Experimental target strength measurements of pteropods and shrimp emphasize the importance of scattering model inputs

Abstract

Shrimp and shelled pteropods are ecologically important organisms that can produce significant amounts of water column backscatter. Few studies have experimentally measured both target strength (TS) and scattering model inputs for individual shrimp and shelled pteropods, especially from the meso- and bathypelagic. We captured animals from net trawls throughout the northeast Pacific and northwest Atlantic continental shelves between 2016 and 2020. We measured morphology, density and sound speed contrasts, and broadband TS (35–75, 110–230 kHz) from tethered individuals at sea and on land in scientific aquaria. Experimentally measured TS was used to test scattering model performance. Differences in measured mean length-to-radius ratios and density contrasts of epipelagic shrimp (10–13 and 1.02–1.03) were statistically greater than for mesopelagic species (8–9 and 1.04–1.05). Differences between predicted and measured shrimp TS averaged over measurement bandwidths were <2 dB, which is similar to previous studies. Conversely, pteropod TS model predictions differed from measurements by >4 dB, with species with elongated shapes having larger differences, likely due to shell shape and uncertainty surrounding material properties. Widely used physics-based acoustic scattering models may significantly underestimate uncertainty in TS predictions for these animals.