Detection of scattered ambient noise by fish: Possible passive perception of potential predators and prey from palpable pressure and particle path perturbations

In fish, a complex passive system has evolved for the detection of acoustic pressure and particle motion that rivals man-made systems. Since the frequency range of best sensitivity for the fish ear occurs where ambient noise is high, it may be assumed that the fish is using the ambient noise to its advantage. Rogers has presented an hypothesis that one fish could perceive nearby fish by recognizing the scattering of ambient noise by the other swim bladders. This characteristic scattered noise could allow for the detection and identification of the scatterer by the receiver.; Two studies were conducted to examine the basic premises of this hypothesis. The first characterized the scattering of sound by a fish by measuring the frequency response in vivo of the swim bladder using NIVAMS (Non-Invasive Vibration Amplitude Measurement System). The single chamber swim bladder of the oscar acted like a damped air bubble resonating in the ambient noise field, scattering significant amounts of acoustic energy. The response of the two chamber swim bladder in the goldfish, however, was found to be more complex.; The second study determined if a fish could sense similarly shaped filtered noise signals against an ambient noise background. Detection thresholds for the goldfish were measured as a function of center frequency of the scattered signal and range to the scatterer. Detection required a narrow band pressure signal to noise of about {dollar}-{dollar}4 dB, independent of center frequency and range. The goldfish did not directionally discriminate scatterers at near threshold signal levels.; Based on the resonance characteristics for the single chamber swim bladder and thresholds to scattered noise, the detection range was calculated to be on the order of the length of the scattering fish. This range could allow escape from predators by prey, indicating detection of scattered ambient noise could be biologically relevant.