Bomb 14C validates Gray Triggerfish (Balistes capriscus) dorsal spine and otolith ageing protocols

Abstract

Gray triggerfish (Balistes capriscus) historically have been aged by counting translucent zones in thin sections prepared from their first dorsal spine because their small, fragile sagittal otoliths are difficult to extract and process for ageing. However, recent research suggests dorsal spine translucent zone counts produce biased age estimates, thus the historical dorsal spine-based ageing protocol results in a systematic underestimation of true age. Here, we employed the bomb radiocarbon chronometer to test the accuracy of age estimates (n = 3 readers) derived from opaque zone counts in whole otoliths, as well as dorsal spine section translucent zone counts produced with the historical ageing protocol and a new method that requires higher magnification to count translucent zones on the margin of dorsal spine sections. Results indicate historical dorsal spine-derived age estimates underestimate age, with the extent of bias increasing with age. There was no evidence of ageing bias for both whole-otolith opaque zone counts and new protocol dorsal spine translucent zone counts. New dorsal spine protocol ageing was slightly more precise among readers (iAPE = 9.4 %) than otolith ageing (iAPE = 10.1 %) and read times were 2–3x faster for dorsal spine sections than whole otoliths. Validation of the new dorsal spine ageing protocol is a critical step in effective production ageing of gray triggerfish. Archived dorsal spine sections can be re-aged with the new protocol to update historical age composition data, and future ageing will not have to rely on the logistically challenging extraction and processing of otoliths.