From the pool to the sea: Applicable isotope turnover rates and diet to skin discrimination factors for bottlenose dolphins (Tursiops truncatus)
Introduction
Marine top predators inhabit one of the most inaccessible and hardest environments to perform ecological research, thus information on their trophic ecology is scarce and lacks in reliability. Predation events are rarely observed and stomach contents are only available for stranded or bycaught animals (e.g. Santos et al., 2001), thus likely providing unrealistic or biased information on feeding preferences. Within this scenario, stable isotope analysis has emerged as a suitable alternative to conventional approaches to reconstruct individuals and populations' diet through mass-balance mixing models (e.g. Fernández et al., 2011, Mèndez-Fernandez et al., 2012, Kiszka et al., 2014).
The reliability of dietary assessments thought isotopic approach relies on the use of accurate diet to tissue discrimination factors and turnover rates (Phillips et al., 2014). Indeed, the use of inappropriate isotopic discrimination factors has been recognized as one of the biggest sources of uncertainty in using mixing models to assess diet (Gannes et al., 1997, Phillips et al., 2014, Wolf et al., 2009). Traditionally, it was assumed that isotopic discrimination factors linking diet and consumer tissues were ca. + 1‰ for carbon-13 (DeNiro and Epstein, 1978) and + 3‰ for nitrogen-15 (DeNiro and Epstein, 1981). However, recent research showed considerable variation in isotopic discrimination factors as a function of various extrinsic (e.g. diet quality or composition (McCutchan et al., 2003, Robbins et al., 2005) and intrinsic factors (e.g. taxa, (Caut et al., 2009, Vanderklift and Ponsard, 2003); or age (Hobson and Quirk, 2014, Minagawa and Wada, 1984)). Taxon and tissue-specific isotopic discrimination factors obtained under experimental trials or studies of wild populations where their diets are well-known are likely to produce the most accurate dietary estimates (Bond and Diamond, 2011, Caut et al., 2008, Gannes et al., 1997, McCutchan et al., 2003, Pilot et al., 2012, Post, 2002, Spence and Rosenheim, 2005).
On the other hand, an accurate knowledge on isotopic turnover rates is mandatory to determine the time window in which researchers can perceive the course of dietary changes (Dalerum and Angerbjörn, 2005). As for isotopic discrimination, turnover rates may vary according to taxa (Boecklen et al., 2011) or tissue type (MacNeil et al., 2006). Therefore, accurate and reliable estimates of turnover rates for specific tissues and different taxa are needed to depict the time frame integrated in diet studies (Dalerum and Angerbjörn, 2005, Martínez del Rio et al., 2009).
To date, few studies have assessed isotopic diet to tissue discrimination factors and turnover rates in dolphins under controlled conditions (Browning et al., 2014, Caut et al., 2011). Previous experimental studies focused on blood (Caut et al., 2011), a difficult sample to obtain in the wild, or on a more appropriate tissue, the skin (Browning et al., 2014), but assessed in short experimental trials without reaching the isotopic equilibrium. Although valuable information was provided by these authors, their isotopic discrimination factors and turnover rates should be taken with caution when reconstructing animals' diet through mass-balance mixing models (see Bates and Watts, 1988, Berges et al., 1994; and Martínez del Rio et al., 2009). Accordingly, the call for comparative laboratory experiments repeatedly recommended by several authors (e.g. Gannes et al., 1997, Wolf et al., 2009) is still needed for dolphins in order to properly interpret isotopic data of wild populations, as well as to make inferences on their diets through mass-balance mixing models. Bottlenose dolphins (Montagu, 1821) are a good model to study these parameters because they are among the most widespread of the small cetaceans, occurring in nearly all tropical and temperate seas (Leatherwood and Reeves, 1983) and they adapt well to captivity, being the most common specie in dolphinariums.
In this study, a controlled feeding experiment was conducted during 350 days monitoring carbon and nitrogen stable isotope ratios in bottlenose dolphins, Tursiops truncatus skin and their diet. This extensive dataset is the longest available so far for cetaceans. Therefore, applicable diet to skin discrimination factors and turnover rates are provided to be used in future diet studies of this species and probably in taxonomically close species such as other small cetaceans in the wild.
Section snippets
Experimental design
Six bottlenose dolphins, one male (Paco) and five females (Clara, Pacina, Sanibel, Ruffles, and Luna) were maintained under a controlled diet during 350 days in “Loro Parque” facilities (Tenerife, Spain). The experimental trial was twofold: (1) dolphins were first fed with diet A [i.e. 90% sprat (Sprattus sprattus) + 10% herring (Clupea harengus)] during 172 days to be confident that the skin of all individuals reached the isotopic equilibrium with their diet, thus establishing a known and stable
Results
Dolphin skin differed significantly between treatments (i.e. bulk vs delipidated samples) in both δ13C (t = − 8.98, df = 94, p-value < 0.01) and δ15N (t = − 7.34, df = 98, p-value < 0.01). Similarly, in fish samples a significant effect of sample treatment and tissue analyzed was detected (see Table 2). Experimental diets differed in both δ13C (mean ± sd for isotopic differences between diets: 2.32 ± 0.23‰) and δ15N values (1.45 ± 0.08‰) (Fig. 1, Table 3), thus allowing appropriate estimates for isotopic
Discussion
Accurate estimations of isotopic discrimination factors and turnover rates are mandatory to properly reconstruct the diet of free ranging individuals and/or populations through stable isotope approaches (reviewed in Phillips et al., 2014). However, reliable estimations are methodologically challenging because individuals must be held on an isotopically fixed diet in captivity for long periods of time (Bond and Diamond, 2011). To date, few studies have provided reliable estimates of these
Competing interests
The authors declare no competing financial interests.
Author contributions
This work represents a component of the doctoral dissertation of J. Giménez. The following tasks were performed: Conceived and designed the experiments: J.G, F.R., J.A., M.G.F., and R.dS.; performed the experiments: J.A.; analyzed the data: J.G., F.R.; contributed reagents/materials/analysis tools: J.G., F.R., M.G.F., J.A. and R.dS.; wrote the paper: J.G.
Funding
This work was supported by the Fundación Loro Parque, CEPSA (Compañía Española de Petroleos S.A.), and Spanish Ministry of Economy and Competitiveness [CGL2011-25543, EcoCet Project; SEV-2012-0262, Severo Ochoa Programme for Centres of Excellence in R + D + I to J.G and R. dS and Juan de la Cierva Postdoctoral program to R. dS].
Acknowledgments
We want to thank the veterinary (Nuhacet Fernández, Cristina Dreisorner and Andrea Cádiz) and the keeper team (Víctor Pacheco, Marco Tulio, Javier Luis López, Francisco Brito, Miguel García, Raquel Albasanz, Félix Afonso, Cristina Ubero and Miguel Luis Sosa) for the valuable help during the dietary treatments and sample collection. We also thank Lluís Jover (Universtitat de Barcelona), Manuela González-Suárez (Estación Biológica de Doñana — Consejo Superior de Investigaciones Científicas) and
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