Any lure-fishing spot can be used to catch raw rainbow fish, such as the one east of the Barbarian Village. (will be on rivers/streams only - none found on lakes or oceans)
A raw rainbow fish has two main uses. Firstly, players can cook it on a range or a fire to make a cooked rainbow fish, a food item. Doing so requires a Cooking level of 35, and grants 110 Cooking experience. With a Cooking level of 63 or higher it is impossible to burn the fish.