Water-quality assessment of the Rio Grande Valley, Colorado, New Mexico and Texas: Fish communities at selected sites, 1993-95

Fish communities at 10 sites in the Rio Grande Basin were
sampled during low-flow periods between 1993 and 1995 as part of
the U.S. Geological Survey National Water-Quality Assessment
Program. The ecology of fish communities is one of several lines
of evidence used to characterize water-quality conditions. This
report describes the fish communities at selected sites in the
Rio Grande Basin and relates the structure of these fish
communities to the physical and chemical characteristics of the
streams. Twenty-nine species of fish representing 10 families
were identified in 25 samples collected during this study.
Species richness ranged from 1 to 13.

Cluster analysis of the 25 samples collected during this
study delineated four groups of sites that were based on the
similarity of the fish communities. The first two groups were
individual sites with low species richness. The third group
contained the most samples, and the fourth group consisted of
samples from the Rio Grande at Isleta, New Mexico, and the Rio
Grande at El Paso, Texas. The shift in community structure of
samples from group 3 to group 4 reflects changes from
predominantly coldwater fishes to warmwater fishes.

Four metrics of biotic integrity (percentages of introduced
individuals, omnivores, tolerant individuals, and anomalies)
were used in this study to provide a broad overview of the
community structure. The relative percentages of introduced
species at the Rio Grande near Del Norte, Colorado; Saguache
Creek near Saguache, Colorado; Rio Grande below Taos Junction
Bridge, near Taos, New Mexico; and Rio Grande at Isleta are
indicative of biological stress on the communities at these
sites. The dominance of omnivores in samples from the Rio Grande
below Taos Junction Bridge, near Taos; Rio Chama near Chamita,
New Mexico; Rio Grande at Isleta; and Rio Grande at El Paso is
an indication of environmental stress at these sites. In 1995,
tolerant species accounted for the entire fish community at the
Rio Grande at Isleta. In all samples the occurrence of anomalies
was less than 2 percent of the individuals, with the exception of
the sample from the Rio Grande at Isleta. On the basis of the
relative percentages of introduced individuals, omnivores,
tolerant individuals, and anomalies, the biotic integrity at the
Rio Grande at Isleta appears to be the most impaired of all sites
and shows indications of potential chemical and physical
perturbations.

Fish communities from three reaches at the Santa Fe River
above Cochiti Lake, New Mexico, and the Rio Grande at Isleta were
sampled in 1995 to assess small-scale spatial patterns in the
structure of fish communities. The spatial pattern at these sites
might be associated with natural variability of the fish
communities or with the presence of habitat features such as
pools.

The total number of individuals and relative abundance in a
sample varied at sites sampled yearly during this study. All
sites, with the exception of the Rio Grande near Del Norte, had a
decline in total number of individuals in a sample. The temporal
decline in the total number of individuals at these site might be
associated with the natural variability within the fish
communities.