Spatiotemporal Population Trends of Notropis simus pecosensis in Relation to Habitat Conditions and the Annual Flow Regime of the Pecos River, 1992–2005
The threatened Pecos Bluntnose Shiner, Notropis simus pecosensis, is restricted to a 333-km segment of the Pecos River, New Mexico. This is a relatively long, undammed segment, but the flow regime is highly modified and the river channel is degraded. Within the occupied segment, upstream river sections are less-degraded with a wider river channel and shifting-sand substrata, whereas downstream river sections have increasingly narrow river channels and compacted, silt-sand substrata. We studied the longitudinal distribution, length-structure, and population status of N. s. pecosensis from 1992 through 2005. On average, individuals were larger upstream and smaller downstream. Highest densities were in upper-middle portions of the study area. Infrequent high-density collections from lower river sections included only small juveniles that were displaced from upstream. In contrast, high-density collections from upper-middle river sections included all life stages. Thus, we concluded that the core population was restricted to upstream river sections. These river sections have relatively diverse habitat when streamflow is perennial, but are largely desiccated during streamflow intermittence. The core population of N. s. pecosensis was evenly distributed between 1992 and 2000, when streamflow was perennial, but became patchily distributed among refugial habitats between 2001 and 2003, in response to low discharge and periodic streamflow intermittence. High density collections resulted from the concentration of fish in refugia. However, N. s. pecosensis percent species composition declined during this period, indicating that refugia were unsuitable. Low density collections observed after perennial streamflow was restored in 2005 indicated a population collapse between 2001 and 2005. Streamflow intermittence is a threat to the core population of N. s. pecosensis, but habitat degradation appears to limit its distribution. Thus, channel restoration and perennial base-flow will both be important for conservation and recovery.Abstract
River sections and geographic features of the Middle Pecos River. The Pecos River is a major tributary to the Rio Grande, which drains to the Gulf of Mexico. Lettered dots represent dams, and hash-marks represent approximate river section boundaries. River sections are numbered from upstream to downstream. The upper and lower boundaries of our study area were the Fort Sumner Irrigation District Diversion Dam (letter D) and Brantley Dam (letter E). This undammed segment is roughly 333 river-km in length.
Annual sampling effort by river section. Seines were used for all collections, so sampling effort is expressed as area (m2) seined. River section graphs are ordered from upstream (river section 0 = letter A) to downstream (river section 7 = letter H).
Notropis simus pecosensis standard length histograms (mm SL) for the entire study period by seasons. Length categories (bars) represent 2 mm SL length-groups. Seasons were four months in length: spawning season = May through August (letter A); post-spawning season = September through December (letter B); pre-spawning season = January through April (letter C). Estimated age-groups are designated by Roman numerals: age-0 (young-of-year), age-I (first year post-hatching), and age-II (second year post-hatching).
Notropis simus pecosensis standard length distribution (mm SL) for the entire study period by river section. Mean SL varied among all river sections (ANOVA of ln-transformed data: df = 6, 19, 243; F = 2,466; P < 0.01; Tukey tests: mean difference > 0.01; P < 0.04 for all pairwise comparisons). Boxplots indicate the median SL (middle line), 25th and 75th percentiles (top and bottom of box), 10th and 90th percentiles (upper and lower whiskers), and 5th and 95th percentiles (upper and lower dots).
Notropis simus pecosensis population density (letter A, fish/100 m2 seined) and adult density (letter B; individuals > 40 mm SL based on Hatch et al. [1985] and Bestgen and Platania [1990]) for the entire study period by river section. Boxplots indicate the median SL (middle line), 25th and 75th percentiles (top and bottom of box), 10th and 90th percentiles (upper and lower whiskers), and 5th and 95th percentiles (upper and lower dots).
Notropis simus pecosensis mean density by season and river section. Error bars represent the standard error of the mean.
Mean annual discharge with standard error bars (m3/s, letter A), number of days per year with zero discharge (letter B), and annual discharge variability (R-B Index, Baker et al., 2004; letter C) based on U.S. Geological Survey gage data from the Near Acme surface-water gaging station for the years 1988 through 2005. Mean density with standard error bars for all N. simus pecosensis (letter D) and adult N. s. pecosensis (letter E) from the core population (river sections 1 through 4) and for all N. s. pecosensis from river sections 5 through 7 (letter F) for the years 1992 through 2005. Mean percent composition with standard error bars for all N. s. pecosensis (letter G) and adult N. s. pecosensis (letter H) from of the core population (river sections 1 through 4) and for all N. s. pecosensis from downstream river sections 5 through 7 (letter I) for the years 1992 through 2005.
Contributor Notes
Section Editor: J. W. Snodgrass.
1U.S. Fish and Wildlife Service, New Mexico Fishery Resources Office, Albuquerque, New Mexico 87109-5831, e-mail: (CWH) ChristopherHoagstrom@Weber.edu, e-mail: (JEB) Jim_Brooks@fws.gov, e-mail: (SRD) Stephen_Davenport@fws.gov. Send reprint requests to CWH.
2Present address: (CWH) Department of Zoology, Weber State University, Ogden, Utah 84408-2505.