Both are farmed; presumably both suffer? Is there any evidence that fish fare better psychologically than mammals overall? Why is there such thing as pescatarianism and not the same for eating only meat but no fish?
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It’s human empathy. Empathy is a powerful evolutionary force for communal herbivores such as humans. But that empathy can be hijacked by animals when humans project concepts of humanity onto those animals. This happens when those animals have traits that are similar to us (or which remind us of humans). Mammals are more similar to us than fish, so we have more empathy for mammals.
Humans sometimes justify differing empathy by discussing the levels of intelligence of animals – but this is just another form of empathy for creatures with humanlike traits, because humans are (relatively) intelligent. So we feel less empathy for creatures that are less intelligent. Consider that most vegetarians will still kill insects, even though an insect’s life is fundamentally the same as any other creature’s.
Straightforwardly, there is no fundamental reason why eating a cow should be any different from eating a dog. But humans have unconsciously and consciously bred dogs to be creatures we can empathize better with. Domesticated animals are the peak in compatibility for human empathy for animals (besides evolutionary cousins to humans like monkeys and apes). Therefore the idea of eating a dog would horrify many people, who project humanity onto dogs, while the same people would have no qualms about eating a cow.
The difference in suffering is irrelevant. The perception of the person eating or not eating them is the only thing that is important.
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