The Battle Trout was introduced with the Mid-Pacific Owl and the Undersea Blast Update.
The item is made by mixing 15 Chemical P, 20 Chemical G, and 10 Mysterious Chemical. It gives they 'Fishy' mod which grants +1 range, and causes you to throw fish instead of punching.
When equipped: "You're holding a dead fish. (Fishy mod added)"
When removed: "You're making better life choices now. (Fishy mod removed)"