The use of multibeam and split-beam echo sounders for assessing biomass and distribution of spring-spawning Atlantic cod in the Gulf of Maine
This research focused on advancing the application of split-beam and multibeam echo sounding to remotely locate and describe spatial distribution, and to provide a relative measure of abundance of the spring-spawning Atlantic cod (Gadus morhua) in the western Gulf of Maine. Specifically, the main objectives of this research were 1) to test the feasibility of a multibeam echo sounder to detect changes in volume backscatter proportional to incrementally decreasing quantities of cod held in a submerged cage, and to compare results to a split-beam echo sounder; 2) to describe the spatio-temporal distribution and estimate biomass of spring-spawning cod in the Gulf of Maine cod spawning protection area (GOMCSPA) by repeated acoustic and trawl surveys; and 3) to determine a predictive relation between target strength and length for 38-kHz and 120-kHz split-beam echo sounders and a 300-kHz multibeam echo sounder, and characterize other factors affecting backscattering of sound. The multibeam echo sounder detected a small and large reduction in volume backscatter proportional to reductions in stocking density of caged cod, while the split-beam echo sounder only detected a large reduction in stocking density. The spatial information from the multibeam echo sounder helped interpret and explain results from the split-beam echo sounder. Repeated acoustic and trawl surveys showed cod were relatively widespread in the survey area in May, but congregated at higher densities in areas adjacent to two elevated bathymetric features. Most cod converged to a single location in June, and were at a higher concentration than observations in May. This congregation decreased in size and density in July. Survey estimates of cod biomass ranged 184-494 mt in May, 138-617 mt in June, and 39-135 mt in July, depending on the estimation method. Based on echo classification and extrapolation, cod biomass to the GOMCSPA ranged 260-466 mt in May, 196-513 mt in June, and 91-198 mt in July. The biomass being protected by the closure may have represented 4-5% of the GOM cod spawning stock biomass at the time of the study based on these estimates. The three echo sounders synchronously collected acoustic data of individual free-swimming captive cod, while the movements of most individuals were observed with underwater video. The standard TS-L equations were TS = 20 log10(L) -- 66.4 at 38 kHz, TS = 20 log10(L) -- 67.4 at 120 kHz, and ⟨TS⟩ = 20 log10(L) -- 71.4 at 300 kHz. The study demonstrated a significant TS-L relation at 300 kHz from aggregated data collected by a multibeam echo sounder with narrow beams over multiple beam-pointing angles and without split-beam target tracking.
Agriculture, Fisheries and Aquaculture, Biology, Zoology, Physics, Acoustics
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