Stable isotope discrimination factors of omnivorous fishes: influence of tissue type, temperature, diet composition and formulated feeds

J. Robert Britton,Georgina M. A. Busst

doi:10.1007/s10750-017-3423-9

J. Robert Britton, Georgina M. A. Busst

Open Access

https://doi.org/10.1007/s10750-017-3423-9

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Abstract

Application of stable isotope data to trophic studies requires understanding of factors influencing the isotopic discrimination factor (Δ) between consumers and their prey resources. This is missing for many omnivorous species, despite their diet and environment potentially impacting Δ. The effects of temperature, diet (including formulated feeds) and tissue type on Δ13C and Δ15N were thus tested experimentally. A temperature experiment exposed three species to identical diets at 18 and 23°C, whereas a diet experiment exposed one species to four diets at 18°C. At 23°C, C:N ratios, Δ13C and Δ15N were generally elevated versus 18°C. After lipid correction, tissue/species-specific differences at 23°C in Δ13C and Δ15N were up to 0.73 and 0.54‰ higher, respectively. Across the four diets, there were also significant differences in Δ13C and Δ15N between a natural diet and diets based on formulated feeds. Δ13C and Δ15N of muscle were 1.51 to 2.76‰ and 3.13 to 5.44‰, respectively. Highest Δ for both isotopes was from a formulated feed based on plant material that resulted in lower dietary protein content and quality. Thus, diet and environment fundamentally affected the isotopic discrimination factors and these factors require consideration within trophic studies based on stable isotopes.

Highlights

The application of stable isotope data to ecology has substantially enhanced understandings of trophic relationships within and between species, including how invasive species impact native communities (e.g. Jackson et al, 2016; Copp et al, 2017) and trophic subsidies influence consumer diets (e.g. Basicet al., 2015; Gutmann-Roberts et al, 2017)
There can be high uncertainty in the discrimination factors of many species (Moore & Semmens, 2008; Elsdon et al, 2010; Busst & Britton, 2016). This uncertainty and variability in discrimination factors between species and tissues suggest that commonly cited values (e.g. D 15N: 3.4 ± 0.98%; D13C: 0.39 ± 1.3%; Minagawa & Wada, 1984; Post, 2002) might not always be appropriate to apply to mixing models
General equations from Thomas and Crowther (2015) suggested that the experimental fishes would have half-lives for d13C of 30 days and for d15N of 32 days, and the experimental period of 150 days meant that isotopic equilibrium should have been reached at its conclusion

Summary

Introduction

The application of stable isotope data to ecology has substantially enhanced understandings of trophic relationships within and between species, including how invasive species impact native communities (e.g. Jackson et al, 2016; Copp et al, 2017) and trophic subsidies influence consumer diets (e.g. Basicet al., 2015; Gutmann-Roberts et al, 2017). Jackson et al, 2016; Copp et al, 2017) and trophic subsidies influence consumer diets (e.g. Basicet al., 2015; Gutmann-Roberts et al, 2017). Their application has been enhanced by the development of ‘mixing models’ that estimate the diet composition of consumers from the stable isotope ratios of their prey (Phillips et al, 2001, 2014). There can be high uncertainty in the discrimination factors of many species (Moore & Semmens, 2008; Elsdon et al, 2010; Busst & Britton, 2016) This uncertainty and variability in discrimination factors between species and tissues suggest that commonly cited values (e.g. D 15N: 3.4 ± 0.98%; D13C: 0.39 ± 1.3%; Minagawa & Wada, 1984; Post, 2002) might not always be appropriate to apply to mixing models

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