Substituting Otoliths for Chemical Analyses: Does Sagitta = Lapillus?

Chemical analysis of fish otoliths has become an important technique in fisheries science with widespread applications. Most research up to this point has focused predominantly on sagittal otoliths, but the underlying assumptions may also apply to lapilli. The goal of this study was to determine whether lapilli and sagittae have the same otolith chemistry and whether one can be substituted for the other for solution-based chemical analysis in wild-captured fish. We compared the stable isotope chemistry (δ13C and δ18O) of paired sagittae and lapilli of juvenile spotted seatrout Cynoscion nebulosus collected from Chesapeake Bay seagrass beds in 2002. Stable isotopic signatures were significantly different in both carbon and oxygen for paired sagittae and lapilli. Both δ13C and δ18O values were higher in the sagitta relative to the lapillus. As a correlate to isotopic analysis, shape measurements were taken on paired sagittae and lapilli for growth comparison. While the 2 otoliths are a similar size at formation, the sagitta grows faster, quickly outgrowing the lapillus during the juvenile stage. We expected that after detrending isotope data to account for fish length, the relation with otolith growth would become clear. Surprisingly, this was not true for either δ13C or δ18O. Carbon isotopic differences appear to be strongly influenced by metabolism, and the relationship of δ18O with otolith growth is obscured by a variable environment. Our results clearly show that sagittae is not equal to lapilli for solution-based whole otolith analysis. Moreover, while the sagitta reflects the juvenile stage environment with greater fidelity, the lapillus reflects the larval stage more strongly.