Thank you! Since you mentioned the decline of eels book that was in Italian, I finally remembered the title too since it wasn't in English neither. It was this book in Estonian: https://www.goodreads.com/book/show/17609869-angerja-teekond.

The title translated to English would be something like "The Journey of Eel" by Aadu Hint. Published in 1950, so I'm rather certain it makes more sense to read the newer books these days.