 The flathead catfish cannot live in full-strength seawater (which is about 35 parts per thousand or about 35 grams of salt per liter of water), but it can survive in 10 ppt for a while and thrive in up to about 5 ppt.The flathead catfish prefers live prey.It is a voracious carnivore and feeds primarily on fishes, insects, annelid worms, and crustaceans.