Use of Fishfinder when Jigging

[Kenken]

Hey guys

Got into jigging this summer with moderate success. I'd really like to improve my fish finding skills especially when using the fish finder. Was wondering if you could help me with a couple of questions.

What depths do you usually jig in? I usually jig at 60- 100m though I was amazed that when steve gb caught that lovely dentex we where at about 25m circa.

I sometimes see some clustered disturbance (many dots) protruding upwards from the bottom on the fish finder. Anyone has any idea what that is? Could it be some type of baitfish? Fish finder doesn't mark it as fish though.

Are there any other signs that you usually look for on the fish finder before attempting to jig....apart from fish arches obviously.

Thanks

ken