One of many issues. The European eel gets hit by just about every possible thing due to its many and varied life cycles. Larval stage is subject to disrupted ocean currents due to climate change, juveniles are subject to an enormous illegal fishing trade (as the most prolific remaining Anguilidae, they are used to supply eel demand in many areas where local species aren’t doing as hot) as they reach European coast, young adults struggle to navigate up waterways due to dams and whatnot, adults spend their lives being fat in the mud and absorb lots of pollutants, then when ready to breed they face the dams again going back out to sea. They’re also under immense pressure from an invasive swim bladder parasite brought over when Japanese eels were introduced to Europe sometime in the 20th century.

And these are just the headline threats for each stage, there’s myriad smaller ones. The reason for their decline is almost certainly a case of lots of small threats overcoming their ability to adapt, rather than one discrete cause.

Smuggling also. In the last decade some cargo of alive eels had been found inside lugagges in airports flying to Asia.

There is an american Nematode parasite also that castrate the European eels.

Plus, overfishing, contamination, invasive species of fishes, drough, engineering of rivers, by-caught, propellers in dam pipes that cut them in chunks while swimming... And can't be breed in captivity.

The animal will go extinct in this century by greed, as usual.