Dezhang Chu

Accurate determination of the target strength (TS) of a fish species is essential for estimating the biomass of fish stocks using acoustic technology. This study estimated the daytime in situ target strength of Pacific hake (Merluccius productus) at 38 kHz using echosounder data collected during hake biomass acoustic trawl surveys and research cruises conducted from 2009 to 2019 by U.S. and Canadian scientists. The intercept term for the 20-log TS regression over fish length at 38 kHz, b_20, was found to be -67.9 dB re 1 m2 (CI: -68.09, -67.72) closely aligning with the currently used value of -68 dB in biomass assessments. Applying the revised b_20 value of -67.9 dB in past stock assessments suggests that biomass estimates would be underestimated by less than 3%, which is well within the typical uncertainty range of fish stock assessments.

The Kirchhoff-Ray-Mode (KRM) model has been used in fishery acoustics for more than two decades to predict backscatter by individuals or aggregations of swimbladdered fish. Backscatter contributions from a fluid-like fish body and gas-filled swimbladder are coherently summed to estimate backscatter from the whole fish. For field applications, where morphologies and orientations of individual fish and their swimbladders are unknown, the coherent KRM model based on a limited number of X-ray images containing precise morphological information on swimbladders, body sizes and orientations may not provide statistically stable and robust backscatter predictions. An incoherent KRM model is proposed, which is independent of the exact knowledge of body–swimbladder acoustic interactions. When backscatter contributions from fish bodies and swimbladders are comparable, differences in coherent and incoherent KRM model target strength (TS) predictions can be as high as a few decibels. Incoherent TS predictions as a function of frequency and orientation are statistically more stable and less sensitive to variations in anatomy among individual fish than those predicted using the traditional coherent KRM model.

Accurate estimates of acoustic absorption in seawater are crucial to the acoustic estimation of aquatic biomass. Estimates of acoustic absorption were obtained via a “pulse-echo” method, implemented using commonly available scientific echosounders and spherical calibration targets over a range of discrete frequencies. Below about 200 kHz, the absorption estimates were not significantly different from those of existing formulas, but at around 333 kHz, the measured absorption was 15 dB km−1 higher than estimated from existing formulas. Measurement variability was about ±2 dB km−1 for all frequencies. This is consistent with an observed anomaly between modelled and measured frequency-dependent biological backscatter. Allowing for this deviation will avoid incorrect spectral-based classification of acoustic targets and improve uncertainty in aquatic biomass estimation.

An acoustic imaging microtome system (AIMS) was constructed to map the internal structure of fish. The system consists of two pairs of high-frequency (4.5 MHz) transmit-and-receive planar arrays, with one pair in the vertical plane and the other in the horizontal plane. AIMS provides a series of acoustic images, analogous to microtome slices, along the length of a fish by combining electronically controlled sequential acoustic transmissions and receptions over 224 channels and a computer-controlled mechanical device that moves the acoustic arrays lengthwise along the fish. AIMS measures the acoustic attenuation (extinction) along the direct ray paths of each transmit-receive channel pair that penetrates the fish body in a bistatic configuration. This results in a 2-D image of the interior morphology with 4- to 5-mm resolution. The images can provide morphological information on the fish's cheekbone, backbone, rib bone, skull, fins, stomach, and swimbladder, which is important for accurately modeling fish acoustic target strength, especially for non-swimbladder-bearing fish species.

A new method is developed in which a single standard target is used to calibrate the system response of broadband monostatic active acoustic systems. Spheres have commonly been used to calibrate narrowband systems. A major challenge involves the resonances of the scattering by the sphere. For narrowband systems, the sphere is designed so that the acoustic frequency is between the resonances. However, this cannot be done for broadband systems as the frequencies will span multiple resonances. Our method completely eliminates the source of the resonances through pulse-compression processing of the signal. Through this processing, the echo from the front interface of the sphere is isolated from other echoes which cause the resonances. The echo from the front interface is generally weakly dependent upon frequency, making it ideal for calibrating a broadband system. The method is a generalization of what was proposed by Dragonette et al. (J. Acoustic. Soc. Am. 69,1186-1189 (1981)). The advances in this new method involve incorporation of signal processing to better isolate the echo, as well as a more accurate treatment of the scattering physics. As a result, the approach is applicable over a wide range of conditions.