Evaluating a native fish species as an alternative to the western mosquitofish, Gambusia affinis, for the biological control of mosquitoes in southern California

Background/Question/Methods

The ecologically relevant field of biological control, which involves the use of natural enemies to control pest populations, began in 1887 in Riverside, California. This concept was initially developed to target agricultural pests but has since expanded to include insect vectors of diseases to humans, namely mosquitoes. The Western mosquitofish, Gambusia affinis, has been introduced worldwide as a biological control agent of larval mosquitoes.  However, the fish is not strictly larvivorous and declines in native amphibian and fish populations, as well as other negative ecological interactions, have been attributed to Gambusia introductions.  The arroyo chub, Gila orcutti, is native to the watersheds of southern California and has been proposed as a replacement for the non-native Western mosquitofish as a biological control agent for mosquitoes in sensitive watersheds throughout the region.  The California Dept. of Fish and Wildlife is interested in using the chub for vector control while helping to re-establish the fish within its historical range.  Because mosquito-eating fish are not strictly larvivorous, they have the potential to impact non-target organisms in systems where they are introduced, which can lead to trophic cascading and other ecological effects. The impact of three different arroyo chub stocking densities on the abundance of larval mosquitoes and the associated invertebrate community in the Santa Ana River were evaluated at a surface-flow treatment wetland in Riverside County, California.  

Results/Conclusions

Invertebrate abundance and community structure did not differ significantly among three stocking densities (0, 1.5 and 6 kg/ha) during a five-week study in cage mesocosms.  Due to mitigating factors, there was an unusually low abundance of larval mosquitoes during the 5-week trial.  However, the absence of mosquito larvae in the system did not lead to a decrease in abundance or diversity of the other aquatic invertebrate and zooplankton taxa present in the wetland, even at the unnaturally high stocking density of 6 kg/ha.  No shifts in community structure or trophic cascading were attributed to the presence of the chub in the system.