Role of material properties in acoustical target strength: Insights from two species lacking a swimbladder

Acoustic target strength (TS) is a key parameter for species identification and stock assessment in fisheries. The TS of fish is influenced by the contrast in acoustic impedance between their tissues and the surrounding water. While the swimbladder is responsible for most of the backscatter in fish with gas-filled swim bladders, the backscatter in bladderless fish is a combination of that of fish tissues. The aim of this paper is to explain the marked differences in acoustic properties reported between skipjack tuna and Atlantic mackerel, two important pelagic fish species that lack a swimbladder. The study measures the acoustic properties (density and sound speed) of their flesh and backbone. These measurements are then used to simulate acoustic backscattering using a numerical model based on the Method of Fundamental Solutions, covering a range of frequencies and fish lengths relevant to fisheries acoustics. The numerical estimation of the TS shows that the differences in the material properties of the tissues predict the reported differences in the reduced target strength of more than 10 dB greater for skipjack than for mackerel at 38 kHz and 120 kHz. The study contributes to the understanding of the complex acoustic field backscattered by bladderless fish species and provides insights into the role of fish tissue material properties in the interpretation of acoustic response differences between species.