Even is deserts, there is still usually some moisture underground. There are some fish eggs that can survive months of being in just slightly damp conditions. A dormant state. Then, when there is a large rain, the fish eggs will hatch and the fish will reproduce and repeat the cycle. Like wise, some fish can survive with just a little moisture for months. They go into a dormant state. Here is a video of a guy digging live catfish out of a 'dry' pond: https://www.youtube.com/watch?v=dvuSR2NLd1s (I have no sources to share and I'm not an ichthyologist, so yeah.)