Use of Stable Isotopes and Mercury to Assess Trophic Positions of Black Carp and Other Large Fishes in the Red-Atchafalaya River System, Louisiana, USA
Leo G. Nico, Amanda Demopoulos, Daniel Gualtieri, and Carla Wieser
Abstract.—The black carp Mylopharyngodon piceus is a large (>1 m long) riverine fish from eastern Asia introduced into the United States via the aquaculture industry. A wild population has been present in the lower Mississippi River basin since the early 1990s, but little is known about the ecological effect of black carp in invaded environments. In its native range, black carp feed almost exclusively on mollusks. In U.S. waters, they likely prey on native mussels, but few wild-caught specimens have been examined by biologists and all have had empty gastrointestinal tracts. In lieu of stomach content data, we examined isotopic values (δ13C and δ 15N) and mercury (Hg) concentrations in muscle tissue of black carp and 10 other large nonnative and native fish species captured in the Red–Atchafalaya River system of Louisiana, USA. Trophic position estimates derived from δ 15N values ranged from 2.0 for grass carp Ctenopharyngodon idella to 4.8 for blue catfish Ictalurus furcatus and flathead catfish Pylodictis olivaris. Adult black carp had a δ 15N value (13.2‰), indicating a trophic level of 3.5. Mean total Hg concentrations ranged from 0.02 µg/g in grass carp to 0.27 µg/g in bigmouth buffalo Ictiobus cyprinellus, in black carp 0.17 µg/g; Hg increased with increasing δ 15N, indicating biomagnification. The limited numbers of taxa and small samples sizes, as well as constraints in methods used, do not allow confirmation that wild black carp are consuming native mollusks. However, our stable isotope results do provide evidence that its diet is similar to other large fish species inhabiting the Red–Atchafalaya system considered to be benthic invertivores, including some known to prey on freshwater mollusks (i.e., smallmouth buffalo I. bubalus and nonnative common carp Cyprinus carpio).