What is the big arch? On March 25, 2016
We all struggle with interpreting the sonar at times.
For example, this big blue arch can be puzzling when you first see it.
I usually run my sonar in splitscreen with down imaging which often gives me the answer.
The down imaging shows the blue arch as a bait ball.
Since the blue arch is unusually shaped and blue and not dense I conclude it is the bait on the edge of the 2D cone. I bet the bait was located near the black circle in the below image. The wider cone of the down imaging shows the bait better.