Where do fish come from? That might seem a stupid question, but when a new pool forms, unconnected to any rivers, fish sometimes appear, seemingly from nowhere. Nobody has put fish in the pool. The pool is not connected with any other waterways that have fish. Yet after a few months or years, there are often fish there.
Centuries ago, people believed that certain creatures could genuinely come from nowhere. We call that idea ‘spontaneous generation’, though they didn’t call it anything — it was just how things were, or so they thought. This covered maggots turning up in meat as it rotted, eels appearing in mud and even mice appearing in grain stores. We now know that maggots hatch from tiny eggs laid by flies, and that eels and mice get in when no one is looking and are quite good at hiding. But fish? How do they get into ponds?
From bird poo, it seems. Scientists in Hungary fed fish eggs to mallards, a type of duck. Then they looked at the duck poo that came out later (sometimes just an hour later) and found that two in a thousand of the eggs went through the duck with no damage and of those, one in six hatched into fish. That means only three in ten thousand fish eggs eaten by a duck might survive to be fish, which doesn’t sound good. But some fish lay millions of eggs at a time and many pools have a lot of fish. Ducks love fish eggs and will gorge on them if they find a lot, so getting the eggs into the fish isn’t a problem. Ducks could be eating billions of fish eggs. Even with such a low proportion of eggs surviving their trip through a duck, that’s still more than 300 hatching fish per million eggs.
Ducks don’t neatly go to a lavatory somewhere when they feel the need. They just drop it from the sky, or into the water they’re sitting on or onto the ground they’re standing on. Lots of duck poo falls on the grass or roads, or other places fish don’t grow. But some falls into water, and some into water with no other fish in. And some of the eggs in that poo will hatch and a few will survive to grow and, if there are other fish of the opposite gender, populate a new pond. So fish don’t quite come from nowhere. They come from poo.