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

Hydrobiologia, Nov 2017

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.

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

Hydrobiologia February 2018, Volume 808, Issue 1, pp 219–234 | Cite as Stable isotope discrimination factors of omnivorous fishes: influence of tissue type, temperature, diet composition and formulated feeds AuthorsAuthors and affiliations J. Robert BrittonGeorgina M. A. Busst Open Access Primary Research Paper First Online: 11 November 2017 8 Shares 889 Downloads 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. KeywordsCyprinidae Stable isotope fractionation Stable isotope analysis Mixing models  Handling editor: Michael Power Electronic supplementary material The online version of this article ( https://doi.org/10.1007/s10750-017-3423-9) contains supplementary material, which is available to authorized users. 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. Bašić et 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). For models to provide reliable dietary predictions requires accurate estimates of the stepwise isotopic change between the consumer and their prey (Boecklen et al., 2011), i.e. the isotopic ‘discrimination’ factor (Δ; Martínez del Rio & Wolf 2005). The issues that affect discrimination factors within and between consumer species include the age and body size of sampled individuals, their diet composition, the tissues analysed and the sample preparations completed prior to analysis (Brush et al., 2012; Locke et al., 2013; Busst et al., 2015). 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 thus suggest that commonly cited values (e.g. Δ 15N: 3.4 ± 0.98‰; Δ13C: 0.39 ± 1.3‰; Minagawa & Wada, 1984; Post, 2002) might not always be appropriate to apply to mixing models. Among consumer species, specific food items within their overall diet can substantially influence their discrimination factors (Caut et al., 2008; Greer et al., 2015). Invertebrate-based diets tend to have lower discrimination factors than diets with higher protein contents (McCutchan et al., 2003). In herbivorous fishes, Δ15N in muscle has been recorded to be 5.25‰ (Mill et al., 2007; Carassou et al., 2008). In the detrivore Prochilodus lineatus (Valenciennes, 1837), Δ13C varied between −1.9 and 3.6‰ and was between 3.3 and 5.7‰ for Δ15N (Sacramento et al., 2016). For species that are highly omnivorous, such as fishes of the Cyprinidae family, discrimination factors of 13C and 15N can be relatively low when fish have diets that are relatively high in fish protein (e.g. 2.0‰ for both isotopes) but significantly higher when high in plant materials that are relatively low in protein (up to 5.6‰ for Δ13C and 6.9‰ for Δ15N) (Busst & Britton, 2016). These data allow testing of the ‘protein quality hypothesis’, which suggests that discrimination factors will increase as protein quality decreases, and the ‘protein quantity hypothesis’, which suggests that discrimination factors will increase as C:N ratios decrease (i.e. as dietary nitrogen concentration decreases) (Pearson et al., 2003). Moreover, for omnivorous fishes that have mixed diets comprising animal and plant items, the varying proportions of these items could have potentially stro (...truncated)


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J. Robert Britton, Georgina M. A. Busst. Stable isotope discrimination factors of omnivorous fishes: influence of tissue type, temperature, diet composition and formulated feeds, Hydrobiologia, 2017, pp. 219-234, Volume 808, Issue 1, DOI: 10.1007/s10750-017-3423-9