In fish, chicken and many other types of farms the conversion coefficient from food to animal is often in the range of 2:1. It takes 2kg of food to produce each kilogram of animal. On the other hand if you studied biology, you learned that about a tenth of the material from one tropic level is captured in the next level. It takes 100kg of krill to make 10 kg of penguin and 10kg of penguin to make 1kg of leopard seal. The conversion coefficient is 10:1. So what is happening and why is it important.
First the what. When you are talking about the situation in nature, you are talking about wet prey and wet predator or even better dry prey and dry predator. Dry-dry is better because different animals can have different percentages of water and what is of interest is how much actual material exclusive of water, transfers from one level to the next. The flip side, of course, which will concern us just now, is how much material goes back into the environment to power the food chain (if in quantities the environment can handle) or pollute the environment (if there is too much of it). Clearly if 10% is captured, 90% is excreeted, secreted and respired out into the environment.
In farms the feed is generally a formulated pellet often using fish meal and soya as its source of protein. The fish meal portion of the pellet is dehydrated ground fish and the soya is the dehydrated seed of the soya bean plant. Depending on availability and on which animal the feed is for, many other components may be added to make the feed, along, usually with a vitamin package, a mineral package and usually a source of starch for energy and binding. The point here is that the pelleted feed, to keep it from rotting in the sack and to reduce shipping costs is dry, typically containing less than 7% moisture.
An uninitiated reader on learning that a salmon farm (samon feed lot actually) achieves a conversion coefficient of 2:1 would be forgiven for assuming that for every kilogram of feed fed to the fish, 500grams in the form of faeces, nitrogenous wastes from excretion and Carbon dioxide from respiration goes out into the environment. Not so. As you can see from the above, the figure is actually closer to 900g of waste per kg of feed. The fish eat the pellets, add water and put on half a kilogram of weight per kilogram of pellets they eat. However, the half kg that they gained consists of 100g from the food they ate and the rest is water.
One shouldn't think that the farmer is pulling a "fast one" in the way he expresses the conversion coefficient. He pays per kg of pellet he buys and receives revenue per kg of animal he sells. Conversion coefficient the way he views it is a perfectly valid method for calculating his profitability. It is only a "fast one" when someone tries to pull the wool over the eyes of the public by falsifying how much of the feed actually finds its way into the environment.
If you want to find out how much waste is shed by a salmon feed lot in order, for instance to work out the equivalent sized town that would contribute the same amount of waste, you will be pretty accurate if you calculate 90% of the feed used by the feed lot.