The roach wounded by the perch approached the native flock, but all the fish scattered away

The roach wounded by the perch approached the native flock, but all the fish scattered away from it. How can you explain this behavior of fish?

The fish was frightened off by the smell of blood.

After the wound, a wound remained on the body of the roach, from which blood could flow. As you know, many fish, especially predatory ones, have a rather powerful sense of smell. If the fish took the wounded roach into their flock, then after a while they would be easily found by some predator, so they scattered. This is part of the instinct for self-preservation.