Water mill from Luttrell Psalter fol 181r British Library

Salmon Suffered from Medieval Water Mills

Salmon was never common food for the masses. Nevertheless, salmon declined after 1400 and is now a priority for nature conservation. New study demonstrates that the culprit was the explosion in vertical water mills from 1200 – 1400.

Contrary to what is believed, salmon was never common on the tables of late medieval peasants. Hitherto overfishing and pollution were believed to be the culprits responsible for the downturn after 1200. However, new research shows that the construction of water mills caused the destruction of the gravel beds in streams, making them unsuitable for salmon to spawn. Whereas it was previously thought that water pollution was the most likely explanation, archival research thus demonstrates that salmon stocks had already dwindled prior to the invention of the steam engine, which caused an explosion in steam ship traffic on the large rivers in Northwestern Europe.

Rob Lenders from Radboud University and his team carried out archival research, as part of which they looked at numbers and prices stated in sources from north-western Europe as of 1260. These data included leases, permits, reports from fish auctions, and the Domesday Book. They also connected a large variety of existing, small datasets.

Half ecologist, half historian

Diagram of watermills and salmon fishing in the lower Rhine - Radboud University
Aggregated indexed salmon decline in the Palaeo-Rhine catchment (circles) mirrored by increasing cumulative numbers of watermills in the Dutch, Belgian and German part of the catchment (squares).
From: Historical rise of waterpower initiated the collapse of salmon stocks. Scientific Report, Nature 2916

“This project turned me into a half historian, as I spent more time rummaging through archives than in the field. All that we observed was decline; never was there a period that showed any increase. I estimate that at the beginning of 1900, 99 % of the numbers from the middle of the thirteenth century had disappeared”, says Lender.

A tale that keeps cropping up since the seventeenth century, provides anecdotal evidence for the abundance of salmon in previous times. The story goes that servants would state in their contracts that they did not wish to eat salmon too often. That said, I haven’t found evidence that backs this up. I’ve got a good bottle of wine waiting for the first person who can provide me with such a contract,” Rob Lenders tells. In fact, at the time when this was said to be the case in Randers near Gudenåen in Denmark, salmon was the “gift” par excellence if you were trying to get special treatment from the authorities (vicars, magistrates, bishops).

Roel Lauwerier from the Cultural Heritage Agency of the Netherlands corroborated Lenders’ archival findings with data from archaeological sites at which fish bones were uncovered. He says, “The Middle Ages shows a sudden change in terms of proportions: the number of locations at which remains of pike are found increase compared to the number at which salmon remains are uncovered.”

Cause of the decline in salmon stocks

Overfishing of salmon to feed a growing population cannot explain the decline, says Lenders. “Data from the year 1311 onwards shows that the salmon stock remained steady in Scotland, a country in which fishing was taking place on an equal scale to other areas and where salmon was an important export article garnering perhaps five times as much as the same amount of pickled herring (although this is difficult to compute as the size of barrels differed).

Water Mills

Medieval Waters Water mill. Radboud University
Source: Radboud University

The team then looked at what changed in the water systems. As of the year 1000, you can notice the widespread construction of water mills, which numbered the tens of thousands once they were all completed. Water mills were also built in Scotland, but there were fewer of them and they had a different design that had a far lesser impact on the riverbeds.”

Rob Lenders identified a clear correlation between the construction of water mills and the decline in salmon stocks. Water mills need dams, obstructing the salmon that are swimming upstream to lay eggs. Thanks to the salmon’s excellent jumping abilities, a number of animals can complete the arduous journey to the spawning area – only to end up in an area that is unfit for spawning.

Lenders continues, “As the water mills and associated dams and pools bring the flow of the river to a halt, this causes a great deal more sediment to settle upstream. The gravel beds that the salmon needed to lay eggs disappeared under a thick layer of sand and silt.”

Consequences for the ecosystem and the future

Salmon in medieval waters. Credit: Radboud University
Source: Radboud University

Cumulative declines in stock likely exceeded 99.9%, indicating that historic Atlantic salmon runs in the Palaeo-Rhine catchment must have once been very impressive. This decline in salmon stocks has had a huge ecological impact. Lenders explains, “The hundreds of tonnes of salmon that once swam up the streams every year carried nutrients from the ocean into the mountains. The decline in bears, wolves and eagles could be explained by the absence of salmon.”

Lenders concludes, “Conversation programmes aimed at the recovery of the salmon population will have little effect, regardless of how clean the water is. Water mill damming permanently changed the spawning areas. Thus, even if you remove the mills and dams, this effect will endure for centuries due to the steps in the riverbed that have been formed. It would be an extremely costly process to remedy this, assuming that that is even possible.”

“The historical perspective also highlights that, for salmon to thrive, restoration efforts need to take into account the entire freshwater system within river catchments, including the capillaries, as well as smaller and larger streams, and not just the main channel and its distributaries, and connections to the sea, concludes the team responsible for this groundbreaking study”, they conclude.


Medieval water power initiated the collapse of salmon stocks (Press release provided by Radboud University)


Historical rise of waterpower initiated the collapse of salmon stocks
By J. R. Lenders et al.
Scientific Reports (2016).


Water mill depicted in the Luttrell Psalter, Add MS 42130, fol 181r. Source: British Library