void FixedUpdate()
{
// apply a physics force in the direction of our destination
rigidbody.AddForce( ( destination - transform.position ).normalized * Time.fixedDeltaTime * speed, ForceMode.VelocityChange );
// if fish is near our current destination, then set a new destination
if ( Vector3.Distance( transform.position, destination ) < stoppingDistance ) {
SetNewDestination();
}
}