A lookdown is very deeply compressed fish. So they look like they weigh much more in pictures than they actually do in real life. Lookdown are in the Jack family. The scientific name is Selene vomer, they range from the Maine all the way to Uruguay. They are sometimes caught by fisherman. This one was caught on a jig at the oil production platforms off the Louisiana coast.
They generally live in shallow coastal waters over sand or mud. When young they have long filaments coming off the first dorsal spine of the dorsal fin.
Fisherman that catch lookdowns often hook them deep due to their method of feeding. when this happens it is best to simply cut your line close to the hook, release the fish and move on to waters that are free of lookdowns.
Below is a couple of pictures of baby Lookdowns. The dorsal fin has long filaments when young. That is not grass or seaweed in the picture.