Quantification of Wide-Area Norwegian Spring-Spawning Herring Population Density with Ocean Acoustic Waveguide Remote Sensing (OAWRS)

Daniel Duane,Nicholas C Makris,Olav Rune Godø

doi:10.3390/rs13224546

Abstract

Norwegian spring-spawning herring are a critical economic resource for multiple nations in the North Atlantic and a keystone species of the Nordic Seas ecosystem. Given the wide areas that the herring occupy, it is difficult to accurately measure the population size and spatial distribution. Ocean Acoustic Waveguide Remote Sensing (OAWRS) was used to instantaneously measure the areal population density of Norwegian herring over more than one thousand square kilometers in spawning grounds near Ålesund, Norway. In the vicinity of the Ålesund trench near peak spawning, significant attenuation in signal-to-noise ratio and mean sensing range was observed after nautical sunset that had not been observed in previous OAWRS surveys in the Nordic Seas or in other regions. We show that this range-dependent decay along a given propagation path was caused by attenuation through dense herring shoals forming at sunset and persisting through the evening for transmissions near the swimbladder resonance peak. OAWRS transmissions are corrected for attenuation in a manner consistent with waveguide scattering theory and simultaneous downward directed local line-transect measurements in the region in order to produce instantaneous wide-area population density maps. Corresponding measured reductions in the median sensing range over the azimuth before ambient noise limitation are shown to be theoretically predictable from waveguide scattering theory and observed population densities. Spatial-temporal inhomogeneities in wide-area herring distributions seen synoptically in OAWRS imagery show that standard sparsely spaced line-transect surveys through this region during spawning can lead to large errors in the estimated population due to spatial and temporal undersampling.

Highlights

Norwegian spring-spawning herring is a critical economic resource for multiple nations in the North Atlantic
One hour before nautical sunset, the Ocean Acoustic Waveguide Remote Sensing (OAWRS) system reveals small, dispersed groups of herring with dimensions that are typically less than 1 km (Figure 2A), and there are few herring groups observed in echosounder measurements from the research vessel towing the OAWRS system (Figure 2B)
Reductions in the ambient noise level observed after nautical sunset are found to be consistent with the predicted reductions in ambient noise due to attenuation from fish (Figure 7)

Summary

Introduction

Norwegian spring-spawning herring is a critical economic resource for multiple nations in the North Atlantic. Since 2013, disagreements over the shifting spatial distribution of this herring stock have prevented these nations from negotiating quota sharing agreements, resulting in a combined catch exceeding the recommended limit by more than 30%. It is important to accurately measure the population size and spatial distribution for Norwegian herring over ecosystem scales. Occupy vast undersea areas that are difficult to sample without significant aliasing in space and time with conventional survey methods [2,3]. We demonstrate the ability of Ocean Acoustic Waveguide Remote Sensing (OAWRS) to instantaneously image Norwegian herring shoals over more than one thousand square kilometers in spawning grounds near Ålesund, Norway. Wide-area OAWRS scattering strength maps are generated by correcting transmissions for source level and

Methods

Results

Discussion

Conclusion