Bioacoustic absorption spectroscopy measurements at the shelf beak off Oregon

Abstract

This paper describes the results of a Bioacoustic Absorption Spectroscopy (BAS) experiment, which was conducted at a biological hot spot at the shelf break off Oregon in August 2012. The location of the hot spot was identified by a NW Fisheries Science Center survey. This experiment included coincident measurements of transmission loss (TL), fish layer depths, fish length distributions, and continuous temperature profiles. The objective was to derive the bioacoustic parameters of hake (a physoclist), the dominant species in this region and other species from measurements of TL vs. range (0–10 km), frequency (0.3–5 kHz), depth (0–260 m), and time of day, and demonstrate consistency with trawl and echo sounder data. TL measurements were conducted between a moving, ship-deployed, broadband source and a 24 element, water column-spanning vertical array, which were provided by the Naval Research Laboratory. This was the first BAS experiment that targeted a physoclist. Previous BAS experiments targeted physostomes. The TL data exhibited absorption lines that were as high as 2 dB/km. Absorption lines at relatively high frequencies, which were observed on near-surface and mid-ocean hydrophones, are attributed to myctophids. Absorption lines, which were observed at relatively low frequencies on near-bottom hydrophones, are attributed to hake.