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

摘要

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 delta ~(18)O decreased from approximately -1.5%o to -3.0%o in early life, followed by an increase to approximately -1%o at an age of 1 yr, and the values remained at thedelta ~(18)O-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 otolithdelta ~(13)C profile followed broadly similar trends to those of delta ~(18)O, showing depletion from approximately -6.5%o to -10.5%in the early life stages, followed by abrupt enrichment up to -6% to -8%o at an approximately age of 1 yr. Otolith delta ~(13)C reached a plateau earlier in the 1st year than diddelta ~(18)O and showed greater variation over the remaining life. Interpretation of otolith delta ~(13)C 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 lifehistory and contribute to our presently limited understanding of the SBT migratory cycle. http://zoolstud.sinica.edu.tw/Journals/48.1/71 .pdf