I found a number of threads in the archives discussing possible ways of fish or fish eggs being transported from one body of water to another. Here is another possibility that has some backing by science. Fish eggs are being eaten by Mallards (in lab conditions) and remaining viable when pooped out!
https://www.sciencenews.org/article/...oped-out-ducks
https://www.smithsonianmag.com/smart...ers-180975230/