With wild fish reserves more and more depleted, fish farming seems like a more sustainable and downright necessary alternative. And it has gotten huge. The problem though, as this Wired article explains, is that the food given to farmed fish is also made from wild fish. In this case, the smaller ones like anchovies that people don’t eat as much. So not only have humans sifted out most of the big fish, we are now vacuuming up the small ones that form the base of the wild food web for all the other creatures in the sea. There are some attempt to develop alternatives such as “feed made from seaweed extracts, yeast, and algae grown in bioreactors”. Basically, if humans are not going to go vegetarian, which might be the most sustainable option of all, we can at least try to eat vegetarian fish.

Fish meal—dried and ground up fish bits—and its more lubricious counterpart, fish oil, are made from cheap species that humans don’t eat that much: sardines, herring, anchovies, krill. But lots of other ocean animals do eat them; they’re kind of the linchpin of marine ecosystems. Lose the forage fish, lose a lot more. And as those forage fish catches are getting smaller, fish meal and oil-based diets are getting more expensive. Since 2012, prices have risen more than 80 percent. “Aquaculture is growing so fast that it can’t possibly continue to use any more,” says Kevin Fitzsimmons, a biologist at the University of Arizona and former president of the World Aquaculture Society. “Forage fish are just maxed out…”

Any kind of plant- or algae-based feed still relies on photosynthesis, and that requires surface area on the Earth, unless and until we go to all artificial light in high rises or in orbit powered by something like nuclear power. Yeast is interesting because it doesn’t require photosynthesis, only some kind of organic input. But if we hit the limit caused by the Earth’s footprint, then go to a technology that is not limited by Earth’s footprint, we will just tend to expand until we hit another limit. And our original natural ecosystems get completely lost somewhere along the way, if they have not been already.