Progress Report

Rio Grande Silvery Minnow Population Monitoring During July 2025

URL: https://webapps.usgs.gov/mrgescp/documents/2025-07-RGSM-Population-Monitoring-Monthly-Report.pdf

Date: 2025/08/18

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

Publication: Contract 140R4025C0013. Requisition 0044018340. Submitted to: U.S. Bureau of Reclamation, Albuquerque Area Office, 555 Broadway NE, Suite 100, Albuquerque, NM 87102

Abstract:

The July 2025 population monitoring efforts were conducted at the 20 standard sites. Five sites were in the Angostura Reach, six sites were in the Isleta Reach, and nine sites were in the San Acacia Reach. For the 2025 monthly trends, data were based on all sites (i.e., standard, additional, and replacement sites) to maintain consistency across all monthly reports. A list of all collection localities is appended (Appendix A). Adult and juvenile fish were obtained by rapidly drawing a 3.0 m x 1.8 m smallmesh (ca. 5 mm) seine through discrete mesohabitats. Larval fish were collected with a 1.2 m x 1.2 m fine-mesh (ca. 1 mm) seine. All fishes were identified to species and enumerated. We used length-age relationships to assign ages (i.e., age-0, age-1, and age-2+) to all Rio Grande Silvery Minnow collected. Age-0 individuals are only present, however, after seasonal spawning events that typically first occur during spring runoff. Figures illustrating fish densities (i.e., fish per 100 m2) were prepared for the ten focal species to facilitate comparisons across reaches.

Standard Sites (n = 20)

During July, sampling covered 8,301.5 m2 (surface area) of water and yielded 1,832 fish. There were no dry sampling sites. Cumulative fish density during July was 22.07 individuals per 100 m2 sampled. The three most common species were Red Shiner (n = 532), River Carpsucker (n = 294), and Western Mosquitofish (n = 226). The sampling sites yielded a total of 16 fish species. Rio Grande Silvery Minnow was present in 26 of the 261 seine hauls that yielded fish and at 8 of the 20 sampling sites. Densities of unmarked and marked individuals were 0.63 (n = 52) and 0.02 (n = 2) individuals per 100 m2 sampled, respectively. Densities of age-0, age-1, and age-2+ individuals were 0.42 (n = 35), 0.05 (n = 4), and 0.18 (n = 15) individuals per 100 m2 sampled, respectively. Based on all July surveys since 1993, its overall density averaged 23.77 (range = 0.26–140.98) individuals per 100 m2 sampled. During July 2025, its overall density was 0.65 (n = 54) individuals per 100 m2 sampled.