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I've recently become aware of the american 'survivalist' or 'prepper' phenomenon.

The general idea seems to be that 'something bad' (exactly what seems to vary) is going to happen, civilization will collapse and the survivalists will run to the hills and live off the land.

It occurs to me that the hills are likely to get over crowded fairly quickly.

Rather than living off the land, is it possible to live off the sea instead?

This isn't a question about the mechanics of living afloat (repairing your boat, fixing leaks, keeping the reverse osmosis plant working) it's more a question of can someone gather/hunt/farm all of the nutrients required to be healthy whilst never having to land again? Obviously fish would be a prime source of food, but would that provide everything a human body needs? Is plant matter essential for survival and if so are seaweeds adequate and can they be farmed? Or are their some essential nutrients that can only be gathered from the land?

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    Kelp forests and other seaweeds generally are found close to the shore. Are you talking about deep ocean, or near shore? Related to that are you only interested in océans or would a large lake work? You might want to research the Uru people of Peru and Bolivia if lake living is acceptable. – Erik Jun 3 at 19:23
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    Not sure if this falls within the scope of your question, but after TEOTWAWKI, the decay of industrial infrastructure might lead to very polluted coastal waters. – No U Jun 3 at 20:36
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    This seems to be mostly an American thing; the population density of the USA is about one fifth the world average, so the hills won't get too awful busy (as long as a few people are prepared to head up to Alaska). – David Richerby Jun 4 at 12:00
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    @MartinBonner then by extension can't we just say "there's a reason why the oceans are uninhabited?" If someone is willing to overcome finding a source of fresh water while living afloat in the ocean, then finding a source of fresh water on an otherwise-uninhabitable island shouldn't be a problem, right? – dwizum Jun 4 at 19:49
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    @MartinBonner If lack of fresh water is the overwheming reason for the uninhabitability then he's in luck, as he notes that he has a reverse osmosis plant to derive fresh water from seawater. Many things that are doable "at sea" will be easier on land (the vessel is generally larger, MUCH more stable, maintains its orientation, is storm resistant (except for very small bits of land) and tends not to sink. – Russell McMahon Jun 4 at 23:01
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Theoretically, it might be possible to survive on fish, rainwater and desalinated seawater. In one 1930 experiment, two men survived for a year eating exclusively meat without experiencing any health issues. This indirectly suggests that meat (and therefore possibly also fish) contain all the essential nutrients, but this was a small and relatively short study from which you cannot conclude this would work for other people.

Potential nutrient deficiencies

Fish contains proteins and fats, but no carbohydrates - which are not essential nutrients. There are some carbohydrates in clams, other shellfish and seaweed. Without plant food, you do not get any dietary fiber, which could cause constipation in many people.

Fish, shellfish and seaweed in combination seem to contain all the essential nutrients, but the question is if you would get them in sufficient amounts from the food you could gather.

The signs of vitamin C deficiency can appear within 1 month of little or no vitamin C intake (below 10 mg/day). Seafood high in vitamin C (>5 mg/100 g) includes certain shellfish, crabs, octopus and roe; kelp contains 3 mg and most fish contain less than 1 mg per 100 g.

You can get calcium from the fish that have soft edible bones (sardines, salmon) and from shrimps. Fish meat is relatively low in calcium.

Iron can be found in sufficient amounts in certain shellfish and fish.

Vitamin B12 is abundant in fish. Salmon contains 4.5 mcg of vit B12 per 100 g; the daily requirement is only 2 mcg.

Potential nutrient and toxin excess

Most fish species are low in sodium (<100 mg/100 g), but shellfish can contain up to 500 mg (Seafood Health Facts).

Certain seaweeds can contain high amounts of iodine.

Mercury in high amounts is found in shark, swordfish, king mackerel, tilefish and bigeye tuna. Common fish low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.

I think you can avoid getting "toxic" amounts of any nutrient by carefully selecting seafood.

In conclusion, it may be possible to survive for a year or more exclusively on seafood, but the question is how healthy is such high-protein, low-carb and low vitamin C diet.

