I've got a little Randall Model 21 with a 3½-inch blade...a little smaller than the Randall Trout & Bird knife...also a little cheaper.

AG Russel out of Springdale, AR makes a trout/bird knife. I love mine. I keep it clipped on the outside of my vest. It has a very secure but FAST thumb release scabbard, which means you can get to it and get it out fast if you need to. At $30, you don't have to worry about losing it in the stream or overboard like you would a Randall.