The momentum of an object is defined as the product of its mass and velocity.
In this case, the mass of the fish is 2.0 kg and its velocity is 1.5 m/s. Therefore, the momentum of the fish can be calculated as:
momentum = mass x velocity
momentum = 2.0 kg x 1.5 m/s
momentum = 3.0 kg m/s
So the momentum of the 2.0 kg fish swimming at 1.5 m/s is 3.0 kg m/s.