Abstract:
The alien invasive silver carp Hypophthalmichthys molitrix established a self-sustaining feral population in an
oligotrophic impoundment, Flag Boshielo Dam, in South Africa. The ability of this population to persist in a dam with
low algal biomass (median annual suspended chlorophyll a = 0.08 µg l−1), and limited access to rivers considered
large enough for successful spawning, has implications for their invasive potential in other systems. Stomach content
and stable isotope analysis were used to assess the trophic ecology of H. molitrix, which was then compared with
indigenous Mozambique tilapia Oreochromis mossambicus, on a seasonal basis during 2011. Hypophthalmichthys
molitrix are generalist filter feeders, with a diet consisting primarily of sediment, vegetative detritus, dinoflagellates
and diatoms. The dominance of sediments in their stomachs suggests occasional benthic scavenging. However,
H. molitrix occupied a higher trophic level (TL = 2.8) than expected, suggesting that this population subsidised
their diet with an unidentified dietary constituent, characterised by enriched nitrogen values. Although the stomach
contents indicated dietary overlap between H. molitrix and O. mossambicus, stable isotopes revealed fine-scale
resource partitioning, despite both species occupying the same trophic level. Nonetheless, the persistence of this
feral H. molitrix population in an oligotrophic impoundment highlights their phenotypic plasticity.
