Isotopic Tools for Assessing Oceanic Versus Reef-Scale Drivers of Planktivorous Megauna Aggregations
Alex S.J. WYATT1*, Rui Matsumoto2, Yoshito Chikaraishi3, Keiichi Sato2, Nao Ohkouchi3, Toshi Nagata1
1Marine Biogeochemistry Laboratory, Department of Chemical Oceanography, Atmosphere & Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JAPAN.
2Okinawa Churaumi Aquarium, Motobu, Okinawa, JAPAN.
3Japan Agency for Marine-Earth Science and Technology, Yokosuka, JAPAN
Stable isotope analyses (SIA) have the potential to provide novel insights into spatial and temporal patterns in the trophic ecology of poorly understood planktivorous megafauna, especially the regional oceanic versus local reef-scale drivers of whale shark and manta ray aggregations that occur along coral reefs worldwide. However, interpreting SIA depends on accurate diet-tissue discrimination factors (DTDF) to quantify diets and trophic positions, with experimental derivations of DTDF rare for such large-bodied organisms. Captive whale sharks Rhincodon typus have provided a unique opportunity to validate a range of SIA, compound-specific isotope analyses (CSIA) and radioisotope approaches in the world’s largest fish and one of three planktivorous sharks. Combining SIA and CSIA with depth-specific radioisotope markers such as iodine ratios (129I/127I) are expected to offer a promising path towards elucidating the regional to local scale divers of planktivore aggregations Although I will focus on the implications of multi-tissue differences in DTDF and turnover times in three captive whale sharks (7.1, 7.2, and 8.4 m in length) the concepts and techniques are highly applicable to studying a wide range of species in diverse environments. An example will be provided of application to a wild caught (4.4 m) specimen of the smallest planktivorous shark, the rarely encountered megamouth shark Megachasma pelagios.