Abstract

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 non-destructively. Here, a diet-switch experiment using the omnivorous fish Cyprinus carpio and plant- and fish-based 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 δ13C 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. δ13C discrimination factors varied by diet and tissue (plant-based: 3.11–3.28‰; fishmeal: 1.28–2.13‰). 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.

Highlights

  • Stable isotope turnover rates represent the change in mass and/or time required for consumer tissues to reflect their new diet, allowing the calculation of an isotopic half-life for the tissue and isotope of interest (Boecklen et al, 2011)

  • The results revealed that epidermal mucus collected from live fish provides a non-invasive and nondestructive tissue for use in stable isotope analyses that can replace the use of dorsal muscle

  • The results revealed that there can be differences in the turnover rates of mucus versus muscle, this was dependent on the diet, and this would need consideration when mucus is used within stable isotope field studies

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Summary

Introduction

Stable isotope turnover rates represent the change in mass and/or time required for consumer tissues to reflect their new diet, allowing the calculation of an isotopic half-life for the tissue and isotope of interest (Boecklen et al, 2011). Along with diet-tissue discrimination factors, turnover rates provide the basis of food web studies based on the isotopic composition of animal tissues (Vander Zanden et al, 2015). Samples taken from fish for stable isotope analysis (SIA) are of white dorsal muscle (‘muscle’). This is because it is considered the tissue that best represents fish diet isotopically (Busst et al, 2015), and provides a relatively long temporal integration of their dietary resources (Vander Zanden et al, 2015). Muscle samples are usually collected from euthanised fishes This is potentially problematic where destructive sampling is either not desirable (e.g. in mark-recapture studies) or permitted (e.g. threatened or endangered species) (Vasek et al, 2017). The noninvasive and/or non-destructive collection of alternative tissues, such as fin and scales, is used (Busst et al, 2015; Busst & Britton, 2016)

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