If Flounder were the only species that declined I might agree it's the environment, predators etc. But Flounder declined the same time Whiting, Ling , Cod , Blackfish and Fluke declined. Fluke were wiped out in the late 1980's during the winter and every other fish went into decline shortly afterward. I saw it with my own eyes, I had a new Downeast Lobster Boat and I wanted to catch all those fish. Had a brief time early in the 1990's when Flounder, Whiting were still pretty abundant. 2 years later they were all gone. The Party Boats all switched to Blackfish in the winter and pounded them down.
The Commercial Fleet couldn't catch enough Fluke to stay in business, they targeted every other fish . If Fluke had been regulated correctly in the 1980's , there's a good chance the Whiting and Ling would have survived. Cod Fishing up North was basically shut down during the 1990's , The New England draggers came down to fish off Long Island. Wonder what they caught ? I had a great Wreck off Shinnecock that was great for Cod. One year it was there and the next it was gone and couldn't be located again. Bet some New England Dragger ran over that Wreck and destroyed it. Some how the Commercials got the Okay to use roller Gear and they flattened great fishing grounds like 17 Fathoms. One group shouldn't be allowed to take everything and leave nothing for other groups. You should be able to go fishing and have a good chance to catch every species and keep a reasonable number for dinner.