Know how each hunter works
A heron is a patient wading hunter. It lands at dawn, walks into water shallow enough to stand in, and stands statue-still until a fish drifts within striking range of that spear of a beak — which means the shallow margins of your pond are its hunting platform. It will also strike from the edge into deeper water if fish cruise close to the rim. A cat hunts completely differently: from the bank, crouched at the edge, hooking at fish that come up to feed near the surface, and it rarely gets wet doing it. Cats seldom empty a pond the way a heron can, but they wound fish that later die of infection, and they teach themselves to return nightly. Design against both techniques, not just the animal you have seen.
Make the edges useless to hunters
The most permanent protection is built into the pond's shape. Steep sides that drop quickly to depth deny the heron anywhere to stand, and water deep enough across most of the pond keeps fish below comfortable striking reach. If your pond has a shallow beach or wide planting shelves at the rim, treat those as the weak points they are: raise the edge with rocks the heron cannot perch on comfortably, or accept that this zone needs the net. A simple line of fishing line strung on short posts around the rim works surprisingly well against herons, which dislike stepping over or through a barrier they cannot see clearly. Keep feeding the fish away from the very edge, so they are not trained to gather at the rim where both hunters wait.
Netting, done properly
A net is the one barrier that defeats both animals outright, but only a net fitted like you mean it. Choose a mesh fine enough that a heron's beak and a cat's paw cannot work through it, pull it taut well above the water surface, and pin or weight every edge — a sagging net that touches the water traps fish against it, entangles birds, and gives cats a trampoline to fish from. Stretch it over a frame or hoops if the pond is wide, and lift it clear when you clean so debris does not drag it down. Check it after storms and in autumn, when leaf fall adds weight daily; a net that sags into the water in November is worse than no net at all.
Cover, caves, and deterrents that rotate
Fish that can vanish survive strikes that catch exposed fish, so build refuge into the pond: a length of wide pipe or a purpose-made fish cave on the bottom, clusters of pots, and planting dense enough to break a hunter's line of sight from above. Lily cover shades the surface in summer and hides the fish beneath it. Deterrents then layer on top: a plastic heron decoy sometimes persuades a real heron that the pond is taken, but only if you move it often, because herons learn a statue within days; motion-activated sprinklers startle both birds and cats and keep working because they move. Rotate and combine deterrents rather than trusting one, and accept that in heron territory the net remains the honest guarantee.
When you need hands, not advice
Fitting a taut net or a framed cover over an irregular pond, or reshaping a beach edge into a steep one, is a half-day of skilled fiddly work, and a badly fitted cover fails at the first determined dawn visit. If the pond's shape makes netting awkward, a local specialist found through the KinsLe Pet Services Directory can fit a frame or cover that protects the fish without spoiling the pond, and can service it when the seasons turn.