The water game that, upon getting pulled out, dies due to lack of oxygen is still fresh. So the cause of death is known. "The slaughter of a water game" is its deprivation from the natural environment including oxygen.
Moreover, HALAL (Permissible food) always comes with TAYYAB (clean, decent, fresh, aesthetically sound), in the Qur'an, of course not decomposed or rotten.
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