Feasibility of Ocean Acoustic Waveguide Remote Sensing (OAWRS) of Atlantic Cod with Seafloor Scattering Limitations

Ankita Jain,Dong Yi,Purnima Ratilal,Nicholas Makris,Anamaria Ignisca

doi:10.3390/rs6010180

Abstract

Recently reported declines in the population of Atlantic cod have led to calls for additional survey methods for stock assessments. In combination with conventional line-transect methods that may have ambiguities in sampling fish populations, Ocean Acoustic Waveguide Remote Sensing (OAWRS) has been shown to have a potential for providing accurate stock assessments (Makris N.C., et al. Science 2009, 323, 1,734–1,737; 54th Northeast Regional Stock Assessment Workshop (54th SAW) US Department of Commerce, Northeast Fisheries Science Center, 2012). The use of OAWRS technology enables instantaneous wide-area sensing of fish aggregations over thousands of square kilometers. The ratio of the intensity of scattered returns from fish versus the seafloor in any resolution cell typically determines the maximum fish detection range of OAWRS, which then is a function of fish population density, scattering amplitude and depth distribution, as well as the level of seafloor scattering. With the knowledge of oceanographic parameters, such as bathymetry, sound speed structure and attenuation, we find that a Rayleigh–Born volume scattering approach can be used to efficiently and accurately estimate seafloor scattering over wide areas. From hundreds of OAWRS measurements of seafloor scattering, we determine the Rayleigh–Born scattering amplitude of the seafloor, which we find has a ƒ2,4 frequency dependence below roughly 2 kHz in typical continental shelf environments along the US northeast coast. We then find that it is possible to robustly detect cod aggregations across frequencies at and near swim bladder resonance for observed spawningconfigurations along the U.S. northeast coast, roughly the two octave range 150–600 Hzfor water depths up to roughly 100 m. This frequency range is also optimal for long-rangeocean acoustic waveguide propagation, because it enables multimodal acoustic waveguidepropagation with minimal acoustic absorption and forward scattering losses. As the sensingfrequency moves away from the resonance peak, OAWRS detection of cod becomesincreasingly less optimal, due to a rapid decrease in cod scattering amplitude. In otherenvironments where cod depth may be greater, the optimal frequencies for cod detectionare expected to increase with swim bladder resonance frequency.

Highlights

Reported declines [1,2,3,4,5] in the population of Atlantic cod have led to calls for additional survey methods for stock assessments, the use of Ocean Acoustic Waveguide Remote Sensing (OAWRS) in the Gulf of Maine [6,7,8]
We show that OAWRS techniques are capable of robustly detecting Atlantic cod above seafloor scattering at and near their swim bladder resonance frequencies
Using OAWRS measurements of long-range seafloor scattering in typical sandy continental shelf waveguide environments along the US northeast coast, we estimate the magnitude squared of seafloor scattering amplitude per coherence volume (Figure 3) over the range of frequencies that are conducive for long-range waveguide propagation and where the scattering amplitude of fish aggregations is high due to swim bladder resonance

Summary

Introduction

Reported declines [1,2,3,4,5] in the population of Atlantic cod have led to calls for additional survey methods for stock assessments, the use of Ocean Acoustic Waveguide Remote Sensing (OAWRS) in the Gulf of Maine [6,7,8]. The combination of conventional methods and OAWRS techniques, has a potential for accurately estimating fish stocks over ecosystem-scale areas [9,22]. We assess the feasibility of OAWRS detection and enumeration of cod in typical continental shelf waveguide environments along the US northeast coast. We do so by combining ocean-acoustic waveguide propagation modeling [9,10] that has been calibrated in a variety of continental shelf environments for OAWRS applications with a model for cod scattering that matches well with measured data [23] for cod scattering

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Discussion

Conclusion