The Recording and Interpretation of Pressures in Prey-Sucking Fish

Abstract

Possible pitfalls of techniques used to record pressures in prey-sucking fish have been analysed by applying control systems and hydrodynamic theory. Fourier analysis revealed a bandwidth of 1 kHz to be generally sufficient for an accurate recording of the overall pressure waveform. The exact bandwidth needed depends on the species, specimen size and intensity of the feeding act. A bandwidth greater than 1 kHz may be needed when secondary fluctuations (e.g. due to vortices) are also to be recorded accurately. Large errors (i.e. of the same order of magnitude as the real pressure) due to the dimensions of the transducer itself cannot be excluded on theoretical grounds. Most reliable pressure records were made with catheter tip pressure transducers, possessing small dimensions and a broad bandwidth (about 3 kHz). Pressure records were made for Amia calva, Salmo gairdneri, Esox lucius, Gadus morhua and Perca fluviatilis. The largest negative pressure peak was measured in Gadus (-42 kPa). Accurate simulations of the pressure records were made using the hydrodynamic model of suction feeding of MULLER, et al. (1982) to which new boundary conditions were added. This model takes into account the unsteady nature of the flow. The good correlation between measured and simulated pressures suggests, but does not prove, that: 1) the model is a good description of the process of suction feeding; 2) the errors in the pressure records obtained with catheter tip pressure transducers are small. The pressure inside the mouth of a prey-sucking fish has velocity and acceleration components due to head expansion and forward motion of the fish. Forward motion strongly influences the pressure profile in feeding events of the two fast-swimming fish studied, Salmo gairdneri and Esox lucius. The literature on pressure measurements in prey-sucking fish is reviewed.