Spatial variability of epi- and mesopelagic backscattering layers remains relatively constant over multi-year monitoring across the southeastern U.S. shelf break

Abstract

Acoustic echosounders can measure the distribution and abundance of pelagic organisms over varying spatial and temporal scales. The spatial variability of 38 kHz backscatter along the U.S. Mid- and South Atlantic continental slope was measured from ship-based surveys (50–100 km2) at seven sites over a four year period. We used variogram analysis to estimate the horizontal spatial structure of backscatter measurements and examined how these varied among sites, across years, and with environmental factors. Average patch sizes were consistently between 2 and 4 km for surveys across time and location. Modeled variogram range varied significantly with the depth of the backscatter layer, but linear effect sizes were negligible (<1m). Chlorophyll a (chl-a) concentration had a significant positive effect on range (95 m), suggesting patch sizes are slightly larger in the epipelagic where chl-a concentration is higher. Incorporating variogram range, sill, and nugget produced some clustering of backscatter spatial autocorrelation with depth, particularly for the deepest sites assessed. These results offer insight into nekton and macrozooplankton backscatter patterns in important shelf slope systems, and provide needed information to design accurate long-term monitoring programs for offshore marine habitats. [Work supported by BOEM/ONR/NOAA under NOPP]