I think most of the world would be aware of it and are likely prejudiced against Fishmen. The fact that Fishmen are incredibly rare to see anywhere on land is evidence of this. I don't think the public would care at all, and since the majority are brainwashed by the government they'd probably support it.
What about the CDs having human slaves, like Kuma or Ivankov (not the most normal examples but there must be others)? Is it just accepted that some people are slaves? It's not just they're slaves, but very mistreated.