It is a combination of how much you fish and the amount of time that you are fishing.

For instance, one tribe I had I found A LOT of mushrooms, so I didn't fish at all, but just researched. Finally, I wanted to increase my population. When I did, it also happened that the mushrooms started getting scarce. I HAD to start fishing.

So I think it was after collecting so many fish and how long I had been fishing, that the algae came. I hope that helps!