Target strength measurements of individual sub-Arctic krill have frequency-dependent differences from scattering model predictions.

Target strength (TS) is commonly used to convert acoustic backscatter from marine organisms to numerical abundance estimates. Shipboard, tank-based TS measurements were made on four sub-Arctic krill species (Euphausia pacifica, Thysanoessa spinifera, Thysanoessa inermis, and Thysanoessa raschii) from the eastern Bering Sea and Gulf of Alaska at discrete frequencies between 42 and 455 kHz. These measurements were compared to scattering model predictions parameterized with data from the same (when possible) individual krill. Statistically significant differences between modeled and experimental estimates at 42, 45, 120, and 131 kHz exceeded 2 dB on average. Variability in the signal-to-noise ratio, animal length, and measurements from two separate narrowband and broadband transducer pairs (at those frequencies) did not account for these differences. Scattering predictions at 120 and 131 kHz were consistent with the expected transition from Rayleigh-to-geometric scattering where models become increasingly sensitive to orientation and body shape variability. Disagreement between modeled and measured TS may be due to using scattering models developed for, and validated on, larger krill (i.e., Euphausia superba) rather than smaller species of krill. Acoustic surveys of smaller (15-30 mm) krill may require further validation of both the generalizability and parameterization of applied scattering models.