Those who deny pain in fish, say that because fish do not have a neocortex as part of their brain, they cannot feel pain. This paper demonstrates that birds and amphibians, who also lack a neocortex, do feel pain and suffer.
"Birds and amphibians lack a neocortex; this suggests that they also do not experience pain. Yet many scientific studies have amply demonstrated pain and suffering in these groups (e.g. birds, Gentle, 1992; amphibians, Stevens, 1992)."