Broadband acoustic quantification of mixed biological aggregations at the New England shelf break

Abstract

At the New England shelf break, cold, less saline shelf water collides with warmer saltier slope water to form a distinct oceanographic front. During the Office of Naval Research Sediment Characterization Experiment in 2017, the front was mapped by narrowband (18 and 38 kHz) and broadband (70–280 kHz) shipboard echo sounders. The acoustically determined cross-shelf velocity of the front ranged in amplitude from 0.02 to 0.33 m/s. Acoustic surveys revealed aggregations of scatterers near the foot of the front. Acoustic backscatter in conjunction with Northeast Fisheries Science Center bottom trawl surveys identified longfin squid (Doryteuthis pealeii) and mackerel (Scomber scombrus) as the most likely scatterers in the aggregations. A mixed species scattering model was developed and further refined by the use of a matching method used for distribution of the lengths of each species. The mean length of squid and mackerel, respectively, using the matching method was 4.45 ± 1.00 and 20.25 ± 1.25 cm compared with 6.17 ± 2.58 and 22.76 ± 1.50 cm from the trawl data. The estimated total biomass of the aggregation was a factor of 1.64 times larger when using the matching method estimated length distribution compared to the trawl length distribution.