I talked in the lecture on ‘Sargassum and the Sargasso Sea’ about the mysterious migration of the European eel (Anguilla anguilla): its putative spawning in the Sargasso Sea and then the return of the young eels (Leptocephali) to European rivers. Over the past few decades the population of the European eel has entered a steep decline, and although this is associated with over-fishing, diseases and changes to natural watercourses, the reasons are not fully understood. The problem has become so acute that the European eel is now classified as a critically endangered species.
The worrying decline in eels numbers is being investigated by Danish scientists, who captured 50 silver eels on their journey back from a river in Denmark to the sea, attached an acoustic transmitter to each, and then released them. The migration of the eels was monitored and the mortality rates at different parts of the river and fjord were estimated. Most of the eels survived the river part of the journey, but didn’t make it out of the fjord, suggesting that fishing is an important contributor to their demise. Read all about it here and the primary paper is here .