Mfcttrf

You have problems, and it isn't food

Food is easy. The ocean is full of fish. So long as your story places the aircraft carriers in the right place, food is irrelevant.

Your problem is nuclear fuel. Those aircraft carriers are big, sealed, bathtubs. Without ventilation, the CO₂ resulting just from human breathing would slowly make the lower decks uninhabitable (IMO). The ventilation is designed for electricity, so electricity is the tall pole in your tent.

And electricity on a nuclear-powered aircraft carrier depends on fuel rods. The period between refueling for a Nimitz-class aircraft carrier is 25 years.. Could that be stretched? Somewhat, but it's not like you can parcel fuel rods out over time. So, 150 years is completely unbelievable without replacement fuel rods, and disposal of the old ones (don't want to radiate the fish!).

If you let the ventilation die out, then you're restricted to living on decks with access to port holes, doors, etc. that open to the outside air, and maybe (maybe) a deck below that. I honestly don't know how far toward the keel portholes go, but you certainly won't have them below the water line. And considering the nature of storms over the ocean, it's unlikely people will want to live on-deck.

Just out of curiosity

• You have a deadly disease that's killing people. Why do you have to wait 150 years? If it's that lethal, you'd only need to wait 5-10 years before 90% of the population was killed off and huge swaths of land became re-inhabitable. 150 years is an awfully long time. One would hope your doctors aboard the aircraft carriers were trying to eradicate the disease and that medical equipment to do so would be among the highest priorities during those early shipments from land. I find that 150 number really hard to choke down.




• This plot is hauntingly similar to The Last Ship by William Brinkley. You might want to go read that novel. Your aircraft carriers would be in radio contact with everyone they possibly could for the entire 150 years, tracking the viability of returning to shore anywhere.

Assuming dirt farming, as opposed to large scale industrial hydroponics, aeroponics, or mycoculture it takes roughly an acre of land to feed a human for a year. We can reduce that somewhat if we assume that the fleet harvests plankton, krill and other high density marine life. Further still if the fleet can take advantage of some of the more advanced techniques available to the modern close space farmer but you're still looking at relatively large volumes of space for farming per person.

I think the fleet is done for; let's look at an example: The flight deck of the Abraham Lincoln is roughly 6 acres and it ships nearly 5700 crew and flight wing.

The bigger problem is that none of the ships will be afloat after more than a couple of decades at most, seawater is one of the most corrosive substances known to man, without regular changes of their sacrificial anodes ships hulls will simply fall apart. Without anti-fouling measures, that require dry dock time to complete or renew shear weight of marine life will sink ships.

There's a lot that goes into answering this, and the answer could turn into either yes or no depending on what assumptions and trade-offs you make.

Firstly, nuclear carriers have an operational lifetime of 50 years, and get a overhaul and refueling at the 25 year mark. This is assuming that the carrier is actively being used for military purposes, if you're content to let it float or cruise at low speed in the middle of the ocean, this timeline can likely be extended to a degree, however there will come a time when the nuclear plant has to be shut down because the lack of maintenance makes it too dangerous to operate even if you've still got fission fuel.

Second bit, there's a lot of stuff happening on an CVN that's just there to keep it in fighting shape: F-16s? Chuck 'em. Avgas? No need for that. Munitions? Garbage. Steam machinery, catapult piping, arrestors, carrier ops, all gone. Machine shops? Cut those back and re-purpose them to ship maintenance. That will free up a lot of space and mass.

Third bit, food and water. Assuming that whatever plague can't be transmitted through consuming fish, the bulk of the ark's diet will be based on fish, seaweed, maybe using algae as a supplement, and hydroponically or aeroponically grown citrus. A meat based diet like those of the Inuit actually provides all the nutrients and vitamins you need as long as the whole animal is consumed - but fruits and vegetable carry vitamin C in much larger quantities and are thus the best source for it. That means you can afford to feed a much larger population than you'd expect if you're assuming that you're growing everything on board.

Fourthly, corrosion and fouling. As others have mentioned, seawater is beastly stuff. But, there are ways around it. Sacrificial anodes have been mentioned, and it's simple enough to just stock up on a whole bunch of those. Wrap them up in airtight bags and attach them to floats then toss them overboard on a line if you've run out of space on board.
Fouling wise, you CAN remove it while still at sea, it's just more of a hassle than it's worth, which is why it's usually done in dock. But there's nothing stopping you from coming to a dead stop and sending out a bunch of divers with electric scrubbers to scrape off fouling. You can take it a step further if you have the equipment, and you can always set it a few years in the future and say they've got ultrasonic anti-fouling hull vibration systems or electrified graphene coatings.

Fifthly, weather. No great way around this. You'd want to keep north, where the water is too cool for hurricanes to form. Not south, because the waves there get really high. Still, you'd need some power to station-keep. By the time your nuclear reactors spool down, the carriers should have working kite sails to be able to maintain some maneuver capability. Their size works for and against them: they're big and thus stable in high seas, but they're also difficult t move with wind alone.

Finally, is this infection magically bound to all pieces of land? There are a lot of uninhabited islands out there that would be a lot more viable for long term habitation than living on the ocean all the time.

FOOD

As others have mentioned, food isn't an issue. You have fish and you have seaweed. You can farm fish and also do things like grow oysters on strings. Some vertical farms on the boats could grow some land vegetables (with compost adding to the soil so it doesn't deplete). Fermenting the vegetables in brine (easy to come by) gives you plenty of Vitamin C, probiotics, and B vitamins.

