Evaluation of cooking effects on otolith stable carbon and oxygen isotope values of teleostean fish Pomadasys kaakan (Cuvier, 1830).

Rationale: For years, archaeologists, climatologists, and ecologists have used stable oxygen isotope values (δ C, δ O) in fish otoliths from archaeological sites to reconstruct the habitats, paleo-temperature, and seasonality of the fish captures. Otoliths from archaeological sites might have been heated when ancient people cooked the fish for food. Therefore, there are debates as to whether the cooking behaviors would cause further isotopic fractionations of the carbonate in the otoliths.

Methods: In this study, we have evaluated the effects of the cooking methods on the otolith δ C and δ O values by comparing the otoliths of the javelin grunter (Pomadasys kaakan) from the same individuals, with the left otoliths taken out before the different cooking processes. Otolith sections of the fish were then made and several subsamples were milled along the microstructures visible in the otolith pairs, mostly annual check rings, followed by the stable isotope analyses.

Results: There were no morphological changes between the cooked and uncooked otoliths. The δ C and δ O values were highly consistent for the otolith subsamples between the cooked and uncooked pairs, suggesting none or trivial effects of the cooking processes on the isotopic values of the otoliths. In addition, some javelin grunters showed lower δ C (-5‰ to -6‰) and δ O (-4‰ to -5‰) values deposited in the wide translucent zone of the otoliths, suggesting seasonal migration of the fish to estuaries during the summer.

Conclusions: This study suggests that cooking processes do not change otolith stable isotopic compositions and will therefore allow for future research to use the otolith δ C and δ O values to better understand the life history and used habitats of the preserved fish remains in the middens.