Because when we aim on the fish to shoot while it is swimming in the water it gets quite difficult to shoot. As because for its movements.
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It is difficult to shoot a fish swimming in water because due to the refraction at water and air interface , the fish appears to be raised , and seems to be close to the surface this is called 'apparent depth'.the shooter aims the gun at at apparent position of the fish , instead of real position . hence it is difficult to shoot a fish in the water.
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