I have two ponds, they are a bit big to net, we do lose some each year especially in the big pond
In the garden we have a koi pond and find the best deterrent are childrens windmills!
The Works have big ones for sale at £2.99 obviously
they work better if its windy. :0)

the other thing is herons like to wade and the smaller koi pond is surrounded by a low brick wall.

A friend of mine used to string black cotton from sticks placed around the pond, at about the 18in. mark, because, as has been said, herons tend to land, then wade in. That stopped it. Black cotton didn't show up, either, so there was no real eyesore created .

And I forgot to add that netting is not particularly safe for wildlife, generally. Nothing worse than finding something trapped, after you've been away for a couple of days.

Worthy just beat me to it. Cord or thread round the edges to stop them landing. And I heard that sometimes a model lifesized heron puts them off as well.

Not here Stera.......they stood side by side, I think he found a friend lol!

Ah well, you can't win 'em all...

Thanks for all the helpful answers.
Further research on the internet discovered something called Pond guard protectors, which are sort of plastic mats which clip together and go round the edge of the pond.
Has anyone used these?
I wonder how easy it would be for the plants to grow through and also for the frogs to get in and out of the pond if I were to use this?