Estimating stable isotope turnover rates of epidermal mucus and dorsal muscle for an omnivorous fish using a diet-switch experiment

Stable isotope (SI) analysis studies rely on knowledge of isotopic turnover rates and trophic-step discrimination factors. Epidermal mucus ('mucus') potentially provides an alternative SI 'tissue' to dorsal muscle that can be collected non-invasively and nondestructively. Here, a diet-switch experiment using the omnivorous fish Cyprinus carpio and plant-and fishbased formulated feeds compared SI data between mucus and muscle, including their isotopic discrimination factors and turnover rates (as functions of time T and mass G, at isotopic half-life (50) and equilibrium (95)). Mucus isotope data differed significantly and predictively from muscle data. The fastest delta C-13 turnover rate was for mucus in fish on the plant-based diet (T-50: 17 days, T-95: 74 days; G(50): 1.08(BM), G(95): 1.40(BM)). Muscle turnover rates were longer for the same fish (T-50: 44 days, T-95: 190 days; G(50): 1.13(BM), G(95): 1.68(BM)). Longer half-lives resulted in both tissues from the fish-based diet. delta C-13 discrimination factors varied by diet and tissue (plant-based: 3.11-3.28 parts per thousand; fishmeal: 1.28-2.13 parts per thousand). Mucus SI data did not differ between live and frozen fish. These results suggest that mucus SI half-lives provide comparable data to muscle, and can be used as a non-destructive alternative tissue in fish-based SI studies.