Catfish (as well as many other species) have a swim bladder. From what i can remember the grunting noise is actually pressure being released from the bladder. They can vary the amount of gases in order to raise or lower their density to match the surrounding water. Useful for holding a specific depth. Some biologists are convinced the fish will also make this sound for courtship purposes, but many beleive this is just the sound of the fish depressurizing itself. Did you also notice a fluid discharge from the fish as well? From what I have seenthis happens simultaneously a lot of the time.
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