Adding to our isotope axes @ IsoEcol 2016 (Dr Wyatt)

Amino acid and radiocarbon insights from captive whale sharks

Alex S.J. WYATT1*, Rui Matsumoto2, Yoshito Chikaraishi3, Yosuke Miyari1, Yusuke Yokoyama1, Keiichi Sato2, Nao Ohkouchi3, Toshi Nagata1

1Atmosphere and 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
*Presenting author

Stable isotope analyses (SIA) have the potential to provide novel insights into spatial and temporal patterns in the trophic ecology of poorly understood organisms like whale sharks Rhincodon typus. 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 R. 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. Diet records over the past five years revealed a diet dominated by North Pacific and Antarctic krill, 44% and 49% of weighted diet for Euphausia pacifica and E. superba, respectively. Despite the well-known diet, SIA of fin tissue from three captive R. typus (7.1, 7.2, and 8.4 m in length) proved hard to reconcile, especially for bulk carbon. In contrast, CSIA of amino acid (AA) nitrogen in the sharks’ tissue was relatively stable over time, despite evidence of variation in AA compositions and δ15N-AA of diet components. Tissue radiocarbon further suggested either long turnover in fin tissues (27 months), or the preferential assimilation of the smaller E. pacifica14C of 3 ‰ compared to -112 ‰ for E. superba). Daily-scale analysis of radiocarbon in R. typus faeces may support the preferential assimilation hypothesis, faeces generally being depleted relative to diet. Together, CSIA-AA and radiocarbon analyses add multiple addtional axes to our isotope space and may alleviate some of the complications involved in interpreting bulk SIA in ecological studies.

IsoEcol 2016 to be hosted in Japan

We are please to announce that we will be hosting The 10th International Conference on the Application of Isotopes to Ecological Studies (IsoEcol 2016) in Tokyo, Japan from April 3-8, 2016.

More information will be made available via the conference website:

Organizing Committee

  • Alex Wyatt, The University of Tokyo
  • Ichiro Tayasu, Kyoto University
  • Naohiro Yoshida, Tokyo Institute of Technology
  • Naohiko Ohkouchi, JAMSTEC
  • Toshi Nagata, The University of Tokyo

Isotope discrimination in captive whale sharks @ IsoEcol 2014 (Dr Wyatt)

Isotope Discrimination in Planktivorous Elasmobranchs Focusing on the World’s Largest Fish, Captive Whale Sharks Rhincodon typus

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
*Presenting author

Accurate diet-tissue discrimination factors (DTDF) are essential for quantifying diets and trophic positions (TP) using stable isotope analyses (SIA), with potential variation between diets, tissues, organisms and environments arguing against untested application of meta-analysis averages (e.g. 3.4 ‰ for bulk nitrogen (Δ15Nbulk), ~0.5 ‰ for bulk carbon (Δ13Cbulk), and 7.6 ‰ and 0.4 ‰ for nitrogen of glutamic acid (Δ15Nglu) and phenylalanine (Δ15Nphe), respectively). Experimental derivations of DTDF in elasmobranchs (sharks and rays) are scarce, with large-bodied organisms difficult to maintain in captivity and non-lethal multi-tissue sampling problematic for both captive and wild individuals. SIA of captive whale sharks Rhincodon typus, one male (8.5 m in length) and two females (7.1 and 7.2 m), fed a mixed diet composed mainly (~ 48 % each) of Antarctic krill Euphausia superba15N = 3.45 ‰, δ13C = -26.3 ‰) and North Pacific krill E. pacifica15N = 5.88 ‰, δ13C = -21.6 ‰), provide an opportunity to examine DTDF in the world’s largest fish and one of three planktivorous sharks. DTDFs estimated based on temporally averaged diets for easily sampled but slow turnover fin tissue were close to previous observations, but varied between individuals, perhaps reflecting differing growth rates with size or physiological differences between the sexes: Δ15Nbulk (2.6, 3.3, 3.1 ‰), Δ13Cbulk (3.9, 4.5, 5.9 ‰), Δ15Nglu (7.6, 6.5, n.d. ‰) and Δ15Nphe (0.3, 0.2, n.d. ‰). Short turnover tissues, such as liver or blood, may be difficult or impossible to obtain for these species, requiring non-lethal isotopic proxies to examine diet and TP at higher temporal resolution. For instance, SIA of faecal material was highly variable but reflected day-to-day variation in minor (<3 %) components of the sharks’ diets. DTDF will be discussed in the context of sampling constraints related to multi-tissue SIA and recent radioisotope approaches for understanding feeding and aggregations of planktivorous elasmobranchs, including recent application to a wild caught (4.4 m) specimen of the smallest planktivorous shark, the rare megamouth shark Megachasma pelagios.