Notice in the depiction that fish #5 is, in fact, in the dark water column of the Si image.... But that same fish (in the other view from behind) is very close to the SI centerline (boat's path)...
Notice fish #2 and fish #3 are not showing in the dark water column of the SI view..but both fish are still relatively close to the SI centerline (boat's path)...
This means that fish have to be very close to the boat's path to show in the dark water column...
Rickie