Life-history studies by non-lethal sampling: using microchemical constituents of fin rays as chronological recorders

Chemical properties of fin rays were investigated in nine fish species to test whether life-history characteristics can be analysed using a non-lethal and minimally invasive methodology. Fish specimens from public aquariums were acquired after fishes died in captivity. Analyses concentrated on exploring the differences between the wild and captive life periods of each fish, which were known from aquarium records. Differences between the two life periods were observed in both the trace-element and stable-isotope compositions of the chemical matrix of the fin ray. Trace-element concentrations in fin rays were compared with those in otoliths using measures of resolved variance and cross-correlation to test the assumption of conserved matrices in the fin ray. Divalent ions and positively charged transition metals (i.e. Fe and Co) had strong associations between the two structures, suggesting conservation of material. Stable-isotope values of δ¹³ C and δ¹⁵ N differed between the wild and captive life periods in most of the fishes, also suggesting conserved matrices. δ¹³ C and δ¹⁵ N were derived from the organic matrix within the fin ray, which may present a stable-isotope chronology. Future studies can use these chronologies to study diet and movement trends on a temporal scale consistent with the entire lifetime of an individual.