2.2 | Stable isotope analysis
We analyzed fish and invertebrate samples at the Stable Isotope Mass Spectrometry Laboratory at Kansas State University, Manhattan, KS, USA, and the Center for Stable Isotopes at the University of New Mexico, Albuquerque, NM, USA. Isotopic values were standardized between laboratories using tetramin® fish food for isotopic calibration and we reported results using the common delta notation (δ ‰) calculated by:
δ ‰ = [(Rsample/Rstandard) – 1)] × 103.
We pre-processed data prior to analyses by removing obvious outliers (i.e., δ13C or δ15N values more than two standard deviations from taxon-specific means). Due to differences in Colorado Pikeminnow and Channel Catfish tissue types sampled between time periods (i.e., muscle tissue from historical samples and fin tissue from contemporary samples), we used mean fin-to-muscle conversions. For Colorado Pikeminnow (δ13C = -1.29 and δ15N = +0.56), we used the conversion from Franssenet al . (2016) and for Channel Catfish (δ13C = -1.08 and δ15N = +0.58), we used Maitland (2020). Additionally, we corrected museum specimens for the effects of formalin preservation which depletes 13C by 1.0 ‰ and enriches15N by 0.53 ‰ (Edwards et al ., 2002). Finally, variable lipid concentration can introduce bias into isotopic analyses because they are more depleted in 13C than proteins and carbohydrates (Post et al ., 2007). Therefore, we corrected δ13C values by the ratio of C:N in samples following Post et al . (2007) using the formula:
δ13Ccorrected = δ13Craw - 3.32 + 0.99 × C:N