Elucidating the mechanism underlying the productivity-recruitment hypothesis in the invasive common carp

Across-ecoregion analyses showed that the recruitment of common carp, Cyprinus carpio (Linnaeus 1758), a globally invasive fish, is strongly influenced by lake productivity: while recruitment was frequent in hypereutrophic lakes, it was invariably absent in oligotrophic lakes. This led to a hypothesis that common carp larvae might have faster growth rates in productive lakes that allow them to outgrow native predators, whereas larvae might encounter nutritional bottlenecks in oligotrophic lakes. We shed some light on this hypothesis by documenting how zooplankton communities found in oligo-, meso-, and eutrophic lakes in Minnesota, USA affected larval carp survival, growth and diet composition. We cultured larval carp in tanks fed zooplankton at naturally occurring densities from three lakes of varying trophic states for 20 days during two consecutive springs. The growth rates were significantly higher (up to 5 times) among larvae fed zooplankton from the eutrophic lake and lowest in larvae fed zooplankton from the oligotrophic lake. Despite their small size (~6 mm), carp larvae selected large zooplankton (0.3–0.6 mm), primarily Bosmina spp., even on the first day of exogenous feeding. This pattern was consistent across all treatments. Rotifers were generally not found in the stomachs of larval carp, despite their high abundance, even if other food items were scarce. The densities of cladocera were highest in the productive lake, especially during one of the two years when larval carp showed very rapid growth rates. Our study shows that larval carp have well defined dietary preferences and that common carp recruitment might be especially likely to occur in productive systems with abundant cladocera populations in which carp larvae are expected to be more likely to escape gape-limited, native predators.