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

Abstract

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 otolith. 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 13C 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 σ 13C distribution in otoliths show an increasing 13C enrichment over the part of the life of the fish when respiration rate can be expected to increase, and a decline in 13C when respiration rate can be expected to be decreasing or stationary. Basic models of 13C fractionation in biological systems predict that 13C enrichment of metabolic CO 2 should occur at higher respiration rates. The similarity between observed and expected 13C distributions in otoliths suggests that the enrichment pattern of 13C in otoliths is an analog of the metabolic rate of fishes, which provides a useful life history record.