The Scandinavian diet is famously hard going for anyone who doesn’t like pickled fish – and a unique archaeological discovery has proved that it was exactly the same more than 9,000 years ago.

The find has revealed that freshwater fish were being fermented on an industrial scale in southern Sweden, through a complicated and distinctly unappetising process involving pine bark and seal blubber, which made the region capable of supporting a far larger population than previously thought.



The discovery was made during the excavation of an early Mesolithic settlement site in Blekinge, on the Baltic sea in southern Sweden. It is the earliest evidence of fermentation being used to preserve food anywhere in the world.



The fish were preserved without salt or storage jars. Instead they were acidified using pine bark, wrapped up with seal fat in seal or wild boar skins, and buried in a pit. The process required a cold climate to help preserve the fish until the fermentation was complete, when they were dug up for consumption. A similar process is still used to produce the Icelandic delicacy of rotted shark meat.



The technique may have been widespread, but the evidence has not survived as fragile fish bones usually decompose without trace: at Blekinge they were preserved by the soil conditions, but even so archaeologists had to use a fine mesh sieve to retrieve them.



Adam Boethius, an osteologist at Lund University, who publishes his findings this week in the Journal of Archaeological Science (under the title Something Rotten in Scandinavia) said: “The discovery is quite unique as a find like this has never been made before. That is partly because fish bones are so fragile and disappear more easily than, for example, bones of land animals. In this case, the conditions were quite favourable, which helped preserve the remains.”



The site yielded vast quantities of fish bones, which have been dated to around 9,200 years old: the archaeologists were excavating them at the rate of 30,000 to the square metre, along with scraps of the pine bark. Under the bone layer they found an oblong pit surrounded by stake holes. The scientists calculate that at least 60,000 tons of freshwater fish were processed at the site.



Scientists had thought that at this early date the population of northern Europe consisted of scattered groups of hunter gatherers, while the Middle East was seeing the emergence of agriculture and larger settled groups. However, such a well developed technique for processing and storing fish stocks could have supported a far larger population. “From a global perspective, the development in the Nordic region could correspond to that of the Middle East at the time,” Boethius said.

