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I am trying to create an island with a large settlement on it, the people of this settlement need to be able to sustain themselves and according to Medieval demographics made easy (MDME)

A square mile of cultivated land (including not only farmland, orchards and pastures, but also the roads and settlements attendant to them) will support around 180 people. This takes into account normal blights, rats, drought, and theft, all of which are common in most worlds.

However, the MDME estimate does not seem to account for fishing. Given that the settlement is surrounded by the sea and therefore will make heavy use of fishing as a way to feed it's population.

How would the land area required to sustain the population change? If fishing can be a substitute for the pastures needed to provide meat to meet human dietary requirements.

Assuming: There is plenty of space to fish and plenty of fish to catch

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I grew up on an island, me, and one which both grew a fair amount of fresh veg and on which fishing was a primary concern.

You've missed several important elements in this discussion:

One: not only salt as a preservative, but the combination of salt and smoking / drying (think both kippered herrings and the Pacific Northwestern Native American salmon jerky) which can create foodstocks which will last for many months. Hell, pemmican too.

Two: many island peoples not only fished but also collected seaweed, which is incredibly nutritious and high in iodine. This collection works well in parallel with gathering shellfish like mussels, winkles and limpets, and in some areas, shrimping.

Three: many islanders also propagate hardy non-cattle ungulates which require minimal supervision, such as goats and sheep - where I grew up we had a herd of local sheep which ate seaweed half their time and grass half their time as they migrated on and off Lihou island via a causeway - their wool was highly prized due to its additional lustre and warmth. These herds also produce goat or sheep milk, which is intrinsically of high nutrition, and makes long lasting cheeses.

Four: Tuna as a nutrition base is somewhat unrealistic (large, fights hard, prefers deeper waters, requires one serious line to handle it, hard to gaffe and return to port) - let's instead take a school fish like mackerel as a better example, as it can be either line fished with small lines or net fished: 1 Kg = 2620 calories, and the nutrition breakdown is similar to most other ocean-going fish - note that it's a higher caloric content per weight than the tuna. Where I grew up we routinely fished mackerel, smelt and whiting, and only occasionally saw tuna far out.

Hope some of this helps.

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    $\begingroup$ Your answer provides some amazing insight, especially points #2 and #3, thank you very much! I have some further questions, since i'm looking at how the land area needed to sustain a population is affected by fishing and such. Would you say that fishing and collection of other sea-based organisms would reduce or eliminate the need for pastures (excluding those used for sheep and their wool/milk as you mentioned). $\endgroup$
    – MB123
    Feb 26, 2019 at 17:58
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    $\begingroup$ Based on the fact that the island I grew up on, Guernsey, is famous for two things: one a specially-knitted fisherman's jumper called a Guernsey which is knitted from oiled wool which keeps you warm even soaked; and the Guernsey Cow, famous for its incredibly high butterfat percentage super-nutritious milk... I'd say reduce, yes, eliminate, probably not. Japan raised Wagyu cattle & rice from 200 AD, but until the 1860's most Japanese Buddhists wouldn't eat meat. Look at the diet of much of Asia: rich in fish and rice; you need arable land for cultivating grains/carb I think. $\endgroup$
    – GerardFalla
    Feb 26, 2019 at 18:20
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    $\begingroup$ So probably a reduction in pastures is possible if the nutrition it normally provides is substituted by the nutrition brought in through the fishing practices but not eliminated due to the usefulness products such as wool and milk. And yeah of course you would still need agricultural land for your grains/carbs and such, hence the distinction between the two. I'm learning so much, thanks! $\endgroup$
    – MB123
    Feb 26, 2019 at 18:44
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I think your question should be broken up into 3 questions. However I am posting this elaboration as an answer because I will show you how to calculate what you want from the answer to the 3 following questions.

Question:

(1) "How much fishing (in Kg/year) per square mile of ocean, is sustainable?"

(2) "How many calories of food per year does an average human need?" A rough answer is 730,000 Calories (2000 daily calories x 365). Tuna has about 1200 Calories/Kg, so a human would need to eat about 608 Kg of fish a year to survive.

