Ayu or sweetfish is a small (7 cm or so) local fish caught in this area, actually its caught in many parts of Japan and Asia.
It’s called sweetfish, because it really has a sweet flavour.
They have been caught in the Naka River (Nakagawa) near Kurobane for hundreds of years.
There are a number of traditional ways to catch Ayu, by fly fishing, in a traditional bamboo trap, both of which are done locally.
But another interesting way is to use a live decoy fish because Ayu are very territorial, so they attack the intruder and get caught.
There is a small shop in Kurobane that sells Ayu, freshly cooked (and decoys if you want to go fishing)
Very typically you see Ayu on skewers being cooked over coals at festivals and other events.