Sound waves characterize ocean front and its inhabitants
Sound waves characterize ocean front and its inhabitants lead image
The continental shelf drops off into the ocean at the New England shelf break. Here, two water masses — cold, fresher water and warmer, saltier water — meet and form a front. This front is teeming with commercially fished animals, and researchers need a reliable method to monitor this biomass.
Loranger et al. used shipboard broadband sonars, which showed there was a dense aggregation of organisms in the warmer, saltier region of the front near the seafloor. The authors processed these sonar signals by adapting a matching method from oil and gas research that combines broadband backscatter measurements with acoustic scattering modeling.
This technique identified the species in the aggregation, longfin squid and mackerel, and measured their size. Both of these species are commercially fished.
The authors compared their acoustic data with data from net trawling, a traditional biomass measuring method. They found the trawl-based method underestimates biomass. Sonar is a less invasive technique for measuring biomass as well.
“The new technique used to identify the species and determine their length can be used to identify other mixed assemblages, which are collections of organisms where more than one species is present, and to acoustically determine the size of species,” Loranger said.
This technique also allowed them to map physical phenomena in the front, including the cross-shelf velocity. These measurements could be used in oceanographic models of the front, and sound waves are a more efficient way to obtain them than traditional methods, such as conductivity, temperature, and depth sensors.
“Mapping oceanographic fronts with shipboard echosounders is a faster, and therefore cheaper, way to study these phenomena compared with using traditional methods,” Loranger said.
Source: “Broadband acoustic quantification of mixed biological aggregations at the New England shelf break,” by Scott Loranger, Michael J. Jech, and Andone C. Lavery, Journal of the Acoustical Society of America (2022). The article can be accessed at https://doi.org/10.1121/10.0014910 .
