"Consuming the bioluminescent compound that makes this fish glow in the dark will increase concentration. Dishes cooked with it will suppress noise when consumed."
The Stealthfin Trout is a fish and material found in Breath of the Wild. They can be found only around the water in and around the Great Hyrule Forest. They can be used in a variety of Recipes, and are used to upgrade the Stealth Mask, Stealth Chest Guard and Stealth Tights.
One method of obtaining the large quantity required for the armor upgrade is to catch all the ones around Lake Saria, and then run into the Lost Woods. Upon being dropped outside, teleport back to Lake Saria, and all the spawns will have refreshed. It's worth noting that the game generally spawns fish as a random distribution of types that can spawn in a given body of water or water feature; consequently individual success may vary.