The fish numbers for suffering only include farm fish, not wild-caught fish if I understand correctly? Regarding the elephant example, it seems a lot of the elephant neurons are in the cerebellum, not the celebral cortex. Humans apparently have three times more neurons in the cortex than elephants, explaining our superior cognitive capacities, and possibly indicating we have more capacity for pain and pleasure.
I understood the numbers to only contain farm fish and no wild fish. Thanks for the fact about elephants, I didn’t know that. A better metric might then be the number of neurons in the cortex. But it would still contain a lot of uncertainty about which regions of the brain are actually causally responsible for suffering and so on.
This might be of interest:
Some whales/dolphins have more neurons in their cortices than humans.
That being said, I’d be reluctant to rely too much on raw counts to decide moral weight. There are many other considerations. Check out Jason Schukraft’s work for Rethink Priorities.