Measurements of target strength of Gulf Stream mesopelagic fishes using autonomous underwater vehicles

Mesopelagic fishes are difficult to study due to their remote locations, patchiness, and net avoidance, but are effectively observed using acoustics. Using shipboard acoustics has limitations when discerning individuals at depth; therefore, critical acoustic information such as Target Strength (TS) of individuals is largely absent. Autonomous platforms allow for sampling in the mesopelagic at resolutions unobtainable via shipboard systems. We present data that were collected in western North Atlantic waters apropos of the New England Seamount chain, specifically in the Gulf Stream (GS) and outside the GS, i.e., Slope and Sargasso Sea, using broadband echosounders on an autonomous underwater vehicle (AUV). Comparisons of TS distributions of individual targets above and in the Deep Scattering Layer (DSL) as well as TS distributions during diel vertical migration (DVM) periods were evaluated. Scattering models indicate resonance peaks of gas bearing fishes at frequencies spanning ∼10–40 kHz are depth dependent, which impact in-situ TS measurements and should be considered for the interpretation of observed mesopelagic fish communities using shipboard systems.