Once the colony is formed, the fish and sea vegetables will need to come from smaller crafts sent out for gathering. Why? Because of the human waste generated by the colony that goes into the surrounding ocean. Yes it helps feed plants and animals but it's not sanitary to harvest directly from it.

WATER

Fresh water is a much bigger problem. Mostly this is for drinking/cooking. While humans feel better bathing in fresh water, it's not required. When there is heavy rainfall, the excess fresh water can be used for laundry and cleaning to get salt residue off.

Rain is uneven and you need enough fresh water storage containers to carry you through to the next heavy rainfall. Dew and condensation collectors can help bridge the gap. And there could possibly even be desalinization technology that works on the right scale to make a difference.

The main issue here though is time. Not just the time between rainfalls, but time ruining your equipment and damaging your storage containers. Plastic containers won't last 150 years (they probably won't last 20 years, especially if exposed to the sun). Metal containers will corrode and develop leaks. They can be repaired, but eventually the patches aren't going to do the trick.

Collectors also will develop holes and leaks. Something as simple as a hanging tarp to allow dew to form overnight and drip into a bucket will not work in a few years because the materials have degraded so much (and can't be replaced). Desalinization equipment will also break down.

CANNIBALISM

Cannibalism can take two forms:

1. Deliberate "culling" of people for food.


2. Making use of the bodies of people who have died.

Either way though, it doesn't make much sense. There's no need to eat human flesh because there will be plenty of fish in the sea, even after 150 years of local eating.

Population control is a big issue though and there won't be any birth control available after the first few years, aside from periodic abstinence (which works better than people think but only if you care, chart, and don't cheat). That's not cannibalism but it may give your story the right amount of horror you want.

There is also the issue of criminal behavior. There could be some prison space (perhaps even a prison boat) but it's not practical to punish people by locking them up long-term. For people where "community service" and plain old shunning/shaming doesn't work, there might be a death penalty.

If you really want some cannibalistic element, use prisoners' bodies to feed the fish farms.

POPULATION

The number of people you can have will depends obviously on the capacity of each ship plus how many ships you have. Some ships might need to be workstations or for storage. If not, then you need to allocate space for those things within ships. Ditto for food and water storage, food processing, and general storage.

Food supplies won't limit the population (it does on Earth now but your ships will only have a fraction of what Earth has). But water supplies will, especially as your storage capacities diminish over time.

Could some number of people survive without resorting to cannibalism? I think the answer is most certainly. Could 7 Billion people survive off harvesting the Pacific Ocean for 150 years? On average humans need 100 grams per day and 1500 calories. That's 700,000 kg per day of protein and a lot of calories. Today, we harvest 90 Billion kg of fish, muscles, and so on from all the seas around the world. So there seems like there would be enough food for a lot of humans. Survivability would be enhanced by colonizing the other oceans too, and not just the Pacific.

I think the biggest challenge would be power generation. They'll need to smelt metal to repair parts of their ships -- 150 years at sea is a long time, and ships rust.

Assuming no issues with the ships ventilation as stated by JBH, here are some solutions.

If your goal is to ensure the survival of humanity

Just do Adam and Eve for each ship. If each ship have a plot of land and fish, a pair (a male and female), they continue the human race; especially if there are multiple ships out in sea. Ideally, you would want a doctor or medical expert to be either the husband or wife and some specific vitamins to make up for the missing resources provided by eating meat.

Since there's only two human per ship, you can even bring in livestock like cows and chicken as they produce useful food items. Heck, if each ship brings a different combination of livestock and/or plants, they can implement a trading system where each ship (or family) can trade with other ships for essentials. This will ensure that each ship is specialized in one produce.

As for who gets to live and who gets to die, well, that's where this gets a bit nasty; it is the survival of the fittest. Those who are the healthiest and have useful skills (i.e. doctor, farmer, fisher) gets to live. While pairing should be compatible/lover, with the end of humanity, lovers can't be choosers.

Save as much people as possible

There are multiple suggestions of fishing/farming, but here is another, very disgusting, albeit doable way.

This is pretty bad, you have been warned

Eating one own poop could help offset some food issues provided that medications are abundant. This is because the digestive system cannot 100% process what we eat fully and thus our waste still have some benefits.
There are health risk with this obviously, but that is where medication comes into play. Meds are generally small and last a long time.
The idea is for the rest of the years, carefully balance fishing/farming and eating poop to survive. And every other or so excretion gets reused as food (obvious for oneself, it is already bad as it is, but force other to eat yours?).

The main idea is not to exhaust the sea and whatever land you have. Over fishing and and planting can greatly tax the resources and led to early depletion of resources. So by limiting the amount of fishing and farming via other means, one can last longer. Also, the waste can be used as fertilizer to help rejuvenate the land and with careful amount, attract micro-organisms which can then attract big predators, leading to more fishes.

The reason I did not mention bring livestock in this case is that humans + livestock = too much waste and thus cause heavy pollution which can easily led to quicker contamination of water and air. Especially cows.

Screw the World

This one is pretty much ignore the disease and move back to land. Humans are very adaptable creatures and a group of people may be able to become immune to the disease.

To help with this crazy idea, lets go even crazier. Radiate people; you got nuclear reaction at your fingertip. Mutation can lead to genes which could help the person survive the disease. What kind and how much? That is up for grab and chance.

Would not recommend if you want to see humanity live; would be a fun simulation tho.