Islamic jurisprudence specifies which foods are halāl (حَلَال "lawful") and which are harām (حَرَامْ "unlawful"). This is derived from commandments found in the Qur'an, the holy book of Islam, as well as the Hadith and Sunnah, libraries cataloging things the Islamic prophet Muhammad is reported to have said and done. Extensions of these rulings are issued, as fatwas, by mujtahids, with varying degrees of strictness, but they are not always widely held to be authoritative.
According to the Quran, the only foods explicitly forbidden are meat from animals that die of themselves, blood, the meat of pigs, and any food dedicated to other than God.
However, a person would not be guilty of sin in a situation where the lack of any alternative creates an undesired necessity to consume that which is otherwise unlawful. (Quran 2:173) This is the "law of necessity" in Islamic jurisprudence: "That which is necessary makes the forbidden permissible."
Dhabīḥah (ذَبِيْحَة) is a prescribed method of ritual animal slaughter; it does not apply to most aquatic animals. The animal must be slaughtered while mentioning the name of God (Allah in Arabic). According to some fatwas, the animal must be slaughtered specifically by a Muslim; however, other fatwas dispute this, ruling that, according to verse 5:5 of the Qurʼan, an animal properly slaughtered by People of the Book is halal. The animal slaughtered must be killed quickly with a sharpened blade.
It must not suffer. It must not see the blade. It must not see or smell the blood from a previous slaughter. The main logic given by Islamic clerics is the significance of life. Many clerics argue that the life, given by God, is not an insignificant gift therefore no entity except God has the right to take this valuable gift away and by mentioning the God's name you imply that command of God.