Nitrogen stable isotope variability in tissues of juvenile tilapia Oreochromis aureus: empirical and modelling results

Abstract

Studies on diet or trophic interactions of organisms based on stable isotopes require accurate estimates of how quickly stable isotope ratios change in the investigated tissues. However, rates of isotope turnover in fish tissues, especially in omnivorous species, are poorly understood. We conducted a diet-shift study using juvenile tilapia to (i) empirically estimate the isotopic turnover rates of nitrogen in the dorsal muscle, liver, fin and backbone; (ii) model the relative contributions of metabolism and growth to the total isotopic turnover in each tissue; and (iii) develop a non-lethal approach for estimating body nitrogen stable isotope ratios for threatened or endangered species. Isotopic analyses were performed using a Flash EA CN elemental analyser coupled to a ThermoFinnigan Delta Plus mass spectrometer. Nitrogen isotopic turnover rates were consistently ranked in the order backbone > liver > muscle > fin due to the relatively lower metabolic rates of muscle and fin tissue. Backbone tissue turned over significantly faster than other tissues, suggesting the potential for a multiple-tissue stable isotope approach to the study of movement and trophic position over different time scales for omnivorous fish. However, fin tissue had the longest half-life, at 57.81days, indicating that this tissue is more useful than muscle as a long-term dietary indicator. The change in nitrogen isotope ratios in dorsal muscle was mainly regulated by somatic growth, but metabolic activity markedly stimulated the turnover rate of backbone. This study is one of a few to demonstrate significant variation in the δ(15) N turnover rates among multiple tissues of a single organism, especially for omnivorous fish. Our results, to some extent, also indirectly contribute to the conservation of threatened or endangered species. Copyright © 2016 John Wiley & Sons, Ltd.