I visited Chernobyl a couple of years ago and was stunned by the amount of large fish in the waterways (a canal now cut off from the Dnieper river, IIRC). It seemed you could almost walk from bank to bank on the swarming fish.
It made me wonder if this is what European waterways were generally like before us humans fished and fished and fished and polluted the Rhine, Danube, Volga, Dnieper, and other major European rivers.
Perhaps. In Nordic countries the sheer abundance of lakes and ponds lead to somewhat similar things.
"Biomass overgrowth of lakes and annual blue-green algae bloom as a result of eutrophication has reduced the overall use of water resources as seen from a point of recreational and leisure use. Also fish stocks have changed so that less valuable fish dominate at the cost of valuable fish like salmon and trout. Selective fishing is an effective way to reduce the overgrowth of less valuable fish populations and at the same time reduce eutrophication of water bodies."
http://www.hoitokalastus.com/hoitokalastus_en.html
I.e. they require certain amount of fishing to remain useful in that regard.
Not just the rivers but the seas as well. There are comments in literature from the 17th century very similar to yours, talking about crossing the oceans on the backs of fish.
It made me wonder if this is what European waterways were generally like before us humans fished and fished and fished and polluted the Rhine, Danube, Volga, Dnieper, and other major European rivers.