Riddle help-- can anyone solve this

A King comes into possession of three expensive and rare fish so naturally wishes them to guarded. To this end he has a pond built a wall built around the pond. Surrounding the pond are a tractor, a bulldozer and a crane. Upon the wall are soldiers keeping lookout, one day one of them notices something amiss. They get a ladder and descend to the pond and find all fish have gone, where?