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 d13C and d15N in bigmouth buffalo
and common carp muscle tissue, along with bulk and compound-specific
amino-acid d15N 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 d15N 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.