Abstract
High-frequency broadband acoustics can provide fine spatial resolution for near-range fish measurements, but the performance and limitations of beam-position-resolved backscatter measurements require careful evaluation. This proof-of-concept study examined one commercially sourced, dead, previously frozen specimen of each of three cultured fishes: golden pompano (Trachinotus ovatus; 24.4 cm), mandarin fish (Siniperca chuatsi; 29.1 cm), and large yellow croaker (Larimichthys crocea; 31.2 cm). A 650–750 kHz narrow-beam system was referenced to a 10.3 mm tungsten-carbide sphere, and matched-filter pulse compression and 1° stepwise scanning were used to estimate a beam-position-resolved backscatter metric along each body. At broadside incidence, the section-summed backscatter indices were −33.61, −22.45, and −33.07 dB for the T. ovatus, S. chuatsi, and L. crocea specimens, respectively. The section-summed abdominal backscatter index, in the region occupied by the swimbladder in the post-thaw X-ray images, exceeded the arithmetic mean of the head and tail group indices by 9.71 ± 1.84 dB (range: 8.27–11.86 dB). Tailward beam positions fell below the noise floor at approximately 12.5% of the expected scan positions for S. chuatsi and 22.2% for L. crocea. A 15° departure from broadside reduced the section-summed index by 3.73–11.43 dB. Kirchhoff-ray-mode (KRM) simulations based on post-thaw dual-view X-ray geometry were broadly consistent with the specimen-level contrast observed among the preserved specimens, but were 0.92–2.74 dB lower than the corresponding broadside section-summed measurement indices at 700 kHz. These differences are not a quantitative validation because the measured and modeled estimators, frequency weighting, geometry, and tissue parameters were not equivalent. These results demonstrate the feasibility of a calibrated beam-position workflow for preserved specimens, while not establishing live-fish target strength, species benchmarks, or biomass-estimation performance.