Leaping trout is a fish which requires 48 Fishing, 15 Strength, and 15 Agility to be caught, providing 50 Fishing experience, 5 Strength experience, and 5 Agility experience. It can be caught at Otto's Grotto or Mount Quidamortem with a barbarian rod and suitable bait after starting Barbarian Training.
Unlike normal fish these cannot be cooked, instead they are gutted by using a knife on the fish. Successfully gutting the fish gives 10 Cooking experience, roe and fish offcuts. Leaping trout are caught with a barbarian rod, which uses fishing bait, feathers, or fish offcuts. You can also catch leaping salmon and leaping sturgeon.
Fishing with the Barbarian rod requires you to speak to Otto Godblessed. Due to the fact that you get Strength and Agility experience Leaping trout are a great way to train fishing. Also Barbarian Assault is just north of the fishing location meaning you can run to it to deposit all your roe.
For an exhaustive list of all known sources for this item, see here