Estimating tissue‐specific discrimination factors and turnover rates of stable isotopes of nitrogen and carbon in the smallnose fanskate Sympterygia bonapartii (Rajidae) |
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| Authors: | D. E. Galván J. Jañez A. J. Irigoyen |
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| Affiliation: | 1. Centro para el Estudio de Sistemas Marinos–Consejo Nacional de Investigaciones Científicas y Técnicas (CESIMAR–CONICET), Boulevard Brown 2915, (U9120ACV), Puerto Madryn, Chubut, Argentina;2. Aquarium Fundación Temaiken, Buenos Aires, Argentina |
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| Abstract: | This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskate Sympterygia bonapartii by feeding six adult individuals, maintained in captivity, with a constant diet for 365 days. TDFs were estimated as the difference between δ13C or δ15N values of the food and the tissues of S. bonapartii after they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: C t95%blood = 150 days, N t95%blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ13C and Δ15N values in blood and muscle using all individuals were Δ13Cblood = 1·7‰, Δ13Cmuscle = 1·3‰, Δ15Nblood = 2·5‰ and Δ15Nmuscle = 1·5‰, but there was evidence of differences of c.0·4‰ in the Δ13C values between sexes. The present values for TDFs and turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed that S. bonapartii has relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ13C values in S. bonapartii blood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ13C = 1·5‰). For Δ15N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ15N c. 3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ15N values could lead to underestimates of actual trophic positions. Finally, the evidence of differences in TDFs between sexes reveals a need for more targeted research. |
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| Keywords: | δ 13C δ 15N elasmobranch fractionation mixing models trophic position |
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