Fish farms play an important role in supplying the modern world’s massive demand for seafood—about half the fish we eat today comes from fish farms rather than being caught in the wild. Aquaculture helps lighten the burden on wild fish populations, and farmed fish have a much smaller carbon footprint per pound than beef. (Of course, fish farms also produce waste and nutrients at concentrations that can wreak havoc on local marine ecosystems.) With all its modern relevance and all the hopes pinned on it for the future, it’s easy to forget that fish farming is an ancient practice.

People around the world have farmed fish since at least 1500 BCE; Egyptian tomb paintings show Nile tilapia being raised in captivity, and in ancient Assyria and Rome, wealthy homes often kept fish and crustaceans in pools called vivariums—a household version of a restaurant’s lobster tank. In China, ancient writers describe raising carp in flooded rice fields starting at around 1100 BCE. But some archaeologists, like Tsuneo Nakajima of the Lake Biwa Museum in Japan and his colleagues, suggest that aquaculture may have started much earlier.

Finding fish from a farm

“Given that rice paddy fields date back to the fifth millennium BCE in China, it might be expected that carp aquaculture has a similar antiquity,” wrote Nakajima and his colleagues. But it’s hard to find archaeological evidence of fish farming; a rice paddy that once housed carp looks about the same as a rice paddy that didn’t. Nakajima and his colleagues suggest that the size of the fish people ate may offer a clue—a clue that points to people capturing and raising wild carp in channels and enclosed areas of marshes starting around 6000 BCE.

At several archaeological sites in China and Japan dating from 6,000 to 5,000 years ago, Paleolithic hunter-gatherers discarded carp bones. When archaeologists plotted the body lengths of those ancient fish on a graph, they tend to form a bell curve, peaking around 300mm—the average length of a wild adult carp when it spawns for the first time. In other words, the discarded bones looked like ancient hunter-gatherers had caught wild carp at their annual spawning grounds.

But historical sources describe how, in more recent times, people in both China and Japan caught carp during the spawning season and then kept them captive in enclosed marshy areas or ponds. The next fall, after the captive carp spawned, people drained the water to harvest the fish. But those harvested fish were a mixture of adults and their offspring, so a graph of their body sizes looks like a hilly curve with two peaks: one around the size of adult fish, and one around the size of young fish the autumn after their spawning (about 100mm).

Nakajima and his colleagues suggested that if they could find a similar pattern of fish sizes at an archaeological site, it might be a sign that people were rearing fish in a similar way. And in fact, that’s what they saw at a Japanese site called Asahi, starting around 400 BCE. By that time, historical documents make it clear that people in Japan had figured out how to farm fish, so that’s not an entirely unexpected finding, but it's an important validation.

Back in time

At a much older site in China called Jiahu, in what is now the Henan Province, the archaeologists saw a similar pattern. Archaeological evidence suggests that people lived at Jiahu from around 7000 BCE to 5700 BCE. If you wanted to spend time at a cutting-edge cultural center in Neolithic China, Jiahu was the place to be. It’s one of the first places where people learned to make wine from fermented honey and rice, and people at Jiahu also made the oldest known musical instruments—carved bone flutes. Bone and tortoise-shell artifacts show that the people of Jiahu carved a set of symbols that may be one of the earliest known predecessors of writing.

Based on the things they left behind, people at Jiahu farmed rice and domesticated pigs, and the moat surrounding the settlement suggests that they were more than capable of digging channels and ponds. If Nakajima and his colleagues are correct in arguing that a mixture of young fish and adults suggests farming, then Jiahu could be the earliest example of aquaculture in the world. In the oldest layers at the site, discarded carp bones matched the size distribution that you’d expect from fishing during the spawning season. Starting around 6200 BCE, however, the size distribution had two peaks, just like at Asahi in Japan.

The rubbish pits of Jiahu also contain a suspiciously large amount of common carp—which, despite its name, should have been much harder to catch than another species called crucian carp. Both species spawn near the shorelines of rivers, but common carp swim away from shore with their young as soon as spawning season is over. Crucian carp hang out close to shore, where they’re easier to catch. But at Jiahu, 75% of the fish bones came from common carp.

“This seems to indicate a cultural preference for common carp, even though these would not have been the most abundant,” wrote Nakajima and his colleagues. They suggest that carp farming could have started in response to people’s preference for the harder-to-catch common carp.

Although people at Jiahu farmed rice, there’s no evidence that they used flooded paddy fields; instead, they probably grew their rice in dry fields, like modern farmers in parts of Thailand and elsewhere. If the people of Jiahu really were raising captive carp, they were probably doing so by enclosing parts of the fish’s marshy spawning grounds and digging channels to control water flow.

Historical documents, including the ancient Chinese guide to aquaculture by Yang Yu Jing, (written around 460 BCE) show that the art of carp farming got more sophisticated over time. By then, paddy fields doubled as carp ponds; that seems to have been the case at the Asahi site in Japan, as well. Nakajima and his colleagues suggest that the domestication of carp farming may have been closely linked to rice paddies. Since the earliest known rice paddies show up around 5000 BCE, there may be reason to think that’s when intensive fish farming started, too—but it’s going to take more research and evidence to say for sure.

Nature Ecology and Evolution, 2019. DOI: 10.1038/s41559-019-0974-3 (About DOIs).