General validation of formalin‐preserved fish samples in food web studies using stable isotopes

Abstract

Summary Stable isotope analyses of carbon and nitrogen are widely used to study food web structure in ecosystems. However, isotopic signatures are affected by the often‐needed chemical preservation of tissues in the field, which impedes or weakens the interpretation of results. The scarcely available correction factors for preserved fish samples are species specific and have so far not been validated for general use. Moreover, no studies have evaluated the effect of preservation on the estimation of metrics typically used in food web studies. We aimed to develop a general correction model suitable for a vast number of fish species and to test whether formalin‐preserved fish muscle δ13C and δ15N values can be used in food web metrics estimations. For this purpose, we used paired formalin‐preserved and fresh muscle samples of 116 fish individuals belonging to 17 species covering a wide range of fish trophic characteristics, from nine tropical streams. Formalin decreased δ13C values by 0·6–1·4‰ (0·94‰ on average) and increased δ15N values by 0·3–0·5‰ (0·33‰ on average). Preservation effect was ecologically significant for δ13C values (as it surpassed natural trophic fractionation) and less important for δ15N values (being minor than natural trophic fractionation). However, preservation did not affect estimation of community‐wide food web metrics. The deviations in preserved samples varied among species and increased with increasing fresh sample C : N ratios and changes in carbon proportions after preservation and with increasing isotopic values (for δ13C). Despite these major deviations in δ13C values, we developed and validated a powerful general linear model to predict fresh fish muscle isotopic signatures from preserved samples. Formalin‐preserved samples of fish can be used in food web studies, simplifying sampling logistics and allowing the use of museum and scientific collection specimens for historical food web reconstruction. Food web metrics based on δ13C vs. δ15N biplots can be directly estimated from preserved samples as the preservation effect seems overall consistent. When more detailed information is needed, or fresh and preserved samples are being simultaneously studied, formalin‐preserved isotopic signatures can be corrected to fresh values using the model developed in this study.