Quantifying ichthyofaunal zonation and species richness along a 2800-km reach of the Rio Chama and Rio Grande (USA)

Ichthyofaunal zonation occurs when lotic fishes are partitioned into distinct assemblages, usually in response to longitudinally distributed habitats. Several studies have documented zonation within the Rio Grande, but this is the first to quantitatively test the zonation hypothesis along a continuous 2800-km river profile, extending from the Rio Chama headwaters to the Gulf of Mexico. Using a large, multi-source dataset, I detected three ichthyofaunal zones: a high gradient (similar to 1.5%) 'upper' zone, a moderate gradient (similar to 0.2%) 'middle' zone and a low gradient (< 0.1%) 'lower' zone. Species richness was lowest in the upper zone and highest in the lower zone, and all zones contained large numbers of nonnative species. However, species richness did not accumulate in a consistent, downstream manner. Instead, it tracked local-scale changes in mean annual discharge. This demonstrates the strong effect of river regulation and irrigation withdraws on fish diversity in the Rio Grande.