(3) "How far could a medieval fishing ship travel for fishing and still be able to return with edible fish?". If we guestimate this we can use the average windward medieval ship speed of about 5knots, and a limitation of 1 week voyages to leave and come-back, to come to the conclusion that this distance is 477 miles.

Assuming you can build as many ships as needed, your answer is then: 

 A = Average height of island + max ship travel distance (477 miles)
 B = Average length of island + max ship travel distance (477 miles)

AreaTotal = A x B
AreaIsland = Average height of island x Average length of island    
FishableArea = AreaTotal - AreaIsland;

SustainableFishingYeildPerYear (in kg) = FishableArea (in miles) x SustainableYeild(kg/mile)
AmountOfPeopleItCanKeepAlive (in humans) = SustainableFishingYeildPerYear (kg) * 1/608 (humans/kg)

I don't know the answer to question 1, so I leave the research to you.

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    $\begingroup$ *"1 week voyages to leave and come-back:" and what do they do with the fish? It will be rotten when they come back. In medieval times fishing was pretty much done with fishing boats which left port and came back on a daily basis. $\endgroup$
    – AlexP
    Feb 26, 2019 at 16:01
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    $\begingroup$ Possibly preservatives: salt exists at this time and could easily be distilled from sea water. But you can change that number to whatever you want. I am not going to defend that number since I chose it out of thin air. $\endgroup$
    – Tyler S. Loeper
    Feb 26, 2019 at 16:01
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    $\begingroup$ Thanks for the great answer! Should be able to figure out #1 and #2/3 help loads. Do you have an idea as to nutritional requirements as i presume a human can't survive solely off fish? I reckon it would be an alternative to the pasture "produced" meat. $\endgroup$
    – MB123
    Feb 26, 2019 at 16:05
  • $\begingroup$ Using Tuna as an example, it has the following vitamins, etc: selenium, vitamin B12, vitamin B6, protein, phosphorus, vitamin D, choline, iodine, potassium, vitamin B1, vitamin B2. - whfoods.com/genpage.php?tname=foodspice&dbid=112 -. Anything else that a human needs would have to come from somewhere else. $\endgroup$
    – Tyler S. Loeper
    Feb 26, 2019 at 16:11
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Fish and shoreline scavenging (shellfish, crabs, seaweed) are a complete diet. You can, in theory, subsist entirely on these.

However, how exactly this looks (and the land resources required) depends massively on the local environment.

  • How strong are local seasons? If very bad, like Iceland or Norway, you need to salt & preserve your food over the winter famine. If weak and broadly pleasant, like in the tropics, this is less of a consideration.
  • How accessible are the fish? If there are on-shore reefs, you can access food by spear-fishing and diving (eg. Bajau Laut tribe in Indonesia). If you are pursuing free-swimming ocean fish then you require substantial boats that can sail independently for several days, which requires good lumber (and, preferably, metal tools for woodworking) and hopefully some cloth/weave for sails and nets. In between these extremes you could use something more akin to rafts made from small trees (paddle to fish stocks within sight of land in non-treacherous waters).
  • How productive are the available waters? Depending on ocean nutrients, there is a variable amount of fish you would be able to sustainably extract.

Which is to say that pleasant local ocean conditions can support villages with little land requirements if you can get your food within the coastal biome. Larger scale ocean fishing operations require forests and low-grade industry to build the tools that fishing requires, which is a non-trivial land & personnel use that would not be possible on tiny islands.

Ballpark, I'd be happy to throw in a few hundred people on the tiniest volcanic squirt of an island if they could access shoreline shellfish and nearby reefs.

On mid-to-large islands, you could supplement farming with fishing and perhaps boost pop. density by 10-25% (also health/famine benefits from a diversified diet), but it comes with a trade-off of land-use and industry. I would expect inter-tribe/inter-national trade and some form of military to co-exist in any society that makes big fishing boats that sail over the horizon for a few days.

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