Acoustical backscattering is used in determining the presence of fish in many bodies of water. Since the characteristics of this backscatter are predominantly determined by those of the swimbladder, knowing the frequency response of a fish’s swimbladder a priori could allow identification not only of size, but also of species. A noninvasive, non-contact ultrasonic measurement system was used to determine the in vivo frequency responses of the swimbladders of two different species of fish, the goldfish (Carassius auratus) and the oscar (Astronotus ocellatus). Experimental results showed a difference in both the resonance frequency and sharpness of resonance of the swimbladder between the two species even though all the animals tested were approximately the same size.

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