Field and laboratory studies suggest that recruitment of the invasive common carp is controlled by native fish in Stable Lakes of the Upper Mississippi Basin.

Abstract

Recent studies have shown that recruitment of the common carp in many lakes of the Upper Mississippi Basin is limited to areas that experience an ecological disturbance that alters the fish community. It has been hypothesized that recruitment of carp is limited by native predatory fish and that carp are only able to recruit in habitats where these native species have been excluded due to winter hypoxia. We tested this hypothesis by comparing the survival of carp eggs and larvae in the presence and absence of native predatory fishes in three experiments. First, we sampled fish diets and carp egg abundance on a daily basis in lakes where wild carp had spawned to identify fish predators and track carp egg abundance in the environment. We simultaneously estimated the date of carp egg hatching using eggs raised in the lab at water temperatures that represented lake temperatures. We found that the bluegill sunfish was the main predator of carp eggs (94% of egg predators were bluegill sunfish), and that egg abundance declined before the estimated date of hatching in areas where bluegill sunfish were present. In our second experiment, carp egg survival was tested in the presence and absence of bluegill sunfish in the laboratory. Carp eggs were fertilized in the laboratory, placed on artificial vegetation in 70 liter aquaria that contained a bluegill sunfish or no fish, and counted twice daily until hatching. We found that the survival to hatching of carp eggs in aquaria decreased from 74% to 15% in the presence of bluegill sunfish (p-value < 0.001). In our third experiment, carp larval survival was tested in the presence of bluegill and green sunfish in the laboratory. Larval carp were raised from eggs fertilized in the laboratory until they reached their free swimming stage and introduced into 1,600 liter tanks with five individuals of a single species of predatory fish or no fish. Larvae were then sampled twice daily for two days. Both bluegill and green sunfish reduced larval carp survival to zero percent after 34 hours (p-value < 0.001). These results suggest that the bluegill sunfish is a voracious predator of the early life stages of common carp and is likely responsible for reducing the recruitment of common carp through predation on its eggs and larvae.