Alvarenga claims he survived on raw bird and fish flesh, turtle blood and his own urine as his boat drifted from Mexico after the engine died.

While the recommended survival rations for those on life rafts is a litre of water and 1,000 calories of food a day, previous survival stories show it is possible to exist on significantly less. A study of those stranded on life rafts during the second world war found that even a daily water ration of slightly over 200 millilitres greatly increased survival chances.

Authorities say they haven't determined how Alvarenga maintained his fluid intake, but Tipton says the sailor could have managed, "if everything goes in your favour and you get periodic supplies of fresh water that you can store, or you're smart enough to use the condensation that appears on the inside of canopies".

Urine, though, is not advisable to drink. Seawater is also not an option, despite anecdotal evidence of people having survived on it in the past.

Jean-Yves Chauve, a French doctor specialising in sailing survival, said he was "sceptical" about the claim of seawater drinking, although he could not dismiss it out of hand. The castaway's food intake from fish, bird and turtle meat would have been protein-heavy, lacking sugar for proper brain function and vitamin C found in fruit and vegetables, he says.

"In the hot conditions of this tropical region, he would have perspired a lot" and lost much moisture,'' the doctor says. "Even with a few drops of rain every now and again, more than a year seems like a very, very long time."

Tipton's survival book lists a series of innovative if grisly ways to obtain fluids: sucking the moisture from the spinal columns of fish and drinking the blood of turtles, the latter being something Alvarenga says he did. A 20kg turtle can provide about a litre of blood - a substance described as tasting like "the elixir of life" by previous castaway Dougal Robertson, a Scottish man who survived 38 days adrift with his family after their schooner sunk in the Pacific in 1971.

Tipton says turtles are relatively easy to catch and can be a lifesaver. "There's also quite a good layer of fat on a turtle, just under the shell. That's quite useful," he says. "You want to try and preserve your proteins. They're the building blocks of life."

Turtle fat is particularly useful because protein can be counter-productive as it requires plenty of fluids for the body to process.

Another unlikely, though useful liquid source, are fish eyes. Brits Maurice and Maralyn Bailey survived 117 days on a rubber life raft in the Pacific in 1973. "They found they started to crave fish eyes, which is not something one would normally do," Tipton says. "It wasn't until after the voyage they realised these are quite rich in vitamin C, which is something you get depleted in when you're adrift, and can of course cause scurvy."

Tipton says that those adrift should resist the temptation to drink anything at all on the first day, because much of this fluid would be lost in urine.

"Once you get past day one or day two the body will switch into conservation mode,'' he says. "As long as you're not swimming about to catch fish or doing lots of exercise, then your metabolism will slow right down." A resting metabolic rate requires 1,000 to 1,500 calories a day. Hunger strikers in the 1970s and 1980s showed researchers how long the body could survive on few calories.

To lessen moisture loss, Alvarenga would may have found shelter from the sun, the experts say. According to reports, his boat had a small crawl-in container on board that could have served this purpose.

Chauve cited previous narrow escapes from which castaways emerged emaciated and physically weak after fewer than six months at sea, contrary to Alvarenga, who appeared in remarkably good shape.

Alvarenga's biggest advantage might simply have been his years of experience at sea.

"One of the things we've learned over the centuries is that people who are regular seafarers are behaviorally adapted to that environment," Tipton says. "It's even down to little things. If you or I were stuck in that situation we'd probably be seasick for the first week, and that would be sufficient to finish us in itself, not only because it's a fairly significant source of dehydration, through vomiting, but also because it destroys your morale."

The experts concur that they had never heard of anyone surviving at sea as long as Alvarenga.

In 2006, three Mexicans made international headlines when they were discovered drifting in a small fibreglass boat near the Marshall Islands, nine months after setting out on a shark-fishing expedition.

"In survival training, we say that the will to survive is the biggest help you can get," says Hilmar Snorrason, chairman of the International Association for Safety and Survival Training, based in England. "The will to survive, when you lose that, you do not have a long time left."

Duke professor of pathology describes medical realities of Alvarenga's ordeal

The story of a El Salvadorean fisherman who says he survived more than a year adrift on the Pacific Ocean raises many medical questions. Claude Piantadosi, professor of pathology at Duke University in the United States and author of the book says Jose Salvador Alvarenga's survival story is possible.

The average is about 100 hours [approximately four days] without water and about five or six weeks without food. You can survive much longer with just a little food, although you'll lose weight and run into vitamin deficiency problems. So it would have been vital for Alvarenga to have collected both food and water during his journey. The Pacific's regular squalls would have provided some rainwater that he could have scooped from the bottom of his boat.

Absolutely critical. You get significantly warmer in direct sunlight and sweat more. The pictures of the boat show a fibreglass box in the middle, which he could have sheltered in.

Over time, the underside of the boat would have become its own ecosystem as barnacles, seaweed and jellyfish collected there, which in turn attracts other creatures. How often can you grab a turtle or catch a fish with your bare hands? I don't know. Bird blood is no more salty than human blood, so would have provided some hydration.

Unlike humans, birds and turtles make their own vitamin C, so fresh meat from those creatures, especially the livers, would provide sufficient vitamin C to prevent scurvy. British sailors used to get scurvy because they [ate] preserved meat which had oxidised and lost its vitamin C.

He'd need to keep mopping himself off and stay dry to avoid that. People on life rafts, or say a piece of floating wood, can develop real problems with macerated skin.

It would be a significant advantage. He could live off his own body fat and muscle for a long time, so long as he was able to get some water, vitamins, micronutrients and a little protein.

The appearances of malnutrition can manifest differently depending on how short you are on calories or protein. Some underfed children in Africa look like stick figures, others get swollen. It's only in end stage starvation that people get really emaciated.

I'm not an expert in psychiatry, but we all [do] have the feature of resilience.

Yes. It's unusual to say the least. As we have gotten more information, it's … likely that he did survive at sea for 13 months.

Associated Press