Well, there is a legend about a lake in Germany that is similar to this, only about fish.
Once upon a time, the new abbot of a nearby monastry was originally from Italy. He was a good and pious man, but he had one weakness: His love for the whitefish of his mediterranean homeland, which was not available in Germany. At one point, the Devil decided to abuse this craving in an attempt for the abbot’s soul (it is the virtuous that the Devil wants to make pacts with - after all, the wicked are already his): In a moment of weakness, he talked the abbot into agreeing that, if the Devil can bring him whitefish before - as always - the first cry of the rooster, his soul would be damned.
As always, the abbot got scared when he heard the Devil soaring back long before the morning and realised that the task would be accomplished. And as always, he escapes damnation by startling the rooster, leading to a premature cry. And as always, the Devil drops his cargo, leaving the oddity to be explained by the legend as a relic of the event - although in this case, it’s whitefish in a lake rather than megaliths or glacial erratics.
(In case you’re wondering: Yes, in German folk tales, the Devil is a moron and it is very much possible to extract oneself a deal with him.)
Thing is, whitefish are actually native to the region. It seems that, at some point, someone mixed up whitefish (German Maränen) with murray eels (German Moränen).