Relationship between volume backscatter from the mesopelagic boundary community and density of organisms in the waters of the Hawaiian Islands

Abstract

Spinner dolphins in the near-shore waters of the Hawaiian islands forage on a mesopelagic boundary community of organisms consisting of myctophids, mid-water shrimps, and small squids. To better understand the foraging ecology of spinner dolphins, it is important to understand the relationship between volume scattering coefficient (Sv) obtained with an echosounder and the density of the organisms in the mesopelagic boundary community. A suite of Simrad Ek-60 scientific echosounders operating at 38, 72, 120, and 200 kHz directed vertically downward was used to collect volume backscatter data from a vessel anchored 1 km from shore. Sv tended to be highest at 38 kHz and lowest at 200 kHz, although at some depths Sv at 70 kHz was slight higher than at 38 kHz data. Immediately after collecting the echosounding data, a profiler with a broadband high-resolution echo ranger projecting a simulated dolphin biosonar signal horizontally was lowered into the same volume examined by the EK-60. The number of organism in the echoranger beam out to a specified range was estimated by counting the number of highlights in the echoes after performing an envelope detection. The relationship between Sv and number of organisms was found to be linearly related.