Don’t really see how it’d make it any more efficient

In a new study out Monday in the journal Fish and Fisheries, researchers say that the vast majority of fishmeal is actually made up of fish deemed suitable for “direct human consumption.” […] Researchers say a whopping 90 percent of that catch is considered “food grade” and could be eaten directly

https://www.npr.org/sections/thesalt/2017/02/13/515057834/90-percent-of-fish-we-use-for-fishmeal-could-be-used-to-feed-humans-instead

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Not to mention there’s other effects of fish farms outside of just the overfishing part that I didn’t even list earlier. They’re actually a big player in mangrove deforestation, for instance

Conversion to aquaculture is the most prevalent driver of mangrove deforestation across the tropics over the last 50 years generating substantial carbon emissions. Preventing further aquaculture expansion within mangrove forest areas will be essential to achieve national emission reduction targets in mangrove-holding countries.

https://onlinelibrary.wiley.com/doi/10.1111/gcb.14774

Or antibiotic usage

High frequencies of antibiotic-resistant bacteria have been reported in sites near aquaculture where antibiotics have been used, demonstrating that modified antibiotics in an aquaculture facility have a high potential to exert selective pressure and increase the frequency of antibiotic resistance in other environmental bacteria [40,41]. In the aquatic environment, 90% of aquatic bacteria show resistance to at least one antibiotic, and approximately 20% were multi antibiotic-resistant. […] An important and at the same time worrying aspect is that the antibiotics used in aquaculture include those used in human therapies, thus inducing resistance to these antibiotics

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8198758/