Isotopic tissue turnover and discrimination factors following a laboratory diet switch in Colorado pikeminnow (Ptychocheilus lucius)

Stable isotope ecology has made great strides in quantifying energy transfer through food webs. However, trophic inferences gleaned from field-collected data can be limited when isotopic turnover and isotopic discrimination factors (Δ13C or Δ15N) are unknown. We quantified isotopic turnover and discrimination factors using an isotopic diet switch in the endangered Colorado pikeminnow (Ptychocheilus lucius). The estimated half-life for δ13C was 62 days or a 33% increase in mass and δ15N averaged 133 days or a 52% increase in mass. Growth and metabolic processes both contributed to rates of turnover, but metabolic processes had a stronger effect in δ13C than in δ15N. Lipid-corrected δ13C values resulted in discrimination factors of Δ13C between 0.67 and 0.82 and Δ15N between 2.31 and 2.93, values similar to other fishes. These results suggest sampling fin tissue may be a useful, nonlethal tool for isotopic studies. Fins also demonstrated enrichment in 13C that was not linked to the diet switch, highlighting the importance of controls in isotopic diet switch studies to verify species- and diet-specific estimates of isotopic turnover rates.