Technical Report
Rio Grande Silvery Minnow Population Monitoring Program Results From February to December 2015
Date: 2016/04/15
Author(s): Dudley R.K., Platania S.P., White G.C.
Publication: Prepared for the U.S. Bureau of Reclamation, 186 p.
Abstract:
The occurrence and abundance of Rio Grande Silvery Minnow has fluctuated widely over the past two decades (1993–2015). Estimated densities (E(x)) derived from mixture models, using October data (i.e., fish per 100 m2 ), were highest in 2005 (44.84) but reached a recent low in 2012 (0.00) and again in 2014 (0.00). While estimated densities of Rio Grande Silvery Minnow were notably lower from 2010–2014 as compared with 2007–2009, there was an improvement in 2015 (E(x) = 0.16).
General linear models of Rio Grande Silvery Minnow mixture-model estimates revealed that variation in the mean of the lognormal density distribution (α), as compared with variation in the probability of occurrence (δ), was more reliably predicted by changes in hydraulic variables over time. The top three models, which accounted for most of the cumulative AICc weight (ca. 93%), were related to the interaction among α and hydraulic variables representing elevated spring flows in the Angostura Reach. Although models related to the interactions among δ and any of the hydraulic variables received much lower model weight, the two top models represented flows during irrigation season in the San Acacia Reach along with elevated spring flows for α in the Angostura Reach. Thus, prolonged high flows during spring were most predictive of increased density and prolonged low flows during summer were most predictive of decreased occurrence of Rio Grande Silvery Minnow over the study period.