Hunter-gatherers transported brown trout into the mountains of Norway at least 7000 years ago so they could be harvested during summer foraging trips into the highlands.

That is the only possible explanation for how fish could have populated isolated mountain lakes that had been covered by a kilometres-thick ice sheet for tens of thousands of years, says Axel Mjærum at the University of Oslo, Norway.

“This is the oldest documented example of human-mediated translocation of fish in the world,” says Mjærum.

Until around 8400 years ago, Lake Tesse, at an elevation of around 850 metres, was covered by the Fennoscandian ice sheet, leaving an ecological blank canvas once it retreated.

Yet before it was transformed for hydropower storage in the 1940s, Lake Tesse was regarded as one of the richest brown trout waterways in Norway, yielding 9 tonnes of the fish annually.

While it has long been suspected that humans had a hand in establishing the brown trout population in Lake Tesse, we had no idea how long ago this happened.

Alternative ideas have included transportation by birds or natural migration of fish and eggs upstream. But most Norwegian mountain lakes, including Lake Tesse, only had brown trout until recently, and geographical features such as large waterfalls make natural dispersal upstream impossible.

Mjærum and his colleagues studied the partially buried remains of elaborate wooden fish traps, discovered in 2022 when the lake level was lowered.

Radiocarbon dating showed that the oldest trap dated from around 4950 BC. By correlating the recovered wood with the pine tree ring chronology available for the central Scandinavian mountains, the team was also able to precisely date some of the posts that were part of the younger traps.

One came from a tree felled between spring 2669 BC and winter 2669-2668 BC, and another 12 years later during the winter of 2657-2656 BC, demonstrating that the traps must have been maintained to keep them in good working order.

“Therefore, it is possible that this form of trap fishing was ongoing in Tesse during much of the period between 5000 and 2650 BC,” says Mjærum.

The transport of fish into the lakes is an early example of people managing the environment to provide food, centuries before the advent of farming in Scandinavia. There is evidence that fish farming began even earlier in other parts of the world, including channels and traps for catching eels built in Australia 8600 years ago.

How people transported the fish is unknown, but they may have used waterskins. People probably stayed in the mountains for part of the summer season, hunting reindeer and fishing, but spent the winters in the valleys or along the coast where the climate was milder, says Mjærum.

“A good and predictable food supply also made longer stays possible, not only for hunters aiming for big game, but for whole family groups,” he says.