Otolith and muscle stable isotope analyses to assess food-web interactions between threatened bigmouth buffalo (Ictiobus cyprinellus) and invasive common carp (Cyprinus carpio)

Long-lived bigmouth buffalo ( Ictiobus cyprinellus) are declining and a species of special concern in central Canada whose densities may be impacted by invasive common carp ( Cyprinus carpio). In addition, the bigmouth buffalo is a culturally important species for the File Hills Qu'Appelle Tribal Council Member First Nations, who have harvested them since time immemorial. Here we analyzed δ13C and δ15 N in bigmouth buffalo and common carp muscle tissue, along with bulk and compound-specific amino-acid δ15 N of otoliths, to examine the food-web interactions of species over the past 100 years. Stable isotope ratios of muscle tissue inferred that the fishes generally occupied different isotopic niches, with bigmouth buffalo exhibiting greater evidence of pelagic feeding (mean ± SD; 88 ± 22% and 42 ± 13% in two lakes), and higher trophic position (3.6 ± 0.3 and 3.8 ± 0.3) than common carp (74 ± 11% and 33 ± 13%; 3.2 ± 0.4 and 3.4 ± 0.3, respectively). Overall, modern otolith isotope values agreed with paired muscle isotope values, while bulk otolith material exhibited increased δ15 N over a century. Apparent isotopic niche differentiation between the species may have arisen from ongoing niche displacement by common carp following watershed invasion ca. 1955. This competition may be more intense for larger bigmouth buffalo.