In a table, I combine various estimates of the number of animals farmed with Rethink Priorities' welfare range or moral weight estimates (Fischer, 2023) or my own guesses for others based on them. The table is also accessible here with calculations and some notes for some of the figures.

Some notes:

1. When central estimates were not available, I've replaced ranges with my own best guess central estimates.
2. The numbers for insect larvae are based on the projection of production by 2030 by de Jong & Nikolik, 2021 (pdf), for production only in North America and Europe and only for farmed animal feed and pet food, although they expected feed to account for most insect farming, and most investment had been for farms in North America and Europe. I use weights and lifespans reflecting black soldier fly larvae. I use Rethink Priorities’ welfare range estimate for silkworms for them.
3. Some of the above estimates may not account for pre-slaughter/pre-harvest mortality, so may understate the number alive at a time or that are killed other than by harvest/slaughter.
4. The figures for “Total weight of animals harvested per year” may be somewhat inconsistent, with some potentially reflecting only meat after removing parts, and others the whole bodies.
5. There are other farmed animals not included above, but the above seems to account for almost all of them (Šimčikas, 2020, Waldhorn & Autric, 2023).

Brine shrimp nauplii

For some background on brine shrimp (Artemia) nauplii (early larvae), see Van Stappen, 1996, The Fish Site, 2019, Brine shrimp - Wikipedia and Aquaculture of brine shrimp - Wikipedia. They are largely used as feed for fish larvae and decapod shrimp larvae. The number estimates are based on unpublished estimates by Aaron Boddy from Shrimp Welfare Project. I assume, as a guess conditional on being sentient at all, brine shrimp nauplii are sentient for one day before they die, roughly between hatching from the cyst (egg) after being added to tanks as a cyst to feed fish or shrimp larvae, and actually being eaten.

I doubt their (potential) mental capacities have been studied much at all. Their average weights as feed are probably around 5*10^-6 grams,^[1] similar to nematodes.^[2] Rethink Priorities has been fairly skeptical of nematode sentience, with representative probabilities around 0.4% to 7%, and lower than for silkworms, sea hares and earthworms, and even lower still than for mature insects and decapod shrimp (Schukraft et al., 2019, Duffy, 2023).

I consider brine shrimp nauplii to challenge expected value reasoning and moral aggregation (see Sebo, 2023). I am fairly inclined to ignore them altogether, perhaps no matter their number, or at least discount them more than linearly. This may be partly due to moral uncertainty and partly due to different attitudes towards risk and aggregation. However, further research on their mental capacities could be valuable. Furthermore, the table includes only nauplii, not the breeding stock, who probably have greater individual moral weights.

1. ^{^}

    180,000 nauplii per gram of (dried?) cysts, according to Alune, 2021 and 0.4*10^-5 grams per 1-mm brine shrimp according to Tong et al., 2000, Table 2.

2. ^{^}

    Around 10^-7 grams per individual based on Bar-On, Phillips & Milo, 2018, Table S1, with 0.02 gigatonnes of nematode carbon and 10^21 nematodes, and assuming nematodes are 15% carbon by weight. “0.04–7.34 µg per individual” for macrobenthic nematodes according to Sharma et al., 2011, which may be larger than typical.