Amistad Reservoir - 2020 Survey Report

Prepared by Randall A. Myers and Mitchell Nisbet
Inland Fisheries Division - San Antonio District

This is the authors' summary from a 33-page report. For a copy of the complete report, use the download link in the sidebar.

Fish populations in Amistad Reservoir were surveyed in 2020-2021 using electrofishing and gill netting. Historical data are presented for comparison. .This report summarizes the survey results and contains a management plan for the reservoir based on those findings.

Reservoir Description

Amistad Reservoir (63,680 acres when full) was constructed in 1969 on the Rio Grande River. It is managed by the International Boundary and Water Commission to provide water for irrigation and hydro-electric power generation. The National Park Service (NPS) maintains nine public boat ramps providing excellent angler access. Water level ranged from 30 to 48 feet below conservation pool elevation during the current study period. Aquatic vegetation occurrence was low during the study period (9%) relative to occurrence in previous years (17-36%).

Management History

Important sport fishes include Largemouth Bass, Smallmouth Bass, catfishes, Striped Bass, and White Bass. Striped Bass have been stocked in most years since 1974. Florida Largemouth Bass (FLMB) were stocked periodically from 1975 to 2006 and annually since 2010 to improve FLMB introgression and trophy Largemouth Bass potential. Angler harvest of all sport fishes has been regulated according to statewide size and bag limits. Since 2004, the NPS has regulated Largemouth Bass tournaments via a tournament permitting program.

Fish Community

• Prey species: Gizzard Shad and sunfishes are the primary prey in the reservoir. Relative abundance of Gizzard Shad and combined sunfishes was greater in 2020 than in previous years. Likewise, Gizzard Shad Index of Vulnerability was greater in 2020 than in previous years. Overall, prey abundance and size was sufficient to support existing predator species populations.
• Catfishes: Blue Catfish and Flathead Catfish are present in the reservoir, but in low relative abundance. Channel Catfish relative abundance was greater in 2021 than in previous years.
• Temperate Basses: In 2021, White Bass relative abundance was substantially greater than in previous years and Striped Bass relative abundance was similar to previous years. Legal-length White Bass and Striped Bass averaged two and three years old, respectively.
• Largemouth Bass: Relative abundance of Largemouth Bass was similar to or greater in 2020-2021 than in previous years. Proportional size distribution values were consistent across years; however, sub-stock fish comprised a larger fraction of the population. Legal-length Largemouth Bass averaged 4.1 years old. Genetic introgression of Florida Largemouth Bass into the population was slightly lower in 2020 than in previous years.

Management Strategies

• Evaluate past stockings of Striped Bass by assessing the fishery in 2022 using a creel survey.
• Continue stocking FLMB annually to maintain high FLMB introgression and trophy potential.
• Conduct electrofishing, gill netting, and vegetation surveys every other year, and a creel survey every 3-5 years.

Performance Report as required by Federal Aid in Sport Fish Restoration Act Texas Federal Aid Project F-221-M-2 Inland Fisheries Division Monitoring and Management Program