The acoustic identification and enumeration of scyphozoan jellyfish, prey for leatherback sea turtles (Dermochelys coriacea), off central California

Graham, T. R., Harvey, J. T., Benson, S. R., Renfree, J. S., and Demer, D. A. 2010. The acoustic identification and enumeration of scyphozoan jellyfish, prey for leatherback sea turtles (Dermochelys coriacea), off central California. – ICES Journal of Marine Science, 67: 1739–1748. Acoustic-sampling techniques were developed to estimate the abundance and distribution of scyphozoan jellyfish, prey for leatherback turtles foraging off central California. Acoustic data were analysed from regions where jellyfish were present or absent from corresponding net tows. Empirical data on bell-diameter distribution (mean 26.2 cm; s.d. 10.6) were used to estimate target strength (TS; dB re 1 m2) at 18, 38, 70, 120, and 200 kHz using a distorted-wave Born approximation scattering model. TS measurements of in situ animals at 38 and 200 kHz averaged −63.9 and −70.5 dB, compared with the model estimates of −54.9 and −64.0 dB, respectively. The model estimates of TS at 18, 70, and 120 kHz were −53.9, −59.9, and −61.8 dB, respectively. Using these TS estimates, an algorithm was developed to extract jellyfish echoes from total acoustic backscatter. Echo-integration methods applied to regions where net catches contained jellyfish resulted in an estimate of mean numerical abundance of 3217 jellyfish (s.d. 2276) with a mean density of 251 522 jellyfish nautical mile−2 (s.d. 57 504) and a mean concentration of 0.003 jellyfish m−3 (s.d. 0.002). The novel methods developed will help characterize prey resources for leatherback turtles off central California.