I was listening to a Dan Carlin podcast, "Globalization unto Death" and he brought up an interesting point. When crews are starving, they talk about how they eat their shoes, rats, and horses but they don't try to fish. Why is that? Did they not have the nets necessary to do so? If so, did it just not occur to them or is there some logistical problem I'm not considering?
This misconception is based on a common mistake made by people who haven't spent much time on the water: that fish are freely available and easy to catch everywhere because hey: ocean! The ocean is full of fish the way the land is full of humans, which is to say there are quite a lot of them but they are very unevenly distributed. Most people think of the ocean as teeming with fish but this is because most people's experience with the ocean is very close to shorelines, and shorelines are hot spots for fish populations just like they are hot spots for human populations, and for the same reason, the easy availability of food.
Fish populations tend to be higher where the ocean is shallower, areas like coastlines and continental shelves that push microbial phytoplankton up from the ocean floor to support a large food chain of species near the surface. These lost ships you are talking about tend to be very far from the coast and far off the sea lanes. Otherwise they would be "found in a day or two and not interesting enough to be remembered in any history" type stories. Any good fishery will have plenty of fishermen by the age of exploration, with the Bering the one possible major exception (and even the Bering was fished by then, just not as heavily as the Newfoundland banks or the major Asiatic regions). The castaways starving stories are stories of men who were entirely familiar with fishing and were handy enough to rig a small net or line from virtually any available materials, but they were stranded in the fishing equivalent of the Sahara desert.
A good source for some overview information on fisheries and their distribution would be the NOAA website, or Bob Stewart's online textbook "Our Ocean Planet".
http://oceanworld.tamu.edu/resources/oceanography-book/fisheries.htm
"They did not commit to the deep the body of John Wilson. They cut it into slices, which they washed in salt water and dried in the sun. They divided it among themselves, partaking of it as a 'sweet morsel.'" "Desperate Journeys, Abandoned Souls," p. 225.
"In the Heart of the Sea: The Tragedy of the Whaleship Essex," Nathaniel Philbrick asked that same question. Why would whalers, men who made their living from the bounty of the sea, not try and fish for something rather than go off and start eating the cabin boy?
It is a delicate question during a time when cannibalism was common in certain circumstances. Remember, there was little hope of rescue back in the day of sail. Even if another ship saw a boat full of desperate souls lost on the high seas, that ship would often lack extra supplies to share.
The lawyer for Alexander Holmes, defending one cannibal sailor in court put it this way:
"This case, in order to embrace all its horrid conditions, ought to be decided in a long boat, hundreds of leagues from shore, loaded to the very gunwale with forty-two half-naked victims, with provisions only sufficient to prolong agonies of famine and thirst. ... Decided at such a tribunal, nature, intuition, would at once pronounce a verdict not only of acquittal but of commendation."
And the jury did so acquit, as was the custom in these cases.
If one is cast upon the wide oceans in a boat with 40-odd folks, a series of unfortunate events have transpired. "Desperate Journeys, Abandoned Souls," shows that often in the historical record there has been a storm or a fire or sudden sinking. In these cases, there was little or no time to rescue supplies or a fishing kit. Even if the lifeboat bound possessed the best fishing tackle and bait on hand, (which was never the case), there was little chance the crew could catch enough to sustain a overcrowded boatload of hungry castaways.
Philbrick writes the crew of the Essex, although practiced at the art of killing whales, didn't seem to know anything about catching little fishes. Puzzling as it may seem for landlubbers, these open boat survivors made no attempt to spear sharks that circled or snatch fishes from the sea. Instead, before too long adrift, they gazed hungrily at the cabin boys' flesh.
Philbrick tells us it is important not to wait to eat your comrades as if you hesitate too long the carcass of a starved man has little to offer in the way of food value as its fat and flesh has wasted away.
In some of the stories, the survivors were left with little choice. There were cases where they lost all, clothes included, as the ship went down. The only food in reach was that rare bird that landed on the boat, the flying fish that jumped in in the night or cabin boy in the bow.
And yes, the cabin boy was usually the first to go. Philbrick states the grown men had families to support back at home and their deaths would be more of a social hardship than the sad loss of a 13-year-old. In the case of the Essex cannibals, the boy's family forgave his murderers after they were recused and returned home to Nantucket.
In the Essex case, apparently fishing did not occur to the crew as a practicable source of food. In many other cases where the "custom of the sea" was practiced, there seemed no alternative as no means of fishing supplies were at hand.
In more modern times the well-equipped sailor has other options. Steven Callahan's "Adrift: Seventy-six Days Lost at Sea," is an example of someone who survived for months spearing and eating little fishes attracted to the shade under his life raft. So it is possible to feed at least one person from fish if there is a method. It is nigh impossible to feed 40.