Geographic patterns in volume-reverberation spectra in the North Atlantic between 33°N and 63°N

Abstract

Volume-reverberation observations were made at 34 stations on Cruise 105 .of R/V CHAIN during its passage from Newfoundland to the Azores, and thence to Portugal and Ireland. The acoustic returns at each station were analyzed in 16 1/3-octave bands extending from 1 kHz to 31.5 kHz to yield column strength spectra. Techniques to maximize the accuracy of these spectra are described. Representative spectra are plotted. Using a measure we have called the cross-standard deviation, we found evidence that the spectra for nearby stations were similar and thus could be classed in geographically-based groups called pelagic regions. This is reasonable since much volume reverberation is sound scattered from animals which in turn are distributed in pelagic faunal regions. We present a matrix showing the similarity of each pair of stations; a small number indicates similarity, while a large number indicates difference in the two spectra. We have identified many of these regions on the basis of physical-oceanographic features such as the polar front. These features are used in defining boundaries for the regions. Subject Classification: 30.40. the front south of our stations had been shown to be a faunal boundary. 0 Since a pattern existed, we felt justified in extending our analysis technique to the remainder of the cruise to look for further patterns and the boundaries they suggested. Acoustic column-strength spectra were determined from broad-band measurements of the deep scattering layers at 34 stations and were computed in 1/3-octave bands from 1 to 31.5 kHz. A few of these stations lay near or within regions which had already been delimited by Backus et al., o but most were in regions where their sampling was being carried out for the first time. The paper is divided as follows: I. Data gathering and processing describes the experimental methods followed in both collecting the data at sea and processing them ashore. Techniques followed at both stages to maximize spectral accuracy are discussed. H. Spectral results is a summary of the prominent spectral features found at many stations. HI. A measure of the similarity of spectra describes the measure we propose to quantify the similarity of spectra. It is called the cross-standard deviation and has been computed for each pair of stations. Low values of the cross-standard deviation are hypothesized to correspond to stations lying in a common pelagic region. IV. Spectral pelagic regions--their description and justification considers the full matrix of cross-standard deviations and various submatrices in order to define different pelagic regions for the portions of the North Atlantic covered by the cruise. l•hysical-oceanographic arguments are adduced where appropriate to reinforce the decisions defining regions.V. Boundaries is an attempt to set some geographic limits to these regions beyond that given by the spectral information. Physical-oceanographic information such as the defining 200-m isotherm for the polar front is used to draw boundaries suggested by the acoustic dat...

Persistent Enhancement of Micronekton Backscatter at the Summits of Seamounts in the Azores

“…Several of these seamounts host commercially valuable pelagic and demersal fish species important for local fisheries (Morato et al, 2008b;Menezes et al, 2013) and act as foraging posts for sea turtles, seabirds, marine mammals, and large pelagic fishes (Santos et al, 2007;Morato et al, 2008b;Silva et al, 2013;Afonso et al, 2014b;Tobeña et al, 2016). However, with few exceptions (e.g., Hargreaves, 1975;Martin and Nellen, 2004), previous studies on acoustic backscatter distribution in the Azores have focused only in open ocean areas (e.g., Moore, 1950;McElroy, 1974;Smailes, 1976;Wade and Heywood, 2001). Here we compare the distribution of micronekton backscatter in open ocean waters and at two seamounts with different physical properties to (i) investigate the influence of seamounts in driving distribution patterns of acoustic scatterers, (ii) determine how this effect varies over diel and seasonal scales, and (iii) discuss physical and biological processes controlling dynamics of micronekton in the study areas.…”