Is the water dog (otter or similar aquatic animal) permissible (to use or consume)?

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The water dog is permissible. This is the position of Malik, Al-Shafi'i, and Al-Layth, and it is implied by the views of Al-Sha'bi and Al-Awza'i. The evidence for this permissibility rests upon the general meaning of the Quranic verse and the Hadith. A narration cited states that Shurayh, a companion of the Prophet (peace be upon him), said: "Everything in the sea is slaughtered (i.e., permissible if slaughtered)." Abu Abdullah (Ahmad ibn Hanbal) also stated that the water dog must be slaughtered (slaughtering it renders it lawful).