open waters of the Sargasso Sea—an area in the middle of the Atlantic Ocean bordered by nothing but floating seaweed and endless blue. For centuries, people didn’t even know where eels came from.

The Atlantic blue crab (Callinectes sapidus), first detected in the Ebro Delta in 2012, is a voracious predator suspected of preying on young eels. The wels catfish (Silurus glanis), another ...