Only grazing animals are required to be slaughtered. Fish is not a grazing animal.
Slaughtering the grazing animals is necessary because the blood must drain out from their body which is not in the case of fish as it has less quantity of the blood.
Therefore slaughtering method is not required for fishes.
Allah knows the best.
WHY CAN WE EAT DEAD FISH AND NOT DEAD ANIMALS (CARRION)?
When the Quran speaks about dead / carrion (animals) ‘maytatu’ (5:3), we need to understand it in the context of the animals that we are allowed to eat and whose life has to be ended in a particular lawful way (thakaytum) (5:3).
This is dealt with in the very first verse of the chapter 5:1, “uhillat lakum bahimatul anaam” (lawful to you ‘grazing livestock’). (5:1).
5:3 ‘maytatu’ (dead) is not talking about ‘fish’ or the ‘catch of the sea / rivers’. It is talking about the type of animals mentioned in 5:1 (grazing livestock).
The catch of the sea (fish etc) are dealt with in another verse by the phrase “uhilla lakum saydul bahr wa ta’amuhu mata’an” (Lawful to you is the catch of the sea and its food as provision)
The catch of the sea does not require the ending of its life to be performed in a particular lawful way(thakaytum) as do the ‘animals’ (grazing livestock bahimatul anaam) in 5:3.
There is a difference between Ending the life of Animal through slaughter and Ending the life of fish.
The life of the animal (5:1, 5:3) has to be ended by a lawful process required by the Quran (thakaytum). The lives of the creatures of the sea are usually ended when you take them out of their environment (sea / river). There is no further process required to make them lawful according to the Quran (5:96).