Biological factors controlling the carbon isotope record in fish otoliths: Principles and evidence

Otoliths are calcium carbonate structures in the endolymph of the inner ear of fish used to estimate age of individual fish based on the annual and the daily periodicities of the internal banding structure of the otolich. The otolith is about 10% by weight carbon and changes in carbon isotope disequilibrium in the endolymph over the life of the fish are recorded in the pattern of C-13 deposition in the otolith. The record of carbon isotope disequilibrium in the otolith can provide useful information about two aspects of the biology of the fish: position in the food chain, and the changes in metabolic rate over the life of the fish. Measurements of delta(13)C distribution in otoliths show an increasing C-13 enrichment over the part of the life of the fish when respiration rate can be expected to increase, and a decline in C-13 when respiration rate can be expected to be decreasing or stationary. Basic models of C-13 fractionation in biological systems predict that C-13 enrichment of metabolic CO2 should occur at higher respiration rates. The similarity between observed and expected C-13 distributions in otoliths suggests that the enrichment pattern of C-13 in otoliths is an analog of the metabolic rate of fishes, which provides a useful life history record.