Little Fish in Motion is Caught by Big Fish

The animation below portrays the inelastic collision among a extremely massive fish and a less massive fish. Before the collision, the little fish is in motion along with a velocity of 5 km/hr and the big fish is at rest. The big fish has four times the mass of the little fish. After the crash, both the big fish and the little fish move together along with the similar velocity. (Collisions such as this where the two objects stick together and move along with the similar post-collision velocity are referred to as inelastic collisions.) What is the after-collision velocity of the two fish?