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The trout of Lake Tesse should not be there.
When the last ice sheet withdrew from southern Norway, it left the high mountain lake as an ecological blank slate. Waterfalls blocked fish moving upstream. Maybe birds carried eggs between lakes, but that can’t readily explain why isolated mountain waters became populated almost exclusively by brown trout.
Yet by about 5000 BCE, people were building substantial wooden traps in Lake Tesse to harvest an established trout population, according to archaeological evidence.
A study published in Antiquity concludes that Mesolithic hunter-fisher-gatherers deliberately carried live trout into Norway’s mountains, creating what the authors call the world’s oldest documented human translocation of fish.
The finding suggests that Stone Age people were not merely following wild food across the landscape. More than a millennium before clear evidence of farming appeared locally, they were redesigning ecosystems to make future meals more dependable.
Poles on a Drained Lakebed
The evidence emerged after Lake Tesse was altered by humans. Since the 1940s, it has served as a hydropower reservoir, and its water level drops each spring. In 2022, hiker and amateur archaeologist Reidar Marstein noticed rows of slender wooden poles protruding from the exposed lakebed.
Archaeologists returned in 2023 and excavated four traps before the water rose again. They collected 113 wood samples, identifying 73 separate poles used in the structures. The best-preserved trap contained 46 poles: roughly 40 formed a circular chamber about 3.5 metres wide, while six others created a fence that funnelled fish inside. Cut marks, split wood and possible lashing traces showed deliberate construction.
Radiocarbon dating placed the earliest traps around 5000 BCE. Tree rings provided unusually precise dates for younger repairs, including two poles cut 12 years apart. Together, the evidence suggests that fishing structures were built, repaired and replaced at Lake Tesse for at least 2,300 years.
The traps can’t tell us when someone released the first trout. However, they date a working fishery that could exist only after people had established a reproducing population.
“This is the oldest documented example of human-mediated translocation of fish in the world,” Axel Mjærum, an archaeologist at the University of Oslo and the study’s lead author, told New Scientist.
A Fishery Nature Could Not Build
Lake Tesse lies about 854 metres above sea level. The surrounding mountains remained under the Fennoscandian ice sheet until roughly 8400 BCE, meaning no freshwater fish population could have survived there from the Ice Age. Trout could colonize some lowland waters after the thaw, but steep rivers and waterfalls barred their path to Tesse.
The researchers also consider transport by birds unlikely. Most high-altitude lakes historically contained only brown trout, not the mix of species expected from random dispersal. Archaeological sites around other mountain lakes have yielded burned trout bones dating to roughly the same broad period, suggesting a regional practice rather than a freak event.
How Stone Age people moved live fish is a mystery in itself. Mjærum has proposed watertight animal-skin bags, although none have survived. The journey may have unfolded in stages: people could stock one reachable waterway, wait decades for the fish to multiply, then carry some farther uphill.
“This wasn’t something they did to get a return the same year, but for the future,” Mjærum told Science Norway. “It took many years before they could begin to reap the benefits of this work.”
Post-Ice Age hunters came into these mountains for reindeer and elk, but large game was unpredictable. A lake full of calorie-rich fish offered insurance, allowing larger groups — perhaps whole families — to stay longer and pursue other resources.
Before Farming, a Future Harvest
Archaeologists have increasingly found that foragers altered landscapes through burning, plant dispersal and animal management. At Australia’s UNESCO-listed Budj Bim Cultural Landscape, the Gunditjmara engineered channels, dams and weirs to trap, store and harvest eels over at least 6,600 years. The Norwegian discovery highlights a different, much more challenging strategy: moving a species across natural barriers so that a new fishery could grow.
It also pushes documented mountain fish stocking far deeper into the past. A 2025 Nature Communications study used parasite DNA preserved in lake sediment to show that people had introduced fish to a Pyrenean lake by the seventh century A.D. Lake Tesse predates that evidence by more than five millennia.
The distinction between hunter-gatherers and farmers, then, becomes less tidy.
“This blurs the distinction between hunter-fisher-forager societies and farming communities,” Knut Andreas Bergsvik, an archaeologist at the University of Bergen who was not involved in the study, told Science Norway. “They managed natural resources, planned ahead, and harvested in much the same way that farmers do.”
Before hydropower reshaped it, Lake Tesse produced as much as nine tonnes of brown trout a year. The researchers argue that people fishing there today are still harvesting the biological legacy of a decision made 7,000 years ago: carry trout uphill, release them into empty water and trust that future generations would return.
This story originally appeared on ZME Science. Want to get smarter every day? Subscribe to our newsletter and stay ahead with the latest science news.
