The specific gravity of mesopelagic fish from the northeastern Pacific Ocean and its implications for acoustic backscatter

Knowledge of the species present, their morphology, and their size distribution is required to infer biomass from acoustic surveys of fish. The gas content and specific gravity of the body (with gas removed), rho(f), was measured for 71 species of mesopelagic fish in the NE Pacific Ocean. Those species that have functional swimbladders when large maintain constant rho(f) with increasing body size. Species without functional swimbladders as adults show decreased rho(f) with increasing body size. The acoustic-backscattering cross-section, sigma(bs), was modelled for all individuals collected from three fish species that differed in the presence of a gas-filled swimbladder. The change in sigma(bs) with increasing body size was markedly different between the three. The low body density of those mesopelagic fish without gas-filled swimbladders greatly reduces their sigma(bs). In species of fish that possess a functional swimbladder as juveniles and in which the swimbladder regresses with growth, the sigma(bs) first decreases, then increases with increased body size. Knowledge of the ontogenetic changes in swimbladder inflation and body density in mesopelagic fish is critical for the construction of the backscattering models used to interpret acoustic surveys.