Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish

Georgina M A Busst,J Robert Britton

doi:10.1007/s10750-017-3276-2

Abstract

Ecological applications of stable isotope data require knowledge on the isotopic turnover rate of tissues, usually described as the isotopic half-life in days (T0.5) or the change in mass (G0.5). Ecological studies increasingly analyse tissues collected non-destructively, such as fish fin and scales, but there is limited knowledge on their turnover rates. Determining turnover rates in situ is challenging, with ex situ approaches preferred. Correspondingly, T0.5 and G0.5 of the nitrogen stable isotope (δ15N) were determined for juvenile barbel Barbus barbus (5.5 ± 0.6 g starting weight) using a diet-switch experiment. δ15N data from muscle, fin and scales were taken during a 125 day post diet-switch period. Whilst isotopic equilibrium was not reached in the 125 days, the δ15N values did approach those of the new diet. The fastest turnover rates were in more metabolically active tissues, from muscle (highest) to scales (lowest). Turnover rates were relatively slow; T0.5 was 84 (muscle) to 145 (scale) days; G0.5 was 1.39 × body mass (muscle) to 2.0 × body mass (scales), with this potentially relating to the slow growth of the experimental fish. These turnover estimates across the different tissues emphasise the importance of estimating half-lives for focal taxa at species and tissue levels for ecological studies.

Highlights

Knowledge on the stable isotope turnover rates of tissues of a consumer species is fundamental for the correct interpretation of their isotopic ecology (Boecklen et al, 2011)
Stable isotope turnover rates tend to be expressed as half-lives, i.e. the time for stable isotope values to reach 50% equilibrium with a new diet (Vander Zanden et al, 2015)
All of the B. barbus individuals increased in length and mass over the first 125 day control feeding period, with mean fork lengths and weights increasing from 79.6 ± 2.8 to 89.3 ± 4.6 mm and 5.5 ± 0.6 to 8.1 ± 1.3 g, respectively

Summary

Introduction

Knowledge on the stable isotope turnover rates of tissues of a consumer species is fundamental for the correct interpretation of their isotopic ecology (Boecklen et al, 2011). Rather than relying on data collected in the wild, an alternative approach is the use of experimental diet-switch studies completed in controlled conditions (Heady & Moore, 2013; Xia et al, 2013a, b; Busst & Britton, 2016) In these studies, diet tends to be fixed so that the food items have relatively consistent stable isotope values that should provide more reliable turnover estimates in the tissues (Logan et al, 2006). Diet tends to be fixed so that the food items have relatively consistent stable isotope values that should provide more reliable turnover estimates in the tissues (Logan et al, 2006) These approaches should provide enhanced understandings of the mechanisms involved in isotopic replacement (Buchheister & Latour, 2010; Heady & Moore, 2013). The data generated favour the testing of different models to determine the best-fitting model that provides the best estimate of the turnover rate, and enable the relative contributions of growth and metabolism to turnover to be estimated (Fry & Arnold, 1982; Hobson & Clark, 1992; Hesslein et al, 1993)

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