Scuba Diving/Can a person survive a free dive (no oxygen)
QUESTION: Can a person survive a free dive (no oxygen) of 350 feet if a trained dolphin takes the person down and back up?
That's an interesting question but there is no definite answer. The ability of a free diver to reach those depths has already been documented so that part of the answer is "yes". The use of a dolphin to bring the diver to those depths has never, to my knowledge, been tried although the dolphin is certainly capable of being trained to do so. I'm not sure of the maximum operating depth of a dolphin but I know whales are easily capable of such depths so it's reasonable to assume that a dolphin could reach 350 feet.
The sport of free diving requires extensive training for the diver to reach those depths and return to the surface. The average free diver could not accomplish this even with assistance from a dolphin or, as is commonly the case, the use of a weighted sled to bring the diver to their maximum depth and then the diver releases the sled and returns to the surface under their own power.
I think you can see from this brief explanation that there are a lot of factors which play into the successful completion of such an activity so my overall answer is "it depends...".
I hope this provides the information you were seeking or at least gives you some insight into the challenges associated with free diving to great depths.
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QUESTION: The average free diver could not accomplish this even with assistance from a dolphin. Is that correct?
Can an adult or even a child travel 20 to 30 miles in the ocean to land, with no live jacket, but can hold on the dorsal fin of a trained dolphin? If an adult or even a child accidentally lets go of the dolphins dorsal fine, than the dolphin will come back.
I answered this question to the best of my knowledge and gave you the reasons for it. There is no "yes" or "no" answer to this question. As I said...It depends on the ability of the diver and the willingness and training of the dolphin.
The second part of your question gets the same answer...it depends. Traveling 20 or 30 miles while holding onto anything (even a dolphin) is so physically demanding on the human that it would take extraordinary strength and stamina to do it. Water resistance is a very powerful force and trying to hang on while being dragged through the water for that distance would be very difficult. It looks very easy in the movies but in real life it is extremely difficult, especially over a long distance. Any water skier who has tried to hang onto a ski rope when falling off a ski will attest to that. Whether or not the dolphin would return to retrieve a human would depend on the dolphin, it's training and it's willingness. Dolphins are not robots and they have feelings and moods just like humans.