Vol. 48 No. 1, 2009

Otolith O and C Stable Isotope Compositions of Southern Bluefin Tuna Thunnus maccoyii (Pisces: Scombridae) as Possible Environmental and Physiological Indicators

Jen-Chieh Shiao^1,*, Tzen-Fu Yui², Hans Høie^3,4, Ulyssess Ninnemann⁵, and Shui-Kai Chang⁶

¹Institute of Oceanography, College of Science, National Taiwan University, Taipei 106, Taiwan
²Institute of Earth Sciences, Academia Sinica, Nankang, Taipei 115, Taiwan
³Department of Biology, University of Bergen, Bergen P.O. Box 7800, Norway
⁴Institute of Marine Research, Bergen P.O. Box 7800, Norway
⁵Department of Earth Science, University of Bergen, Bergen P.O. Box 7800, Norway
⁶Institute of Marine Affairs, College of Marine Sciences, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

Jen-Chieh Shiao, Tzen-Fu Yui, Hans Høie, Ulyssess Ninnemann, and Shui-Kai Chang (2009) This study evaluated the use of stable isotopic signatures stored in otoliths to study the migratory life history of the southern bluefin tuna (SBT, Thunnus maccoyii).  Otolith powders were sequentially collected from the edge to the core for stable oxygen (O) and carbon (C) isotopic analysis.  All specimens (n = 11) analyzed showed a 3-stage profile of stable O and C isotope compositions.  Values of δ¹⁸O decreased from approximately -1.5‰ to -3.0‰ in early life, followed by an increase to approximately -1‰ at an age of 1 yr, and the values remained at the δ¹⁸O-enriched level with moderate fluctuations for the remaining lifetime.  Hatchling SBT appeared to stay in the spawning ground for approximately 1 mo and then migrate southward, experiencing a temperature decrease of 8-10°C to about 20-25°C until 1 yr old.  Water temperatures varied between approximately 15 and 25°C with no evident trend over the remaining life.  However, the ambient water temperature might have been overestimated during the subadult to adult stages of the SBT when their thermal conservation ability was fully developed.  The otolith δ¹³C profile followed broadly similar trends to those of δ¹⁸O, showing depletion from approximately -6.5‰ to -10.5‰ in the early life stages, followed by abrupt enrichment up to -6‰ to -8‰ at an approximately age of 1 yr.  Otolith δ¹³C reached a plateau earlier in the 1st year than did δ¹⁸O and showed greater variation over the remaining life.  Interpretation of otolith δ¹³C values is complicated, but the results suggest progressive transitions of ontogeny, metabolism, and trophic levels for the SBT.  Otolith stable O and C isotope analyses provide continuous information about the migratory life history and contribute to our presently limited understanding of the SBT migratory cycle.

Key words: Thunnus maccoyii, Southern bluefin tuna, Otolith, Stable isotope, Migratory life history.

*Correspondence: Tel: 886-2-33663227.  Fax: 886-2-33663744.   E-mail:jcshiao@ntu.edu.tw