A quote from Wikipedia:
"Use as animal feedEighty percent of the forage fish caught are fed to animals, in large part due to the high content of beneficial long chain omega-3 fatty acids in their flesh. Ninety percent is processed into fishmeal and fish oil. Of this, 46 percent was fed to farmed fish, 24 percent to pigs, and 22 percent to poultry (2002). Six times the weight of forage fish is fed to pigs and poultry alone than the entire seafood consumption of the U.S. market. One of the most promising alternatives to fish oil as a source of long chain omega-3 fatty acids and certain amino acids is algal oil from microalgae, the original source of these fatty acids in forage fish.
According to Turchini and De Silva (2008), another 2.5 million tonnes of the annual forage fish catch is consumed by the global cat food industry. In Australia, pet cats eat 13.7 kilograms of fish a year compared to the 11 kilograms eaten by the average Australian. The pet food industry is increasingly marketing premium and super-premium products, when different raw materials, such as the by-products of the fish filleting industry, could be used instead."
These stats give some basic dimensions of the forage fish fishery and their use, and would need a closer look at the studies quoted, but you can use these to do a number of interesting things for farmed fish/humans:
1. Feed conversion rate. Fish farms like to crow it is getting close to 1 to 1, ignoring, that live wet fish are about 4 to 5 kg to make 1 kg of fish meal.
2. If half is fed to farmed fish, rather than humans, that is a real waste.
3. The real issue is that omega fats can be raised from algae, an alternative, and make the fish farm/sustainability issue start to approach something that can use the two words in the same sentence without it being an oxymoron.
4. You can compute the actual weight in millions of tonnes of the forage fish used for fish meal.
5. You can compute the actual number of forage fish fed to a farmed salmon during its lifetime, making it concrete. I would be surprised it was less than 100 dead forage fish per kg. So 100 deaths for one first world human meal, well, 101, because we would have to kill the farmed fish to eat it in the first world.
6. The Scotland science on the stomach biota show it can be manipulated to allow carnivores to tolerate more vegetable source fats.
7. 80% of killed forage fish are fed to animals rather than humans who need the food. You can calculate the actual tonnage 'taken' from humans.
8. And so on, limited only by your imagination.