That depends on what you want to happen. If you don't mind a number of fish being eaten and some others being scared and the potential for it adding parasites into your pond and fish then leave it alone and watch. They are beautiful birds. On the other hand you can run them off with constant harassment if you can be there to chase it around. Dogs are good for chasing off wading birds if the pond is small. If you want to get rid of it and can't chase it off then all I can tell you is some here suggest that shoot , shovel and shut-up is an effective method.