Technical Report
Rio Grande Silvery Minnow Population Monitoring Program Results From December 2006 to October 2007
Date: 2008/08/22
Author(s): Dudley R.K., Platania S.P.
Publication: Report prepared for the U.S. Bureau of Reclamation, 159 p.
Abstract:
Population monitoring efforts of the Middle Rio Grande fish community over the past decade have documented vast changes (i.e., order of magnitude increases and decreases) in the abundance of Rio Grande silvery minnow. Recent monitoring efforts show that the October density of Rio Grande silvery minnow was significantly lower (p<0.05) in 2007 compared to 2005. However, the October density of this species was higher (p<0.05) in 2007 than in 1996 or 2000-2004. The Angostura Reach yielded most of the Rio Grande silvery minnow in October of 2007, followed by the Isleta Reach, and the San Acacia Reach. This was in contrast to population monitoring in October of 2006, when the largest catch rates were recorded in the San Acacia Reach.
An analysis of sampling variation at all 20 sampling sites revealed that overall sampling variation for Rio Grande silvery minnow was low and consistent among sites. Values of the coefficient of variation (CV) were often <0.7 for individual sites and nearly always <1. The overall CV value for collections of this species in the Middle Rio Grande was quite low in 2006 and 2007 (0.38 and 0.35, respectively). It is likely that the population monitoring sampling protocol combined with modest numbers of Rio Grande silvery minnow during 2006 and 2007 accounted for these low and consistent CV values. The notable changes in densities among years (i.e., often >1,000% ) greatly outweighed the reasonable value of relative precision (ca. 35-38%) observed during this intensive sampling effort. This indicates that the current sampling approach should provide a reasonable trend estimate of increasing or decreasing population size of Rio Grande silvery minnow over time.