So what is this? It's an interesting Japanese fishing technique that dates back centuries. Ayu is a trout native to Japan. The fish feeds on underwater plant material in his own territory. Say 100 square feet. The Ayu does not like invaders and will angrily fight to protect its turf. One of the fighting techniques involves bashing into the belly of an invader to drive it away.
Ayu fishing means "a fish brings another fish." In practical terms this is what you do. You start with one Ayu, threading mono through its mouth and gills. The fish is alive and only slightly hurt. At the end of the mono is a hook hanging out under the fish. The angler draws the Ayu (let's call this one #1) into the territory of another Ayu (fish #2). Fish #2 gets angry because of the invader and belly butts fish #1. During the scuffle #2 gets hooked by the hook hanging off of #1. #2 fish gets hooked in the dorsal fin (usually). You have used #1 to catch #2. Now #2 becomes the fish which will catch #3 and so on.
It sounds kind of cool. It is a favorite summer-time pleasure of the Japanese. They eat the small fish.