Impacts of a native fish on non-target invertebrates in constructed wetlands in southern California

Wetlands created for wastewater treatment are often planted with dense stands of vegetation to increase the uptake of excess nutrients. This vegetation creates microhabitats where large numbers of mosquitoes are produced. Previous studies have shown that Gambusia affinis, the mosquitofish, is not an effective control agent in wide bands of California bulrush (> 3 m). We examined the effects of vegetation patch size on an alternative biocontrol agent, the arroyo chub (Gila orcuttii). The arroyo chub is native to southern California and is a species of special concern due to the impacts of urbanization and introduced fish. Twelve ponds at the UC Riverside Agricultural Experiment Station received additions of Schoenoplectus californicus (California bulrush) in one of two arrangements such that 12 0.1m² (or single plant) plots, 4 0.4 m² (or four plant) plots, and 2 0.9 m² (or nine plant) plots were present. Six ponds received additions of 30g of G. orcuttii. Ponds with fish had significantly fewer larval mosquitoes than ponds without fish in February, March, and April, but were not statistically significant in overall production of mosquitoes. While there was not a significant fish X vegetation plot size interaction, the 0.4 m² consistently had more mosquitoes than the 0.1 m² plots, though neither significantly differed from the largest plots. The arroyo chub did not have significant impacts on non-target invertebrates. These results suggest that Gila orcuttii could be a useful alternative to the mosquitofish as part of an integrated mosquito management program.