بسم الله الرحمن الرحيم
It is known that the ocean fish that live exclusively in the seas and oceans are permissible to eat due to the hadith, “هو الطهور ماؤه الحل ميتته”, “It is pure and purifying its water [i.e. the seas] and its dead animals are permissible to eat”[i.e. that which dies in the oceans from its animals exclusive to living within it can be eaten even without slaughtering], with that regard Shaikh Saleh al-Fawzaan was asked about the entity recognised and known as the “mermaid” and the ruling upon it in terms of hunting it and eating it since it is also from the fish of the sea etc.
Questioner, “Shaikh, some experts and specialists in sea-life and fish mention that there is a fish that has a head like the head of a woman, and she has hair and a face like a woman, so is it permissible to eat it…and that is what they term as the “mermaid”?
Shaikh al-Fawzaan, “There is the “human of the ocean”, there is something from a type of fish upon the appearance of humans, they call it the “ocean human” [otherwise termed as "mermaid"] and it can be eaten, it is from the items that can be caught and eaten from the sea, yes, even if it was upon the appearance of a man or woman”.
[Official website of the Shaikh]
It is important this issue is understood properly, the question may be detailed, however the Shaikh states, “there is something from a type of fish upon the appearance of humans”. This therefore does not dictate in any way that an individual should be confused by the common perceptions of mermaids portrayed through stories and their likes of almost actual human women who are completely and fully a woman from the upper body and completely fish like lower body, and that they talk, and that they are of human size etc etc… none of that is confirmed by the Shaikh here. Rather he states there are fish that resemble the human, similar in appearance in some ways hence known as “insaan ul bahr” [sea-human], and that is similar to what some of the fuqahaa mentioned in their books in earlier times, i.e. of a type of fish having some form of resemblance to human features…essentially it is a fish.