Notice the Pectoral Fins have been ripped off. The Pleco is a non native invasive species. They are slow moving and easy to catch. But they lock their pectoral fins outward, to prevent the bird from swallowing it. If it does swallow the fish with lock fins, it can cut their digestive tract and kill the bird.
The clever Blue Herons have adapted. They grab the fish by the pectoral fin and shake it like a dog, to rip the fin off. Now it is safe to eat the fish.