In Celtic lore, Finn Eces required help in capturing a specific type of this animal from Finn McCool.
The Polynesian god Ika-Roa takes the form of an elongated one of these, who gave birth to all of the stars in the heavenly sky. In the Book of Tobit, Tobias kills one on his journey to Media and uses its liver, heart and bladder to make medicines. Matsya, the first avatar of Vishnu has the appearance of one of these and warns Manu about the coming flood. In Celtic lore, Finn Eces required help in capturing a specific type of this animal from Finn McCool. For 10 points, name this animal often used as a symbol for Jesus Christ, which makes up the bottom half of a mermaid.
The Polynesian god Ika-Roa takes the form of an elongated one of these, who gave birth to all of the stars in the heavenly sky.
{fish} [prompt on {salmon} of knowledge around the Finn McCool clue]