This is an interesting topic! If there's one thing I've come to understand is that pretty much nothing is impossible and....sometimes the answers to mans questions are the simplest.

My experiences.

I've built ponds with no incoming or outgoing water (spring fed) and within 2 or 3 weeks there are minnows in it. The only sign of life I see during the early growth (first few weeks) are raccoons, mink, herons, frogs, turtles and puddle ducks. I have one tiny pond that is about 12 feet round and it is one of my best minnow growing ponds.

I've also seen snapping turtles with leeches on their backs with fresh mud travelling from pond to pond.

We have many birds of prey and I've seen a fish fall out their grip..in another pond.

While in the arctic we flew some biologists into landlocked lakes to study char and trout...they could have been located by higher water levels or.....?

And a pond/swamp I trap often (about 10 acres wide and 6 feet deep) was drained by some otters, in the end there were thousands of max 4 inch long sunfish, private property in the same family since the 1800's and no oral history of it ever being stocked. There is nothing above that pond for them to come from and there is no way they could have swam up to that swamp.

My remote cabin in Quebec has walled in it...many of them, those were brought in by one fella in a backpack in the 1950's, its the best walleye lake I've ever fished.

Just my experiences.

Cheers.

Lou