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

TitleThe acoustic identification and enumeration of scyphozoan jellyfish, prey for leatherback sea turtles (Dermochelys coriacea), off central California
Publication TypeJournal Article
Year of Publication2010
AuthorsGraham, Tanya R., Harvey James T., Benson Scott R., Renfree Josiah S., and Demer David A.
JournalICES Journal of Marine Science
Volume67
Pagination1-10
EndNote Rec Number2683
KeywordsTurtles, acoustic survey, echosounder, leatherback turtle, multifrequency, scyphozoan jellyfish, target strength
Abstract

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 1m2) at 18, 38, 70, 120, and 200kHz using a distorted-wave born approximation scattering model. TS measurements of in situ animals at 38 and 200kHz averaged 263.9 and 270.5 dB, compared with the model estimates of 254.9 and 264.0 dB, respectively. The model estimates of TS at 18, 70, and 120 kHz were 253.9, 259.9, and 261.8 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 mile22 (s.d. 57 504) and a mean concentration of 0.003 jellyfish m23 (s.d. 0.002). The novel methods developed will help characterize prey resources for leatherback turtles off central California.