Marks on fishfinders can be interesting to interpret. If I was seeing a lot of marks like the one on the left and the center, I'd check the angle that the transducer makes to the water level as my boat sits on the water. Thinking that it was tipped backwards too far. I'm having trouble visualizing in 3-D how a fish can be first seen in the cone shallow then go deeper. (unless he was dive-bombing towards the center of the cone) If tipped too far backwards then the first two fish could have been swimming horizontally but perpendicular away from your boat. (hence further out of the cone) If a perfectly vertical cone passes over a stationary fish, it will show up deeper then appear shallower then deeper again. Like an inverted arch. (checkmark). If he was coming up from the bottom then the mark would start deep then go shallower. i.e mark sloped the other way than on your screen. Someone help me on this?
Also remember that length of the mark does not necessarily mean length of fish, rather it means he spent a longer time in the cone. i.e. the one on the right was probably a follower.