To assess organisms forming mesopelagic scattering layers in the Red Sea, we took advantage of their reactions to light. We used a remotely operated vehicle (ROV) equipped with LED lamps for herding the acoustic targets down to the bottom (700 m), while concurrently monitoring the event by shipborne and deployed echosounders as well as video footage from the ROV. In essence, entire mesopelagic scattering layers at 38 kHz were moved downwards until the organisms became trapped and concentrated at the bottom and identified as fish from video images. However, responses to the artificial light source appeared to include both repulsion and attraction. An individual-based model reproduced the herding event by assuming a dichotomous response to light where targets close to the light source are attracted, while targets further away are repulsed. We hypothesize that attraction is associated with the artificial light acting as a point source (beam light), while the repulsion is associated with the artificial light acting as diffuse light.
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