How long has smoked eel been a delicacy in northern Poland?
Since the neolithic, when the ice melted and the first humans appear on the scene. Smoking is a very ancient method of food preservation, and eel is a prime candidate for smoking because of its seasonal super-abundance. Without much effort, you could have easily caught hundreds of pounds of eel in a single day at the right time of the year in prehistoric streams all over what is now Poland. Kind of like salmon, another prime candidate for smoking.
The acquisition of nicknames of this sort is impossible to pin down except in those extremely rare cases where the reason for the sobriquet has been documented. The original bearer may have fished for or sold eels, or not. He may have loved eating eel. Or was known for loathing it. He may have been skinny or limber like an eel. Or he may have been fat or clumsy, very unlike an eel. Or he might have had a huge...., um, "eel". Or a really small one, for that matter.
Nicknames had a bizarre logic of their own, and irony was often employed. Think of the nicknames of the kids you grew up with, and the often twisted "logic" behind them.