Long-term change in the biotic integrity of a shallow tropical lake: A decadal analysis of the Lake Chapala fish community

We describe the successful application of a 4-decade analysis of an Index of Biotic Integrity (IBI) demonstrating its assessment value when applied to the fish community of Lake Chapala. We based our analysis on fish community and fishery composition and on the proportion and density of native and exotic species for each decade. Fisheries landings and IBI values document a long-term downward trend since the 1970s. Data from the 1990s and 2000s reveal a dramatic drop in the IBI score to 20 points (out of a possible 100), a rating of very poor. The fisheries collapse and the loss of 11 native species as a consequence of habitat degradation and invasive species have reduced native fish biomass and richness. Fluctuations in rainfall, principally, and water-extraction related to human activities have had a major impact on the lake. Additionally, water quality deterioration has been caused by a lack of wastewater treatment. The fish community has been further modified by large increases in the number and biomass of exotic fishes. Although the exact degree of decline in biotic integrity is impossible to determine because of uncertainty about the representativeness and accuracy of some historical information, the analysis of the temporal changes in the IBI provides compelling evidence for an accelerating drop in ecosystem health. The application of IBI, which is broad in scope and has flexible data requirements, can serve as a template for assessing trends in other water bodies with a history of fisheries data but limited standardized monitoring of the fish community.