Medieval water power initiated the collapse of salmon stocks

July 20, 2016
Medieval water power initiated the collapse of salmon stocks
Natural river bed with exposed gravel upstream. Credit: Radboud University

Salmon largely disappeared from the Netherlands due to the construction of water mills, ecologists from Radboud University conclude (Scientific Reports, 20 July). 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 contamination was the most likely explanation, archival research demonstrates that salmon stocks had already dwindled prior to the invention of the steam engine.

Salmon have always been cherished and are now a priority for nature conservation. 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

"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.

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."

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."

Medieval water power initiated the collapse of salmon stocks
Upstream sedimentation due to water mill dams. Credit: Radboud University

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.

We 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."

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 need to lay eggs have disappeared under a thick layer of sand and silt."

Consequences for the ecosystem

The 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."

The future

Lenders concludes, "Conservation programmes aimed at the recovery of the population will have little effect, regardless of how clean the water is. The spawning areas have been permanently changed by the water mill damming. 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."

Explore further: Kodiak bears track salmon runs in Alaska

More information: H. J. R. Lenders et al. Historical rise of waterpower initiated the collapse of salmon stocks, Scientific Reports (2016). DOI: 10.1038/srep29269

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10 comments

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AlexNC
1 / 5 (2) Jul 20, 2016
Wow, what a pessimistic closing. Dam removal and fishery remediation is not out of reach. Just look at the US for examples of many projects that have proving this fact. The main obstacle is always money, but that doesn't mean it is not possible. Many successful projects in the United States and all around the world prove the last paragraph to be completely wrong.
gkam
1.4 / 5 (7) Jul 20, 2016
Those without knowledge do not understand what we are doing to our Life-Support System.
mark0101
1 / 5 (3) Jul 20, 2016
Salmon don't need gravel beds to reproduce when humans are involved. The gravel is only used to keep the eggs safe.
Just catch the salmon, squeeze out their eggs, and grow them in a tank where they are safe till they are big enough to fend for themselves.
Psilly_T
5 / 5 (2) Jul 20, 2016
Salmon don't need gravel beds to reproduce when humans are involved. The gravel is only used to keep the eggs safe.
Just catch the salmon, squeeze out their eggs, and grow them in a tank where they are safe till they are big enough to fend for themselves.

and what we just do this for the next species this happens to and then the next on so on? forget fixing the problem we will just manage it ourselves? slim chance that would work out. At best we could keep enough alive to just eat and not have anything wild. And the cost of trying to catch every wild salmon for their eggs so they're species can recover is ridiculous.

Maybe we have been removing dams here in the states but the article clearly states Alex that the rivers in question are ruined as far as salmon spawning will go. The amount of money you would need to go into the river invasivle and try to "clean up " the steppes the mills made is laughable sorry =/ And the damage we would cause along the way too high
Surveillance_Egg_Unit
not rated yet Jul 20, 2016
"Lenders concludes, "Conversation programmes aimed at the recovery of the salmon population will have little effect, regardless of how clean the water is." - from the article

Don't they mean "Conservation"? This is the second time I have noticed this same kind of error in a Physorg article.
Surveillance_Egg_Unit
not rated yet Jul 20, 2016
Salmon don't need gravel beds to reproduce when humans are involved. The gravel is only used to keep the eggs safe.
Just catch the salmon, squeeze out their eggs, and grow them in a tank where they are safe till they are big enough to fend for themselves.


Bad idea. Salmon have been procreating the natural way for millions of years. Squeezing the female salmon for their eggs wouldn't make sense.

(cont'd)
Surveillance_Egg_Unit
not rated yet Jul 20, 2016
"Salmon Run" from Wikipedia

"The eggs of a female salmon are called her roe. To lay her roe, the female salmon builds a spawning nest, called a redd, in a riffle with gravel as its streambed. A riffle is a relatively shallow length of stream where the water is turbulent and flows faster. She builds the redd by using her tail (caudal fin) to create a low-pressure zone, lifting gravel to be swept downstream, and excavating a shallow depression. The redd may contain up to 5,000 eggs, each about the size of a pea, covering 30 square feet (2.8 m2).[53] The eggs usually range from orange to red. One or more males will approach the female in her redd, depositing his sperm, or milt, over her eggs.[54] The female then covers the eggs by disturbing the gravel at the upstream edge of the depression before moving on to make another redd."

Clean up the rivers; remove obstructions; and let Nature take its course.
rrrander
not rated yet Jul 21, 2016
Overfishing wiped out cod stocks in the north Atlantic. The cod prior to this were so thick, according to the first Europeans to come to N. America "you could walk on water by stepping them." SURE overfishing could have wiped out salmon stocks in the Netherlands.
EnricM
not rated yet Jul 21, 2016
Wow, what a pessimistic closing. Dam removal and fishery remediation is not out of reach. Just look at the US for examples of many projects that have proving this fact. The main obstacle is always money, but that doesn't mean it is not possible. Many successful projects in the United States and all around the world prove the last paragraph to be completely wrong.


I wouldn't call it "pessimistic". We are talking about a change that happened 700 years ago and that changed the ecosystem for good. I am not able to tell how this impacted the diversity but I assume that the alteration of riverbeds has also had positive effects allowing other species of vegetation and animals to thrive.

In the USA the rivers were not so deeply affected by the changes such as here in Europe and this is why it is a difficult task that may just not be worth it.
ACoffeeDrinker
5 / 5 (1) Jul 21, 2016
"Lenders concludes, "Conversation programmes aimed at the recovery of the salmon population will have little effect, regardless of how clean the water is."

Try instead CONSERVATION programmes. There is a already to much talk anyway.

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