NJMC Naturalist Brett Bragin supplied the answer:
"That looks like a gizzard shad to me. I recall catching a few adult gizzard shad in Kearny FW marsh in 1987 or 1988.
"As usual, it is hard to do a positive ID from just a photograph."
Rest of post follows.
Brett continues: "In order to be certain you need to pull up the dorsal fin to see if the last ray (closest to the body) is extended into a long filament (as in the above photo of a large adult taken from the lower Hackensack River)."
Ron also wondered how the fish got into the [presumably landlocked] Kerant Marsh.
Brett replies: "The gizzard shad are native to NJ and are widespread. They are found in freshwater, brackish, and marine waters in the Atlantic, Delaware River/Bay and Hudson river drainages, as well as inshore areas of the Atlantic Ocean.
"I suspect that they somehow found their way into the Kearny Marsh from the Hackensack and/or Passaic River via culverts and leaking tide gates. Another possibility is that they got there by being carried into the marsh during flooding from large storms that caused the man-made impediments (e.g., RR embankments, Route 7, etc.) to be less of an impediment than they normally are.
"But then again, you just never know; maybe some fisherman using them as live bait dumped his bait bucket at the end of a less-than-productive day while fishing in the marsh."