Thanks for checking out my mod! As for your questions:
You actually can't! At least not really. Fish don't respawn (this is part of base factorio), and as such, fish are only a temporary source of certain letters like F, G, and H for the early-game. You can get these letters renewably once you've delved further in the tech tree, though.
At the very start, you can get Ms from decompiling the burner letter decompilers you can get from the starter recipe. Further down the tech tree, more options open up (e.g., petroleum gas barrels, or uranium like you noticed). Something important to note is that, at every point in progression, you will always be able to reliably acquire the letters to make the next science pack. So, if you're feeling stuck, factoriopedia can be very helpful ^w^
