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Novel applications of animal-borne Crittercams reveal thermocline feeding in two species of manta ray

Joshua D. Stewart*, Taylor Smith, Greg Marshall, Kyler Abernathy, Iliana A. Fonseca-Ponce, Niv Froman, Guy M. W. Stevens

*Corresponding author:

ABSTRACT: Many marine species rely on oceanographic dynamics to aggregate prey sources and facilitate feeding opportunities. Numerous studies have demonstrated the importance of oceanographic fronts in the movement and foraging ecology of both predatory and filter feeding marine species. Fewer studies have investigated the importance of vertical pycnoclines (e.g. thermoclines) as foraging queues and prey aggregators. Manta rays, large batoid filter feeders, are believed to rely heavily on mesopelagic and non-surface associated zooplankton prey based on telemetry data, stomach contents, stable isotope and fatty acid analyses. However, few direct observations exist of non-surface feeding in manta rays. We developed minimally-invasive attachment methods for animal-borne video cameras (‘Crittercams’) on two species of manta ray in Mexico and the Maldives with the objective of capturing feeding behavior at depth. We achieved retention times of up to four hours using an active suction attachment with a sealant in oceanic manta rays, and up to five hours using a J-hook attachment on the upper jaw of reef manta rays. We observed feeding by both species on high-density zooplankton prey that was associated with the thermocline, suggesting that this prey aggregator may be important to the foraging ecology of both species. However, we also captured a variety of social and non-feeding behaviors that occurred within the thermocline, suggesting that telemetry-based temperature and depth data alone cannot facilitate an evaluation of the relative importance of thermocline-associated feeding. We analyzed the impact of different attachment methods on camera retention time, and discuss other relevant applications of these minimally-invasive attachment methods.