Old newspapers track porpoise populations across the Baltic Sea

Harbour porpoises were once found across a much wider area of the Baltic Sea than they are today, including regions where they are now rare or absent. This is shown in a new study that uses centuries-old Swedish newspapers to reconstruct past distribution patterns.

Researchers from the Swedish Museum of Natural History, Lund University, and the Natural History Museum of Denmark analyzed digitized Swedish newspapers from the late 1700s to the early 1900s. These revealed how the distribution of the Baltic Proper harbour porpoise (Phocoena phocoena) has changed over time. 

“It’s exciting to see these old newspaper reports describing porpoises in areas where you don’t expect to see them today,” said Magie Aiken, Postdoctoral Researcher at the Swedish Museum of Natural History. “These are not scientific surveys. They are everyday observations, but considered together they show a very different Baltic Sea.”

Old papers with new insights

The old newspapers show that porpoises used to occur along the entire Swedish coastline, including the northern-most parts, the Gulf of Bothnia, where the species is now rarely observed. In the Baltic Sea, news reports are less frequent than on the west coast of Sweden, but still indicate a wider historic distribution than today. Many of the northern Baltic reports are from spring and summer, suggesting that these areas may once have been used as feeding areas by porpoises moving in from the southern Baltic or Belt Sea.

Trawling libraries for a fresh catch

The team searched the National Library of Sweden’s digitized newspaper archive using keywords related to harbour porpoises, and screened more than 100,000 search results. This produced around 1,500 newspaper reports containing relevant references. From these, over 1,400 reports of individual porpoises were extracted and verified. Based on the recorded locations and publication dates, the team examined changes in reported porpoise distribution over time.

Reconstructing lost biodiversity

Today, porpoises are largely absent from parts of the northern parts of Baltic Sea where they were once recorded, accounting for a loss of about one third of their historic range. The Baltic Proper population is classified as critically endangered with only around 500 individuals. Current surveys also indicate a dramatically shrinking distribution. Without historical information, it is difficult to know how much the species’ range has changed over time, as present conditions become the default reference point for what is considered “normal”.

This study is part of a growing effort by scientists to pay closer attention to historical sources such as tax records, fishing logs, recipes, and newspapers to reconstruct past marine ecosystems. This new approach is known as Marine Historical Ecology.

The findings identify areas where porpoises once occurred and where they could potentially return to if environmental conditions improve. Historical records provide a baseline for understanding how species distributions have changed from the past and how this can help guide conservation efforts in the future.

Reference

Aiken, M., P.Erikson, C.Kinze, and J.Carlström. 2026. “Historical Range Contraction and Extent of Harbour Porpoises (Phocoena phocoena) in the Baltic Sea Revealed by Archival Newspapers.” Ecology and Evolution16, no. 5: e73475. https://doi.org/10.1002/ece3.73475.