Technical Report
Rio Grande Silvery Minnow Population Estimation Program Results from October 2010
Date: 2011/05/31
Author(s): Dudley R.K., White G.C., Platania S.P., Helfrich D.A.
Publication: Prepared for U.S. Bureau of Reclamation, 40 p.
Abstract:
Systematic monitoring of Rio Grande silvery minnow, Hybognathus amarus, and the associated Middle Rio Grande fish community has been conducted since 1993 and has provided relevant, quantifiable, and timely information regarding the status of this species both spatially and temporally. In contrast to the Population Monitoring Program, which continues to provide necessary year-round documentation of trends for the entire ichthyofaunal community, the Population Estimation Program has provided an annual estimate of the Rio Grande silvery minnow population during a single time-period (October) since 2006. Estimating population size required employing statistical techniques that were subject to a series of assumptions. Estimates of the number of Rio Grande silvery minnow are presented within the context of those assumptions, especially given the inherent variation in the density and distribution of organisms within their environment. The objectives of this study were to 1) Develop and implement methods that provide statistically robust population estimates of Rio Grande silvery minnow, 2) Provide a population estimate of Rio Grande silvery minnow based on fish densities stratified by mesohabitat for 20 sampling units, 3) Develop site occupancy rates for Rio Grande silvery minnow populations over time, and 4) Calculate a population estimate of Rio Grande silvery minnow using Population Monitoring Program data, controlling for mesohabitat, and compare this value to that generated in Objective #2.
Data collected during the 2010 Population Estimation Program indicated that the ichthyofaunal community in the Middle Rio Grande between Angostura Diversion Dam and Elephant Butte Reservoir was numerically dominated by cyprinids and included ten native fish species. Red shiner was the most abundant native species collected (N = 794), followed by flathead chub (N = 391), Rio Grande silvery minnow (N = 268), and fathead minnow (N = 32). The most abundant introduced species were channel catfish (N = 394) and western mosquitofish (N = 388).
The best model for the mesohabitat-specific depletion data (based on the lowest AICC value) was by mesohabitat and location (for BW, PO, SHPO, and SHRU) and was supported by a high model weight. The capture probability estimates (i.e., proportion of fish removed per depletion pass) for the different mesohabitats ranged from 0.6858 (shoreline pools) to 0.8444 (runs). The associated standard errors for estimates were consistent among mesohabitats and ranged from 0.0157 to 0.0382.