The best way to immediately differentiate whether you have a river or sea herring (good to know for livelining
) is by taking your finger and drawing it from the tail to the head right along the ventral side of the fish. If it feels smooth, it's a sea herring, likely an Atlantic. If it feels rough and ridged, it's a river herring. You can feel these belly scutes in all of the anadromous herring- try it on the next pogie you snag.
Very tough to tell the difference between an alewife and a blueback- the easiest way is to bisect the fish and examine the color of the gut lining- An alewife is pink, and a blueback is black. Not recommended to do this in the line of sight of any enforcement
From the relative thickness of the body, position of the dorsal, oversized eye, smooth belly scutes and extremely flaky scales I'd say juvenile Atlantic herring.