On Touch Release change Sprite velocity

Question

I am having problems working with Androids Motion event system, I'm trying to implement a simple control system where pushing down on the left side of a screen decreases a Sprites y velocity whilst pushing down on the right hand side of a screen increases a Sprites velocity.

____________________
|         |         |
|click    |    click|
|down   Sprite    up|
|_________|_________|

if nothing is pressed velocity is reset back to 0.

I have not been successful with this to work so I've temporarily resorted to a different system, below is the code I have.

@Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();

        int action = event.getAction();
        if(action == MotionEvent.ACTION_DOWN){
            if(x < this.getWidth()/2)
                this.fish.vy = 2;
            if(x > this.getWidth()/2)
                this.fish.vy = -2;
        }

        if(action == MotionEvent.ACTION_UP){
            this.fish.vy=0;
        }

        return false;
    }