Broadband measurements of the acoustic target strength of mesopelagic fishes

The mesopelagic zone (∼200–1000 m) is defined by depths where small amounts of sunlight still penetrate but light levels are insufficient to support photosynthetic activity. This region is one of Earth’s largest ecosystems and is home to a diverse community of marine animals. Biomass estimates for mesopelagic regions, based on acoustic measurements, suggest small fishes at these depths may dominate total fish biomass. These estimates, however, are generally made using surface echosounder measurements at 38 kHz. These measurements are subject to numerous confounding factors including unknown species compositions, physiology, target strength distributions as a function of depth, calibration, and low signal-to-noise measurements. To address these challenges, in addition to questions that require non-acoustic technologies, a towed, integrated sensor platform, the Deep-See, was developed to measure the mesopelagic zone at depth (Lavery et al., 2019). This talk focuses on the acoustics package, which includes commercial and custom split-beam channels from 1–500 kHz. System calibrations and target strength spectra are presented for towed depths up to 700 m, where individual targets are still present in relatively high abundance. Target strength measurements as a function of frequency are considered with an emphasis on their relationship to narrowband, surface-based measurements.