There are as many theorists who would explain the marked decline in numbers of the Europen Eel as there are valid theories for the decline in Atlantic Salmon numbers.
Whilst many of the theories surrounding the decline of Atlantic Salmon are centered around the Salmon farming industry and the damaging effects that Sea Lice and fish farm escapees can have on a river systems native fish, the theories for the decline in Eel numbers are many and all seem to hold at least some degree of plauseability.
It would be interesting to see someone come up with a definative reason for the Eels decline which i think has been some 85-90% in the last 40 years or so.