Why certain species of fish become invasive is poorly understood and a key obstacle to restoring many of the world's ecosystems. In this study we tested whether variation in biotic resistance exerted by native predators might explain the reproductive success of the common carp, a large and fecund invasive species that typically spawns in outlying and unstable shallow habitat. An initial three-year study of the relative abundance of youngof-year (YOY) carp in interconnected lakes in the Upper Mississippi River Basin discovered that YOY carp are only found in shallow waters that experience winter hypoxia (winterkill) and have low densities of the native egg-predators that otherwise dominate these locales. A follow-up experiment tested if native fish predation on carp eggs could explain this distribution. It found that while carp eggs survived in winterkill lakes, they only survived in non-winterkill lakes when protected by a mesh that excluded fish. Large numbers of carp eggs were found in the stomachs of native fish inhabiting lakes that did not winterkill. We conclude that common carp, and likely many other highly mobile and fecund invasive fish, have evolved life histories to avoid egg predators and can become invasive when they are absent.

Field and laboratory experiments tested the hypothesis that recruitment of the invasive Common Carp Cyprinus carpio is often controlled by the feeding activity of egg and larval predators in interconnected lakes of the upper Mississippi River basin. The survival of naturally spawned carp eggs was monitored in a seemingly typical lake by sampling the abundance of such eggs in spawning areas and in the diets of fish caught at these locations. Over 95% of the carp eggs found attached to spawning substrate disappeared within 4 d of spawning, at the same time that large numbers of eggs were found in the stomachs of Bluegills Lepomis macrochirus. Egg predation closely paralleled but lagged egg disappearance. Concurrent laboratory studies showed that carp eggs hatched after 4.75 d at lake temperatures, demonstrating that most (but perhaps not all) of the eggs had been eaten by egg predators prior to hatching. A second laboratory experiment found that Bluegills readily find and consume carp larvae while Black Bullheads Ameiurus melas do not, suggesting that predation on larvae contributes to carp recruitment. These data, together with previous studies of the survival of carp eggs placed into lakes and the distribution of young carp relative to various species of native fish, add strong support to the hypothesis that the recruitment of carp in interconnected lakes of the upper Mississippi River basin is often controlled by native fish predators.