Big Fish in Motion Catches Little Fish

Inelastic Collision: The animation below portrays the inelastic collision among a extremely massive fish and a less massive fish. Before the collision, the big fish is in motion along with a velocity of 5 km/hr and the little fish is at rest. The big fish has four times the mass of the little fish. After the collision, 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?