Hearing in most fishes tops out in the low kilohertz. American shad and their clupeid relatives are exceptional: behavioural and physiological experiments show responses to sounds well above that ceiling, extending into the tens of kilohertz and in some tests beyond one hundred, a range overlapping the echolocation clicks of toothed whales.
The anatomy involved is unusual. Clupeids possess a gas-filled auditory bulla connected to the inner ear, an arrangement that appears to couple high-frequency energy into the ear far more effectively than the standard fish hearing apparatus. In experiments, ultrasonic playback elicits a startle and directed avoidance rather than a general disturbance response.
The predator-detection interpretation fits well, since dolphins hunt clupeid schools and their clicks fall in the detected range, but it is an inference from correlation between the hearing range and the predator signal, not a demonstration that avoidance improves survival in the wild. The ability has been trialled practically, with ultrasonic deterrents proposed to steer shad and related fish away from power plant intakes, with mixed results.