Broadband backscatter measurements of individual Hawaiian mesopelagic boundary community animals

Abstract

Broadband simulated dolphin echolocation signals were used to measure the ex situ backscatter properties of mesopelagic boundary community animals. The signal had a peak frequency of 100 kHz, 3-dB bandwidth of 35 kHz, and duration of 80μs. Subjects were captured by trawling with a 2-m-opening Isaacs-Kidd midwater trawl. Backscatter measurements were conducted on the deck of the ship in a 2000-L seawater plastic tank with the transducer placed on the bottom, pointed upwards. Anesthetized animals were suspended in the transducer’s beam by nylon monofilament lines connected to a support device constructed out of dowels. Backscatter measurements were obtained in both the dorsal and lateral aspects for seven myctophids and only in the dorsal aspect for the other 13 myctophids, 6 shrimps, and 3 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 the body of the subject and reflecting off the opposite surface of the animal. The squid echoes consisted mainly of a single highlight but sometimes with a low-amplitude secondary highlight. The target strength had a rippled pattern in the frequency domain showing a distinct null.