Electric fence. Seriously, it works great. Look up a Fi-shock SS-750.
That model is "pet safe". Go with a pulsed output. It is less dangerous, and actually more scary to animals.
My first version just had a bunch of 24" poles made of 1/2" PVC pipe stuck about halfway in the ground, and a single wire running along the top, around the perimeter of the pond.
Since then, I've made a decorative fence using copper pipe (even the uprights, but I connected the uprights to the horizontal parts with CPVC tees cut open, and zip ties, so I had electrical insulation from the uprights).
These kinds of fishing birds like to wade into the water, so if they don't have a good way to walk in, they leave.