I says it's entirely plausible some fish eggs could stick on the bottom of a birds foot and be transported a short distance in the moist feet that are folded up. Eventually from body of water to body of water even at short distances the eggs could get transported farther and farther. Fish eggs are sticky for a short period of time. All the bird has to do is step in a nest.
Case in point: One of the biggest goldfish farms in the country here in Martinsville,Indiana has rare goldfish. The goldfish farm has hundreds of heron visitors. They've given up on controlling them. The rare gold fish are in ponds several miles away but were never planted by humans in those ponds.
Explanation?
If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.
