It is often assumed that animal protein is not reasonable in a space habitat. Certainly if we try to feed high-quality grains and soy to an animal and then eat it we will have wasted significant amounts of calories, protein, electricity and other resources. Meat requires finding ways to use biomass that would otherwise be wasted.
When speaking of animal feed there are several factors of interest. First is the feed conversion ratio (FCR), or the amount of food required to produce 1 unit of live weight. Next, each animal requires a certain range of protein in their feed; fish need around 36% while chickens need about 20-22% and rabbits need around 16%. Shredded vines and moderate-protein waste (averaging 10% digestible protein) can be mixed with insects (40-45% protein), spirulina algae (60% protein) or fermentation byproducts to boost the protein content. Also of concern are the other bulk properties (moisture, fiber, total energy content) and vitamin/mineral content. Use of spirulina as a protein supplement for feed is a subject of current research; while amounts up to 40g per kg feed are safe for chickens it is likely there are unknown limiting factors (just as there are factors limiting human consumption to 50g per day or less). Numbers for the rest of this post will assume the use of insect protein; spirulina is much more efficient, so there is room for improvement here.
The edible yield of most plants is around 50% (though some plants of interest are lower and lettuce is higher). That means on average there is a kilogram of waste per kilogram of food; this waste is adequate for feeding to insects like crickets or fly larvae (at an FCR of 1.7 or so) which can then be fed to fish or chickens as supplements. Fish FCR is about 1.5 (for primary feeders; predatory fish are much higher) and edible yield is 32%. Chicken FCR is 1.8 to 2.2 and edible yield is about 33-36%. Rabbit FCR is about 4 and edible yield is about 46% (58% carcass and 80% meat). Rabbits can consume high-cellulose matter that fish and chickens cannot digest, so they can still be productive in a balanced system even though their yields are lower. Bones from all of these animals can be used to make stock before being further processed. Inedible animal biomass can be processed and fed back into insect or algae colonies. Sterilized bone meal is particularly useful as a supplement for egg-laying chickens.
Each person's 2.5 kg of food per day also yields 2.5 kg of ag waste. (If people's food intake were to contain animal protein then their ag waste might be more like 2.2-2.4 kg per day.)
For fish, 85% of that waste can be fed to insects which are then added to the remaining 15% waste and fed to fish, yielding 1.64 kg of 36% protein feed or 1.09 kg live weight or 350 grams edible fish fillets. On a per-kg waste basis that's 140 grams meat.
For chicken, 44% of the waste is fed to insects which are then added to the remaining 56% waste and fed to chickens, yielding 2.05 kg of 21% protein feed or 1.03 kg live weight or 350 grams edible meat. On a per-kg waste basis that's 140 grams meat.
For chicken eggs (see below), that same feed yields 827 grams of eggs (736 grams unshelled). On a per-kg waste basis that's 290 grams 'meat'.
For rabbit, 26% of the waste is fed to insects which are then added to the remaining 74% waste and fed to rabbits, yielding 2.23 kg of 15% protein feed or 558 grams live weight or 260 grams edible meat. On a per-kg waste basis that's 100 grams meat.
There is always the option of raising human-edible insects. With an FCR of 1.7 and an edible yield of about 80%, the same 2.5 kg ag waste could yield 1.47kg live weight or 1.18kg edible insect. On a per-kg waste basis that's 490 grams 'meat'.
Fish are very space-efficient; yields of channel catfish in excess of 1kg per m³ per day (321 grams of fillets) have been reported. Three hundred-gram servings a week would require just under 1 m² of half-meter deep fishtank. This would be a large system shared across many people; there are certain minimum populations to maintain for breeding. These values do not include the space required for waste management or breeding stocks.
Chickens need room to run. Some flocks can become aggressive and attack their own members; chickens will occasionally cannibalize each other if they are severely crowded. Ideally they would be housed on a bed of grass or growing greens and rotated periodically so they don't kill the bed. In a habitat this may not be possible. The amount of space needed can range from 0.05 to 0.2 m² in commercial farms, while hobbyists often call for 0.4 to 0.9 m². Free-range chickens would require 16.3 m² per bird. Chickens are typically harvested after 6 to 8 weeks (1.8-3.0 kg); that works out to 90-100 grams of meat per day per m² at 0.2m² per bird or half that for 0.4m² per bird. Three 100-gram meals of chicken meat a week would require 2.52 m² of coop and run plus a bug box. Chickens also require litter or some means of keeping their area clean and dry; shredded waste fibers can be used for this, but there are other demands for that material.
For egg-laying chickens the space requirements are the same but the feed has to be calculated a bit differently. Breeding flocks need around 1 rooster per 15 hens. Laying hens require about 20 weeks before they start laying, then lay for 10-12 months before moulting. Each chicken can lay an egg about every 26 hours but their cycle is light-sensitive. Just after beginning to lay, the percentage of chickens that lay an egg each day will rise rapidly to about 95%. Over the next year that number will gradually drop to around 70%. Productivity drops by about 10 percentage points per year while feed consumed slowly increases as the hens gain mass. The size of the eggs grows as the size of the hens grow; first-year hens lay eggs from 55 to 65 grams each. The daily average mass of egg per chicken is 48.875 grams over year 1. So, each hen requires feed for 72 weeks, produces for 54 of those weeks and yields 18.475 kg of eggs (about 308) over that period. They require 8.06 kg of feed to reach week 20, then eat about 100 grams of feed daily (37.8 kg). In total, 45.86 kg of feed yielded 18.475 kg eggs or an FCR of 2.48. Eggs are about 11% shell so the edible FCR is 2.79, far better than the slaughtered animal yields. At 0.2m² per bird that's 32.6 grams unshelled egg per m² per day. Meat chickens are more space-efficient, while laying chickens are more feed-efficient.
Rabbits need roughly as much room as chickens, though the range is narrower; 0.2m² is a good starting point. They are less aggressive and less likely to attack each other or have other behavioral problems. Broiler rabbits are typically harvested at 8-9 weeks or when they reach 2-2.2 kg. Normally rabbits are fed a compressed alfalfa pellet feed and straw. To reach the quoted FCR and yield values these must be fed a balanced feed with supplemental straw, though not necessarily alfalfa. These animals also produce a modest amount of skin that could be tanned to leather for gloves, grips, laces, etc. Rabbits can be raised for fiber, but the breeds that are productive for this have poor meat yield; these are best considered pets as the inputs are very high for the small quantities of fiber yielded.
Note that intensive production of animals using ag waste as described means that same waste will not be available for other uses like plastic. Use of spirulina will allow capturing nutrients from further down the waste stream (ie. sewage and biogas waste streams plus concentrated CO2 wastewater from life support) and could free up waste cellulose and sugars for fermentation.
I may consider pigs, ruminants and milk animals in a future post. Information available on them is much more comprehensive.