It was already a special day for Caleb Newton and Phil Wilcox — the two friends enjoying a bass-fishing bachelor party on the Potomac River. But when the men spotted a long shadow swimming among the reeds, they knew things were only going to get more interesting. Gliding among the grass was a northern snakehead, a gruesome animal known as a “Frankenfish” by some. And it was big. But it wasn’t until they wrestled it into the boat that they realized it might be the biggest anyone has ever caught with a hook and line. “I knew it was a...