Technical Report

Monitoring of the Rio Grande Silvery Minnow Reproductive Effort During 2015 in the Rio Grande and Selected Irrigation Canals

URL: https://webapps.usgs.gov/mrgescp/documents/Dudley%20and%20Platania_2015_Monitoring%20of%20the%20RGSM%20Reproductive%20Effort%20during%202015%20in%20the%20Rio%20Grande%20and%20Selected%20Irrigation%20Canals.pdf

Date: 2015/09/25

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

Publication: Prepared for U.S. Bureau of Reclamation, 37 p.

Abstract:

Systematic monitoring of the reproductive output of Rio Grande Silvery Minnow at multiple sites in the Middle Rio Grande has been conducted annually since 1999. Previous studies demonstrated May and June as the primary period of spawning activity. The 2015 study was a continuation of the long-term monitoring effort 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 2013–2015.

Rio Grande Silvery Minnow mixture-model estimates (E(x)), using standardized egg passage rate data (E_p = eggs · second^-1 ) from 2003–2015, were highest in 2011 (5.40 x 10¹ ) and lowest in 2004 (9.62 x 10^-4 ). Values of E_p (i.e., daily estimates of the number of eggs passing the site per second) are indicative of the relative spawning intensity across years, corrected for annual differences in flow magnitude. There was a decline in the densities of eggs collected from 2011–2013, followed by an increase in 2014. The estimated values of E(x) did not differ significantly (P > 0.05) from 2014 (5.41 x 10⁰ ) to 2015 (7.05 x 10^-1 ).

General linear models of Rio Grande Silvery Minnow mixture-model egg passage rate estimates (Delta (δ) and Mu (µ)) revealed that variation in δ and µ was only weakly predicted by changes in hydraulic variables (allowing for random effects) over the study period (2003–2015). The top model (δ(Year) µ(Year)) received over 99% of the AIC_C weight (w_i). The next four models, which accounted for < 1% of the cumulative w_i, were primarily related to the interaction among δ, µ, and hydraulic variables representing elevated spring flows in the Angostura Reach. No models relating to the interaction among δ, µ, and previous year October fish density data received appreciable values of w_i (i.e., no models with w_i > 0.01%). Thus, prolonged high flows during spring were most predictive of increased egg passage rate estimates of Rio Grande Silvery Minnow over the period of study.