Technical Report
Rio Grande Silvery Minnow Population Monitoring Program Results From May to December 2013
Date: 2014/04/30
Author(s): Dudley R.K., Platania S.P., White G.C.
Publication: Prepared for U.S. Bureau of Reclamation, 142 p.
Abstract:
The abundance of Rio Grande Silvery Minnow has fluctuated widely over the past two decades (1993–2013). Mixture model density estimates (E(x)) for this imperiled species, using October sampling site density data (i.e., fish per 100 m2), were highest in 1995 (36.04) and 2005 (44.84) and lowest in 2002 (0.08) and 2012 (0.00). While these extremes indicated general periods of elevated or reduced abundance over time, there were exceptions to these trends where densities quickly declined and rebounded within a few years (e.g., 2005–2007). Most recently, the estimated densities of Rio Grande Silvery Minnow were significantly lower (P < 0.05) in 2010–2013 as compared with 2007–2009. Population monitoring efforts in 2013 yielded very few Rio Grande Silvery Minnow (E(x) = 0.03) from only three of the 20 sampling sites.
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 the period of study (1993–2013). The top model (δ[Year] α [ABQ>2,000+random]) received 39% of the AICC weight (wi) and had a scaled r2 value of 0.63 (P < 0.001). The top four models, which accounted for most of the cumulative wi (ca. 76%), were related to the interaction among α and hydraulic variables representing elevated spring flows in the Angostura Reach. Although models relating to the interactions among δ and any of the hydraulic variables received much lower values of wi, 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 period of study.