.

The victims were plucked from the water to the deck of the charities' first search and rescue vessel, the Aquarius, where medical teams battled to bring each one back to life with CPR.

"The sailors were traumatised by the fear of not getting them all out (of the water) -- in fact, they saved everyone," said Rajablat, who spent six weeks on the ship and wrote about her experiences in her book, "The Shipwrecks of Hell".

The Aquarius saved 30,000 lives during its near three years of operations.

Rajablat built up her experience in that time and slowly put together a team of about 15 people whose job it is to help prepare others like the crew on the Ocean Viking.

'A closed world' of horrors

Three psychologists in their 60s were in Marseille in the south of France for the first-ever obligatory briefing before the Ocean Viking set off.

At first, the crew members in their early- to mid-30s, "were a little bit suspicious," said psychiatrist Marie Lepine.

"They told themselves, it's the 'blabber of old ladies' -- that's what they call us," she joked, saying they got used to them afterwards.

Frederic Penard, operations director at SOS Mediterranee, said the crews "are our treasure... and they get worn down by the traumatic shocks".

"It is not normal to see people die at sea -- support, listening (to them) have become key."

Rajablat added: "On board, it's a closed world where there are only horrors."

The migrants who are rescued are often traumatised and in distress, after having fled violence, misery and war in their home countries in perilous conditions.

A world apart

Births sometimes occur on board the rescue ships -- half a dozen were recorded on the Aquarius -- but as the mothers' conditions are so fraught it is hardly a moment to celebrate.

"It's a world apart, centred on the disaster, a little surreal too. Returning to land causes another problem," which can be equally traumatic, said Lepine.

"It's very difficult to return to normal life and believe everything will be fine," said 39-year-old Alessandro Porro, who has completed four missions on the Aquarius.

"People die at sea, it's like a plane that crashes and sinks and it's not normal. This psychological support is good for us."

The rescuers' backgrounds vary from merchant seamen, to former soldiers, paramedics, rescue workers, nurses.

"The ones who help the group the most are the military," said psychiatric nurse Marie Ablatir.

Western armies are well versed in the prevention and detection of post-traumatic syndromes in their ranks. But above all, "they help the group stay focused on its goal: to save people", she said.

Most rescuers from SOS Mediterranee have gone on other missions but others decided to quit.

"We were not prepared for that," said one sailor from France.