As a book-l'arning biologist, I know something about the life-cycle and economic history of eels Anguilla anguilla but I'm a crap field biologist and lacked the persistence and contemplative habit required to fish effectively. Tom Fort makes the point that anyone in Europe who has walked beside a river, mere, beck, drain or culvert has been within spitting distance of some eels but very few of us have seen them. His book was first published in 2003 and that point is unlikely still to be true for anyone born in the present century. Because like so much of nature's bounty, eels have not proved to be inexhaustible in supply.
After a wigging from my pal P, I have now sorted out the inventory of anadromous species of fish: These are fish which spawn in fresh water but spend some part of their life-cycle at sea. Catadromous fish, including eels, mate and spawn at sea and live the adult part of the existence in freshwater. Whichever way they are going, the transition between a marine salinity of 35 ‰ = 3½% and the 0.2% = 2,000 ppm generates an osmotic shock which requires a host of physiological actions to maintain the internal salinity at a comfortable 9‰ = 0.9%. Eels, salmon and mammals, including us have essentially the same level of salt in our blood plasma and lymphatic circulation. They say that is because when vertebrates evolved the salinity of sea-water was about 1% and pumping excess sodium and potassium ions through the membranes wasn't an issue. The last 450 million years has swept a lot of soluble minerals into the sea. In that time also the Atlantic Ocean has opened up from a crack to a pond 5,000km wide. Gradually the adult eeels have had to travel imperceptibly further each generation to reach the trad spawning grounds.
The first part of Fort's book tells how scientists from Italy (🇮🇹 Giovanni Grassi) and then Denmark (🇩🇰 Johannes Schmidt) showed where adult eels go when they leave their home rivers, lakes, drains and marshes for a last sex-fuelled vacation. That hot coupling destination is the wide Sargasso Sea on the Bermuda side of the Atlantic in the middle of the North Atlantic Gyre. You're quite right to give side-eye to the flag-emojis because wot-in-heck does nationality have to do with the great enterprise of Science? But 100 years ago, chauvinism and flag-wagging was very much to the fore in funding the research to solve the catadromous details of this important species in European feeding.
The second half of the book turns a spotlight on the catastrophic collapse of eel fisheries across the whole North Atlantic 'market'. Lough Neagh, the Rivers Thames and Severn, the Delaware River system, and Commachio where the Po debouches into the Adriatic. I won't even attempt to summarize the details but the story repeats itself. Demanding and cash-rich external influences seduced local fishers into eating their seed-corn. Globalization is a great disruptor of the commons: when resources are shared among neighbours there is a chance for sustainable development even if that looks like same-old same-old rather than "progress". Inexhaustible supply is a tenable position if those involved tweak traditional methods or harvest. If outsiders sweep in to feed at the same trough, often with different industrial-sized techniques, the balance is likely to get upset and the resources trampled into the mud. It may be a small-small thing, seemingly unrelated, like installing street-lights on bridges. Eels prefer to travel on dark and stormy nights: 200W of sodium light across a stream mouth act like daylight on Dracula: no way up there. But what do I know?? my pal Russ actually fished for eels in Waterford.