Historical Modifications to Fish Assemblages in West-Central Mexico

We analyzed the historical trends and current status of fish assemblages in freshwater ecosystems in Central West Mexico (Colima and Jalisco states) using species-specific ecological information and a fish-based index of biotic integrity (IBI) as framework. The IBI helps analyze compositional, structural, and functional data from a biological community to infer ecosystem health. Using data from over 230 collections made between 1955 and 2008 at more than 140 sites in both lotic and lentic environments, and ecological guild information for more than 100 primary, secondary and peripheral species, we were able to describe how fish communities have been modified by ongoing anthropogenic effects. We detected a significant increase in the number and geographic spread of exotic species through time, and a decrease in the number and abundance of native species, especially carnivores, benthic, and endemic species. The most frequently encountered exotics within the area were tilapias of the genus Oreochromis. These changes translated into losses of biological integrity for many sites, but other sites have maintained IBI scores through time. Regionally, fish communities in sites located near or in urban and agricultural areas have been severely altered, while some of those located in protected and/or mountainous regions maintain relatively intact fish faunas. Specifically, sites located in relatively high gradient streams and relatively small rivers draining directly to the Pacific Ocean had comparatively healthier fish communities than other sites in the region.