Mood et al., 2023 had an earlier estimate:

The FAO (2022c) describes a system in which 4,500 hatchling carp are concurrently stocked to feed each mandarin fish. If this is typical then mandarin farming overall consumes an estimated 3,000 billion feed fishes, i.e. 3.0 × 10¹², based on an estimated 674 million mandarin fish (Table 3).

From the FAO source (archived):

According to field practice, live foods should be at a density of 3 800–4 500 fish per m². Moreover, the size of the live food should be well controlled to keep pace with the growth rate of the cultured fish. In order to provide an appropriate quantity of live food, provision procedures are suggested as: 1 to 4 days after pond fertilization, silver carp and bighead carp hatchlings are stocked as live foods at 1 500 per m²; one week later, the same quantity is again stocked; and again an additional week later, stocking of the same number is performed. The third stocking should be accompanied by the stocking of mandarin fish of size 3–4 cm at an average rate of 1 fish per m².

So 1 mandarin fish per m² vs 3 800–4 500 feed fish per m²?

Why do you think it was naive instead of a good bet that happened to not work out?

Another two:

1. Rethink Priorities, but could be hard to evaluate. You'd also have to restrict funding to the animal department.
2. THL UK, which fundraises separately from THL and gets limited funding from THL.

I had Perplexity do a review and compare PETA's report with Welfare Footprint's research here, and also critique and review the report Perplexity generated here. Summary from the second link:

Where Models Agree

Another: Animal Policy International

I'm guessing this is on your radar, but I think it's worth checking basically every Coefficient Giving, Navigation Fund and EA Animal Welfare Fund grantee over the last ~2 years.

You could focus on those that received at least $X (e.g. $100K) if you're looking to filter quickly, but we should also be looking to get good orgs to consider more ambitious plans.

Several more invertebrate agriculture uses here.

From 2000 to 2023, the number of species comprising 85% of aquaculture production grew from 14 to 22.

Does this account for the >1 trillion fish fry artificially propagated in China per year, a large share of which are probably fed live to mandarin fish? See my post here, and some (higher) estimates here. My sense is that these fish aren't counted in the FAO stats, because they're not slaughtered for food, and fish fed to mandarin fish are from a smaller number of species. From my post:

Li and Xia (2018) wrote “Almost all prey for mandarin fish is provided through artificial propagation”, and single out mud carp as the favourite feed fish, although others are reported elsewhere, e.g. FAO:

Common live foods for mandarin fish include mud carp (Cirrhinus molitorella), Wuchang fish (also called Chinese bream, Megalobrama amblycephala), silver carp (Hypophthalmichthys molitrix), bighead carp (H. nobilis), grass carp (Ctenopharyngodon idellus), crucian carp (Carassius carassius), common carp (Cyprinus carpio), stone moroko (Pseudorasbora parva) and other wild and trash fish. Wuchang fish fry is preferred at the start of food intake, then feeding bighead and silver carp follows. When body length reaches 25 cm, common and crucian carps are fed.

And “silver carp, bighead, grass carp, Wuchang fish or tilapia fry” (Kuanhong/FAO, 2009).

I agree with those benefits but there's no mention here of potential costs? Maybe you don't think those are significant? 

If we're assuming the post would be good quality, then I don't expect the costs (to me) to be significant, but I'm open to reasons otherwise. If the posts are sometimes low quality or repetitive, then AI could enable more of them, and that would be bad. I'd lean towards allowing 100% AI written posts and seeing what happens to the EA Forum, i.e. tracking the results and reassessing. 

Maybe the voting system, minimum karma to post, and throttling based on recent net negative karma posts/comments are enough to handle this without negatively affecting engagement. Banning 100% AI-written posts is a blunt tool, and it seems worth trying other things.