Broadband backscatter from individual Hawaiian mesopelagic boundary community animals with implications for spinner dolphin foraging

Abstract

Broadband simulated dolphin echolocation signals were used to measure the ex situ backscatter properties of mesopelagic boundary community (MBC) in order to gain a better understanding of the echolocation process of spinner dolphins foraging on the MBC. Subjects were captured by trawling with a 2-m-opening Isaacs-Kidd Midwater Trawl. Backscatter measurements were conducted on the ship in a 2000 L seawater tank with the transducer placed on the bottom pointed upwards. Backscatter measurements were obtained in both the dorsal and lateral aspects for seven myctophids and only in the dorsal aspect for 16 more myctophids, six shrimps, and three squids. The echoes from the myctophids and shrimps usually had two highlights, one from the surface of the animal nearest the transducer and a second probably from the signal propagating through body of the subject and reflecting off the opposite surface of the animal. The squid echoes consisted mainly of a single highlight but sometimes had a low amplitude secondary highlight. The backscatter results were used to estimate the echolocation detection range for spinner dolphins foraging on the mesopelagic boundary community. The results were also compared with multi-frequency volume backscatter of the mesopelagic boundary community sound scattering layer.