Microchemical analyses of fish otoliths have revolutionized fisheries science. Molecules deposited within otoliths may originate from ambient water and diet, with molecular concentrations being subject to subsequent physiological alteration after exposure. Analyses of otolith microstructure and incorporation of inorganic elements have led to major advances in stock assessment and fisheries ecology. However, the use of otoliths for microchemical analyses has drawbacks. Specifically, otolith removal from live specimens requires specimen sacrifice, which may be forbidden in the case of protected species. In addition, otoliths rarely contain sufficient concentrations of organic matter to allow reconstruction of food-web relationships via multiple stable isotopes, and otolith microstructure can be difficult to interpret in some species. Here, we review alternatives to otoliths that can provide microchemical analytes for life-history studies in fishes. Our focus is to describe advantages and disadvantages to the use of each alternative structure, with particular attention paid to trace-element analysis for inorganic matrices and stable-isotope analysis for organic ones. In general, the chronological analysis of elemental and isotopic values within each structure depends on the inert nature (or lack of molecular turnover) of the tissue. Structures with high turnover rates or those that are metabolically active will not effectively record elemental or isotopic compositions over time. Here, we provide an assessment of the use of bony endoskeleton, fin spines, fin rays, scales, and eye lenses as alternatives or complements to fish otolith analysis.
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 δ C and δ N differed between the wild and captive life periods in most of the fishes, also suggesting conserved matrices. δ C and δ 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.
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