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    Adding to the list of potentially missing nutrients: Calcium? (especially for children) There might also be an issue of getting too much nutrients. I'm specifically thinking about salt here, but there may be other issues. – Matthieu M. Jun 3 at 18:39
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    The Inuit survive largely on meat and fish and are able to get enough vitamin C to prevent scurvy, though they may still be vitamin C deficient – Johnny Jun 3 at 21:37
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    The man in the 382 day fast was also given vitamin supplements (including vitamin C). Whereas the all-meat diet study did not include those (since one of the main things it was studying was whether vitamin deficiency would occur). So the existence of the fasting study (and lack of scurvy in the same) does not suggest that the time frame of the meat-only study was too short. It is also worth noting that the subject in the fast study lost 270 pounds over the course of the study, so nutrients were provided from fatty tissue being broken down. – Ray Jun 3 at 21:47
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    For calcium, you can eat fish bones. Sardines are known to be a source of calcium because the bones are soft enough that you can just eat them directly. – user3757614 Jun 4 at 2:47
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    Yes, it does. If you click on the 3rd link ("Fish") in my answer, you can see that 100 g of salmon contains 4.5 mcg of vitamin B12. If you click on "sufficient amounts" link, you can see that daily requirements for vit B12 are 2 mcg per day. – Jan Jun 4 at 9:17
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... can someone gather/hunt/farm all of the nutrients required to be healthy whilst never having to land again?

Nutrients yes, healthy not likely.

A 1000 days is a grueling journey.

Reid Stowe, whom used to have a website called (Beyond) 1000 Days at Sea: The Mars Ocean Odyssey, lived at sea (without contact with land) for 1,152 days (equals 3 Years, 1 Month, 3 Weeks, 5 Days, 13 Hours, 26 Minutes and 39 Seconds).

Stowe is an experienced mariner and was the principal designer and builder of the Anne, a 70 ft (21.3 m), 60-ton (54,400 kg) gaff-rigged schooner which he sailed on this voyage. The purpose of the enterprise was to remain on the open ocean, without resupply or pulling into any harbor, for a period of one thousand days.

He took a lot of supplies with him and grew simple plants (like beanspouts) while at sea.

The heavily laden schooner passed through New York Harbor and into the open ocean by the evening of April 21 2007. ... On Day 658, Reid Stowe broke the world record for the longest non-stop ocean voyage, previously held by Jon Sanders, if one disregards Nansen's Fram expedition, during which the schooner Fram lay trapped on ice for nearly three years, and the crew was away from land for at least 1067 days.

Reid Stowe and his support team have since accomplished one of their goals of a person sailing on the open seas without resupply for 1000 days, as well as breaking the 1067-day record set by the Fram in 1896. January 16, 2010 was officially the day of the 1000-day mark, while March 24, 2010 equalled the 1067-day mark. Subsequent to the first 306 days with Soanya Ahmad, Reid Stowe also broke the record for the longest solo sea voyage without resupply, on Day 964 (Dec. 11, 2009). Furthermore, as a two-member male-and-female crew, Stowe and Ahmad could also lay claim to the longest non-stop voyage on the ocean by a man and a woman since Bernard Moitessier and his wife Françoise completed a 126-day voyage in 1966, from Tahiti to Spain.

None of the records claimed by Stowe were ever officially recognized. The World Sailing Speed Record Council explains on their website: "We concentrate on speed record attempts and claims, and no longer recognize "human condition" categories which can expand to such an extent that almost anyone would be able to claim a record of some sort."

Throughout the journey, Stowe maintained contact with the New York City–based support team via an Iridium phone. Stowe employed a VHF marine transceiver for ship-to-ship communications. Volunteers maintained a web site so that the general public could follow the progress of the voyage. The entire route of the schooner Anne was verified daily by GPS tracking, and the manufacturer of the equipment has made the database available online.

Guinness World Records:

13 March 1988
The longest distance sailed non-stop by any vessel is 71,023 nautical miles (131,535 km; 81,732 miles), a feat achieved by Australian Jon Sanders between 25 May 1986 and 13 March 1988. Starting and finishing in Fremantle, Western Australia, Jon made a record three consecutive non-stop solo circumnavigations of the globe – one west and two east – and was at sea for a total of 657 days on his 13.9-m (44-ft) sloop Parry Endeavour.

No doubt you could load a cargo vessel with a lifetime worth of supplies but there are practicalities such as maintenance of the vessel and your own health.

Pescetarianism or pescatarianism (/ˌpɛskəˈtɛəriənɪzəm/) is the practice of adhering to a diet that incorporates seafood as the only source of meat in an otherwise vegetarian diet. Most pescetarians are ovo-lacto vegetarians who eat seafood along with dairy products and eggs, often colloquially defined as "fish but no other meat".

Yes, it is considered a healthy diet.

If you can make your own repairs at sea, as Stowe did, and never need to visit a doctor (or a hospital) then you can live at sea indefinitely.

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