Technical Report

Monitoring of the Rio Grande Silvery Minnow Reproductive Effort During 2014 in the Rio Grande and Selected Irrigation Canals

URL: https://webapps.usgs.gov/mrgescp/documents/Dudley%20and%20Platania_2014_Monitoring%20of%20the%20RGSM%20Reproductive%20Efforts%20During%202014%20in%20the%20Rio%20Grande%20and%20Selected%20Irrigation%20Canals.pdf

Date: 2014/09/30

Author(s): Dudley R.K., Platania S.P.

Publication: Prepared for U.S. Bureau of Reclamation, 39 p.

Abstract:

Systematic monitoring of the reproductive output of Rio Grande Silvery Minnow at multiple sites in the Middle Rio Grande was first conducted in 1999 and has continued annually (except 2005) since 2001. Previous studies demonstrated May and June as the primary period of spawning activity. The 2014 study was a continuation of the long-term monitoring of Rio Grande Silvery Minnow spawning in the downstream-most river reach just upstream of Elephant Butte Reservoir. Additionally, five upstream sites (three in the Isleta Reach and two in the San Acacia Reach) provided data on the entrainment of Rio Grande Silvery Minnow eggs into the Middle Rio Grande irrigation canal network from 2013–2014.

Rio Grande Silvery Minnow mixture-model estimates (E(x)), using standardized egg passage rate data (E_p = N [eggs] · second^-1) from 2002–2014, were highest in 2002 (9.58 x 10² ) and lowest in 2004 (9.62 x 10^-4). There was a steady decline in the densities of eggs collected from 2011–2013, followed by an increase in 2014. The estimated value of E(x) was significantly higher (P < 0.05) in 2014 (5.41 x 10⁰ ) as compared with 2013 (5.17 x 10^-2). Simple estimates of mean densities, using the method of moments, were similar to values of E(x) over time.

General linear models of Rio Grande Silvery Minnow mixture-model egg passage rate estimates (Delta (δ) and Mu (µ)) revealed that variation in µ and δ was only modestly predicted by changes in hydraulic variables (allowing for random effects) over the period of study (2002–2014). The top model (δ(Year) µ(Year)) received about 30% of the AIC_C weight (w_i) and had a scaled r² value of 0.40 (P < 0.001). The next four models, which accounted for about 66% of the cumulative w_i, were related to the interaction among µ and hydraulic variables representing elevated spring flows in the Angostura Reach (e.g., ABQ>2,000 and ABQmax). No models relating to the interaction among δ, µ, previous year October density data, or previous year flows during irrigation season in the San Acacia Reach received appreciable values of w_i (i.e., no models with w_i > 2%). Thus, prolonged high flows during spring (as opposed to prolonged low flows during summer) were most predictive of increased egg passage rate estimates of Rio Grande Silvery Minnow over the period of study.