Long-Term Changes in Fish Assemblage Structure Associated with Hydrological Alteration in the Lower Rio Grande/Rio Bravo del Norte (USA/Mexico)
The lower Rio Grande watershed below Falcon Dam has been 95% developed for agriculture, urban, and industrial uses. The river has been extremely altered to make this possible, with the addition of two more downstream dams, a series of five rock weirs, and numerous water diversions. This regulation has resulted in an extremely altered flow regime and fish fauna since the early 1950s. There has been a significant general retreat and decline of primary freshwater fishes over time, and we identified three significantly different faunal groups across the fragmented watercourse. However, the overall species richness of the region did not change significantly over time, likely due to an ongoing upstream intrusion of estuarine and marine-derived taxa, as well as the increase in the number and spread of non-native taxa. Despite no overall change in species richness within the region, we identified a significant trend in the species richness of the two most diverse primary freshwater fish families, Leuciscidae and Centrarchidae. Leuciscid richness significantly declined and centrarchid richness significantly increased over the 68-year period. Fluvial native leuciscid species that require a natural flow regime became extirpated or extinct, while lentic-adapted native and introduced centrarchids have thrived. The flow regime of the lower Rio Grande has been severely altered since impoundment of Falcon Reservoir. Median monthly flows have declined for all months, maximum flows and high flow pulses have declined, and base flows have increased. Also noteworthy were the increased number of hydrograph reversals post-impoundment. The streamflow regime is of central importance in sustaining the ecological integrity of rivers, and its disruption in the lower Rio Grande corresponds to a vastly different contemporary fish fauna than what historically occurred.
Map of sampling locations (red dots) from sampling conducted in 2017–2019.
Species observed in the lower Rio Grande by river reach and year. A line through time indicates a species has been found in that reach; black points indicate that there was a sampling event which occurred that year but the species was not observed; reach-specific colored points (blue = B; green = C; red = D) indicate that the species was collected that year in that reach. For a more detailed view, this figure is available as supplemental material (see Data Accessibility).
Visualization of changes in fish community composition in NMDS space within the three reaches of the lower Rio Grande. Reach B (blue), Reach C (green), Reach D (red). Note that communities cannot be compared visually across facets. See PERMANOVA results for comparisons among communities. Small points mark the ordination of individual sampling events, while large points show the decadal centroid, with lighter shaded points indicating earlier decades and darkly shaded points being more recent decades.
Changes in mean species richness of the two dominant families of primary freshwater fishes in the lower Rio Grande, Centrarchidae and Leuciscidae, over 69 years of surveys. Points indicate the mean number of species observed each year with 68% and 95% credible intervals. The darkly shaded region shows the 68% credible region, and the lightly shaded region shows the 95% credible region around the estimated change in mean species richness.
Flow duration curves for the Rio Grande at Brownsville. Curves indicate the percent of time specified discharges were equaled or exceeded during a given period. Dashed line represents flow regime before impoundment by Falcon Reservoir (1934–1953), and solid line represents post-impoundment discharge (1955–2022).
Contributor Notes
Associate Editor: M. P. Davis.