In Islamic law, fish do not require the same conditions for slaughter as land animals. There is no need for the person catching the fish to be a Muslim, and there is no requirement for the fish to be slaughtered. Fish that die due to natural causes, such as extreme temperatures, ice trapping, or water pollution, are considered halal. The hadith, “The water of the sea is pure, and its dead (animals) are halal” (Abu Dawud, Tirmidhi, Ibn Majah, Nasa’i), supports this.
However, in the Hanafi school, if a fish dies naturally and floats to the surface belly-up without any apparent cause, it is considered haram because it may have died from disease, which could be harmful to humans (Sarakhsi, Al-Mabsut, 11/247).