Indirect effects and biological control of mosquitoes by mosquitofish.
Gambusia affinis reduced the abundance of the mosquito Culex tarsalis in a rice field in the San Joaquin Valley, California. The fish also reduced the abundance of other aquatic invertebrate taxa, including predatory insects and zooplankton. The reduction in the abundance of predatory insects (e.g. Odonata), due to the fish, reduced mosquito mortality due to insects. The reduction in the abundance of alternative prey (mainly zooplankton) increased predation rates on mosquitoes by individual fish. A low abundance of zooplankton increased the predation rates of individual predatory insects. Successful control of mosquitoes thus resulted from a direct negative effect by G. affinis that more than outweighed indirect positive effects of the fish on mosquitoes. In other studies, the introduction of mosquitofish increased the abundance of mosquito larvae; indirect positive effects of the type demonstrated in this study could account for these results.