Ancient bone remains shed new light on herring fishing in the Baltic Sea. Herring was an important commodity 400 years earlier than previously thought.
Throughout history, herring fishing has created great wealth along the entire coast of Norway. It has been even more important in Sweden, Denmark, and the Baltic Sea.
Large-scale trade in «silver of the sea» dates back to the Middle Ages. In fact, even further back. A new study dates trade in herring in the Baltic Sea back to the Viking Age in the 9th century.
Previously, historians believed that extensive herring trade began around the year 1200.
"Our study shows that this trade already existed around the year 800, 400 years earlier," says research fellow Lane Atmore at the Department of Biosciences at the University of Oslo.
She is one of the researchers behind a study published today in PNAS. It shows that bones from herring caught between Sweden and Denmark have been found as far east as Truso in present-day Poland. Truso is known as an important trading center for the Vikings.
"The genetic signature of these bones shows that the fish are adapted to higher salt content in the water than is found this far into the Baltic Sea. This means that they were fished around the Kattegat and then transported into the Baltic Sea," says Atmore.
The low salt content in the Baltic Sea means that herring from the Kattegat stock will have great difficulty adapting to the waters further east.
"Fish that are adapted to high salt levels have never been found this far into the Baltic Sea," says Atmore. to Titan.uio.no.
Herring is difficult to trade in
Her research colleague, Associate Professor Bastiaan Star, has previously studied cod trade in the same area.
"We have seen that cod found in the trading town of Hedeby in present-day Germany traveled all the way from northern Norway. The new study shows that it was not only cod that was traded, but also herring, a fish that is much more technologically demanding to trade," says Star.
Herring (Clupea harengus) is a much fattier fish than cod. It is therefore much more difficult to store and trade if you do not have the necessary technology.
"It spoils very quickly if you don't salt or smoke it. You need access to a lot of salt and wood to be able to process and transport it, you need extensive trade networks, and you have to catch enough fish to make it worth the investment," explains Atmore.
"I believe fish was traded over greater distances than previously thought. We can now date this because these herring bones have been dated with certainty to between 800 and 850 AD," says Star.
"We cannot prove that it was the Vikings who transported the herring from one place to another, but now we have herring bones from a place where we know the Vikings traded," says Atmore.
Biology and archaeology hand in hand
Both Atmore and Star are biologists. In this study, they have collaborated closely with archaeologists. One of them is Professor James H. Barrett at the NTNU University Museum.
"The herring industry in the Baltic Sea was one of the most important trades in medieval Europe," says Barrett.
"By studying the genetics of both archaeological and modern herring bones, we have discovered the earliest known evidence of the growth of long-distance trade in herring, from the salty waters of the western Baltic Sea to the Viking-era trading center of Truso in northeastern Poland," says Barrett.
The study also shows what has happened to herring stocks in recent times.
"The economic and political effects of the herring industry have been well documented, but the ecological consequences have been much debated," says Barrett.
Spring showers and autumn showers
The different herring stocks spawn in different locations. This is how they have adapted to different levels of salinity in the water. But they also spawn at different times.
"There are two main stocks of Atlantic herring. One spawns in the spring and the other spawns in the fall. Since they spawn in different areas and at different times, there is not much interbreeding between the stocks. Therefore, they are genetically different from each other," explains Atmore.
With modern technology, and after long and patient months in the lab, she is able to see where each individual fish comes from and how stocks grow and shrink. And they can see how this is affected by the fishing industry.
"Early on, from around 800 AD, we see mostly autumn-spawning herring from the western Baltic Sea stock in these archaeological finds. It was this stock that was the target of the famous fishery around 1200," says Atmore.
The collapse 100 years ago
Later, it has been the opposite.
"In recent times, spring spawners have been fished, and this stock collapsed in the 1920s. In the Baltic Sea today, 90 percent of commercial fishing is for spring spawners," says Atmore.
"It's not that autumn sprats have completely disappeared from the Baltic Sea, but they are no longer of commercial interest. They are still there, but not in the same numbers as before," says Star.
He has no doubt that the fishing industry has had a major negative impact on herring stocks.
"There is a consistent pattern of overfishing that has taken place over the centuries," says Star.
"Our findings provide new and compelling methods for testing the archaeological hypothesis that human impact on rich fish stocks in Europe began as early as the Middle Ages. And that different herring stocks were exploited continuously over time," says Barrett.
This also means that the ecology of the Baltic Sea is changing. The greatly reduced population of autumn spawners are spawning in other places and at other times.
"They are also larger than spring spawners and eat slightly different food. When the population of fall spawners is greatly reduced, the ecology will change," says Atmore.
"The Baltic Sea is a much more enclosed sea compared to the North Sea. Some of the consequences of human activity and climate change are amplified in such a small system," says Star.
Scientific article:
Lane Atmore et al.: Population dynamics of Baltic herring since the Viking Age revealed by ancient DNA and genomics, PNAS, October 2022.
