Digging a reservoir
Whether it’s raining or not
When water is filled
There are cases where fish appear ‘spontaneously’.
Where did this fish come from?
There is no waterway.
One of the reasons is
birds eat fish eggs
Come to the reservoir and defecate
There are surviving eggs
Hatching…
Of course, this only applies to fish eggs of strong survival types…
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Spawning carp (left) and black horse. Mallard ducks enjoy skimming and eating carp eggs on black horses. https://www.hani.co.kr/arti/animalpeople/ecology_evolution/950582.html