If you dig a pond on the mountain and don't put any fish in it, will there be fish inside after ten years?

Let’s start with the answer: Yes, fish will appear, and not too long after. If the pond doesn’t dry up completely, fish can appear within a year at the earliest. After a few years, not only fish but also various microorganisms, aquatic plants will develop, eventually forming a small, self-sustaining ecosystem.

So, where do the fish in the man-made mountain ponds come from? Is it really like the old folks say—“a thousand-year-old grass seed, ten-thousand-year-old fish seed”?

Before we begin, we need to address a widely circulated folk saying: “A thousand-year-old grass seed, ten-thousand-year-old fish seed.” The idea is that fish eggs are incredibly resilient, capable of lying dormant in the soil for tens of thousands of years, and hatch as soon as water appears. This is also a common explanation among elders for why ponds they dig by hand often have abundant fish.

But this saying is completely wrong. Fish eggs are actually quite delicate. Unlike bird eggs with hard calcium shells, fish eggs are covered with a soft membrane. The main function of this membrane is to allow water and oxygen to pass freely—yes, fish eggs need to breathe, and they must do so underwater.

What happens if we take fish eggs out and leave them exposed on the shore? Scientists have conducted experiments: exposing fish eggs directly to air for 15 minutes results in all of them dying. That’s right, just 15 minutes. Not a thousand years, not even fifteen minutes.

What if we keep them moist? For example, wrapping them in damp aquatic plants or moist soil? Experiments show that fish eggs attached to moist materials can survive for about an hour, with around 20% still alive afterward. Remember this data; it will be useful later.

Therefore, the myth that “fish eggs can live for ten thousand years” can be completely dismissed. But where do the fish in the pond come from? Don’t worry, let’s keep looking.

No organism can appear out of nowhere. Since fish don’t come from the soil, they must have been transported there. In nature, the most effective “transporters” are waterbirds.

As early as the 19th century, Darwin was curious about this phenomenon. He once soaked a duck’s foot in an aquarium to observe whether freshwater mollusk larvae could attach. He found that these tiny creatures could grip the duck’s foot tightly and survive 12 to 20 hours in a humid environment. Darwin hypothesized: a duck could fly six or seven hundred miles within that time, easily bringing aquatic organisms to remote ponds.

This hypothesis was confirmed over a century later.

First evidence: feathers and feet

In 1925, German researchers discovered that some fish eggs (like pike and perch eggs) are sticky and can adhere firmly to waterfowl feathers or feet.

Although the probability isn’t high, there are tens of millions of migratory waterbirds worldwide. Each bird may visit dozens of different water bodies annually. This “accidental” transportation network is quite extensive. Usually, they carry eggs of fish that spawn on aquatic plants or floating debris.

Second evidence: digestive tract

If sticking to feathers and feet is routine, the next discovery is even more astonishing: fish eggs can not only stick to birds but also be ingested and survive the digestion process!

In 2020, scientists conducted a rather intense experiment. They fed mallard ducks with fertilized eggs of carp and crucian carp (about 500 per duck), then collected the ducks’ feces. The results showed that live fish eggs were found in the droppings! About 0.2% of the eggs survived the passage through the digestive system and remained viable after hatching.

Though 0.2% seems low, let’s do some math: a single carp can lay up to 1.5 million eggs at once. A duck might swallow thousands of eggs. Even if only one in a thousand survives, that’s still several fish fry. Plus, the global population of wild ducks is in the tens of millions, flying between different water bodies daily, feeding, drinking, and excreting.

When we dig a small pond on the mountain, to many waterbirds, it’s just a “rest stop.” Today, one bird drops a few fish eggs, tomorrow another brings some, and after a few years, fish in the pond is no longer surprising.

Moreover, a 2023 study published in Biological Bulletin provided further evidence. Researchers surveyed newly built artificial lakes that were off-limits to the public and had no fish introduced artificially. The surprising result: over 80% of these lakes contained the same species—European perch.

They found that perch spawn during periods when waterbirds gather in large numbers, and their eggs are sticky, laid in shallow waters, making them easy targets for birds to eat or stick onto feathers. Genetic analysis showed gene flow among perch populations in different lakes, matching the flight range of waterbirds.

This completes the chain of evidence: waterbirds are the main force behind the spontaneous appearance of fish in ponds.

First cause: heavy rain and floods

If the pond is near a river, heavy rains can cause river water to overflow, creating temporary channels. Fish can then migrate along these water flows. This method only works for ponds close to rivers.

Second cause: tornadoes

It sounds like a movie plot, but it’s real. Records from the US, Australia, the Philippines, and other places document phenomena like “fish falling from the sky.” The principle is simple: when a tornado passes over water, it can suck up water and fish into the air, then drop them elsewhere. Although most fish suffer in the process—dying mid-air or upon landing—some lucky ones may fall into ponds.

Third cause: terrestrial migration of fish

Most fish die quickly after leaving water, but some, like the familiar blackfish (Amur sleeper), can absorb oxygen directly from the air. This gives them some ability to migrate on land, especially in humid conditions. They can crawl quite far on land, but the distance is limited, and they still need nearby water sources.

The last cause is humans. Since ancient times, people have released fish for blessings or other reasons. Today, with water resources dwindling and overfishing being a major problem, some may deliberately release fish into mountain ponds to prevent overharvesting.

Let’s look at data: in 2025, a team of European scientists published a solid study. They surveyed 123 newly created ponds in Central European agricultural areas that initially had no fish. The result: over 58.5% of these ponds contained fish! Of these, 40% had fish within the first year—meaning, within a year of digging, fish had already appeared.

Furthermore, the age of the pond had no consistent correlation with the presence of fish. Some old ponds had no fish, while some new ones quickly did. This suggests that fish appearance is random and depends on whether the pond is on a waterbird’s flight path.

The study also found that ponds closer to human settlements are more likely to have fish. This seems counterintuitive, as wild animals tend to avoid human areas, but the researchers suggest that human settlements attract more waterbirds, possibly due to the abundance of artificial water bodies like ponds, reservoirs, and aquaculture sites.

What are the most common fish in these ponds? The research identified roach and crucian carp (especially the silver crucian carp) as the most prevalent. This is likely because these species are hardy: they reproduce quickly, adapt well, produce many eggs, and have wide distributions.

In summary, fish in mountain ponds don’t appear out of nowhere. They are mainly brought by birds—an unintentional, natural process. Thanks to the fish’s ability to hitchhike on waterbirds, they can spread across various water bodies, becoming dominant inhabitants of freshwater ecosystems.