The herons catch a fish, usually one that is on the upper size range of what they like to consume, then they drop while trying to handle it. That is only part of the transplanting part. The heron needs to drop the fish so it can get into another body of water. Well the heron sometimes fly with fish in beak either to another location on same body of water or a perch near another body of water close by. The fish can be dropped in flight or from new perch. If fish able to reach open water without too much damage, then you have a stocking event of one fish.
I have seen herons drop green sunfish several times, most because I was the reason they opted to fly with a fish in beak. When ponds close together or near a stream, then odds pretty good that a fish will eventually be transplanted from one body to the next over time. Farthest I have seen heron carry a fish before dropping was about 100 yards.\
Then there is the business of fish moving overland during heavy spring and summer rain events. Multiple fish move at a time with those and they are much more predictable.