Definitely a tightly packed bait ball indicating that it was being harrassed by a predator species.
In other words, herring usually travel in schools that are readily apparent and easy to mark on your sounder. However, when "somebody" is feeding on a number of herring, they tend to display classic schooling behavior (well defined tight ball) which is how I interpret your screen shot.
Salmon, as all bony fishes (Class Osteichthyes) have a swim bladder containing air which marks very well as bright arches.
Sharks on the other hand are a cartilaginous fish (Class Chondrichthyes) and remain neutrally bouyant in the water column due to a large, oil filled three-lobed liver) The oil in the liver has a slightly lower density than water and therefore sharks do not always light up your screen the way salmon and herring do!
Hope that answers your question and good luck with your Lowrance!