Technical Report
Rio Grande Silvery Minnow Population Estimation Program Results From October (2006 to 2008)
Date: 2011/02/10
Author(s): Dudley R.K., White G.C., Platania S.P., Helfrich D.A.
Publication: Prepared for U.S. Bureau of Reclamation, 66 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 provides an annual estimate of the Rio Grande silvery minnow population during a single time-period (October). 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 based on repeated sampling in November, 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, and 5) Synthesize the results of the first three years of this study (2006 to 2008) into a single comprehensive document.
The ichthyofaunal community in the Middle Rio Grande during October (2006 to 2008) was numerically dominated by cyprinids. The native ichthyofauna consisted of seven species (red shiner, Rio Grande silvery minnow, fathead minnow, flathead chub, longnose dace, river carpsucker, and bluegill) in 2006 and 2008 but included smallmouth buffalo in 2007. Red shiner was the most abundant native species collected in 2006 (N = 7,885) and 2007 (N = 18,826) but was the second most abundant species in 2008 (N = 909). Rio Grande silvery minnow varied widely in abundance among years (range = 339 to 3,122) and was the most abundant native species in 2008 (N = 1,576). Other commonly collected native species included fathead minnow and flathead chub. Longnose dace, rivercarpsucker (with the exception of 2007), smallmouth buffalo, and bluegill were often the least abundant native fishes. The most abundant introduced species collected in all years were common carp, white sucker, channel catfish, and western mosquitofish.