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

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