Physicochemical mechanisms of FT-NIRS age prediction in fish otoliths

Context. Fourier transform near-infrared spectroscopy (FT-NIRS) is of interest to fisheries managers for rapid age prediction in fish otoliths, yet the underlying prediction mechanism is unknown. Aims. To better understand drivers of FT-NIRS age prediction, we evaluated FT-NIRS spectra and age prediction models for otoliths of red snapper, Lutjanus campechanus, related to otolith structure, mass, and constituents (calcium carbonate (CaCO3) and protein). Methods. Spectra were collected from a set of whole otoliths (n = 84, 0-28 years) and again sequentially after grinding to powder and subsampling a fixed mass of each ground otolith. Protein content was also measured (n = 26) and related to spectra. Key results. Age prediction was diminished in ground and fixed-mass otolith models, but remained within 2 years of traditional ages. Protein content (0.43-0.92% weight) increased significantly with age, implying a concomitant decrease in CaCO3 content. FT-NIRS models predicted protein content to within 0.04%, but protein variability hindered modelling. Spectral characteristics of both CaCO3 and protein are evident in otolith spectra and are implicated in age-prediction models. Conclusions. Changes in otolith composition, mass, and structure underlie FT-NIRS age prediction, but compositional changes inform the majority of age prediction. Implications. These results provide a foundation for understanding FT-NIRS age prediction.