Granger Reservoir - 2020 Survey Report

Prepared by Muhktar Farooqi and Patrick Ireland
Inland Fisheries Division
San Marcos/Austin 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 Granger Reservoir were surveyed in 2020 using electrofishing, trap netting and in 2021 using gill netting. Historical data are presented with the 2017-2021 data for comparison. This report summarizes the results of the surveys and contains a management plan for the reservoir based on those findings.

Reservoir Description

Granger Reservoir is a 4,009-acre impoundment of the San Gabriel River in Williamson County, Texas. The reservoir is located approximately 40 miles northeast of Austin, Texas, within the Brazos River drainage basin. It was constructed in 1980 by the U. S. Army Corps of Engineers (USACE) for purposes of flood control and water conservation. Granger Reservoir has a drainage area of approximately 709 square miles and a shoreline length of about 29 miles. High turbidity and fluctuating water levels have deterred the establishment of aquatic vegetation in the past. Reservoir bank slope is relatively flat and small changes in water level (1-2 feet) can have a large impact on the abundance of shoreline habitat and river/reservoir connectivity. Granger Reservoir was classified as “infested” with zebra mussels in December 2019.

Management History

Important sport fish include White Crappie, White Bass, and Blue Catfish. The last creel survey was conducted in the spring of 2005 and showed White Crappie was the most sought-after species (61.5% directed angler effort) followed by catfishes (16.8%), White Bass (5.1%), and Largemouth Bass (2.5%; Bonds and Magnelia 2005). Blue Catfish were stocked in 1995 and 1996 to provide additional angling opportunities and utilize an abundant shad population. No additional stocking has been conducted since 1996. In collaboration with the Brazos River Authority (BRA), a fishery assessment (habitat availability, river/reservoir connectivity, and access at various lake levels) was conducted at Granger Reservoir for a multi-year system operating plan for the Brazos River Basin (Farooqi and De Jesus 2013). Based on these multiple assessments, threshold recommendations were provided to decrease potential impacts to the fishery during future basin-wide water level manipulations. The Granger Reservoir management threshold recommendation was 504 ft. above mean sea level (MSL), which is the same as conservation pool. A critical threshold for fisheries applications was set at 498 ft. MSL. As a consequence, other than under exceptional circumstances, water level fluctuations at Granger Reservoir will be minimal and therefore have little impact on the fishery. Low to negligible vegetative coverage has been descriptive of Granger Reservoir.

Fish Community

• Prey species: Gizzard Shad and Threadfin Shad were the dominant prey species in 2020. Catch rates for Gizzard Shad and Threadfin Shad had significantly increased compared to previous surveys.
• Catfishes: Blue Catfish was the dominant catfish species. Total catch rate was much higher than in the previous two surveys and body condition of all stock-size fish collected was high, indicating very heavy fish. Most Blue Catfish were of harvestable size (12 to 19 inches) and the largest fish measured 32 inches in length. The abundance of Channel Catfish had increased since the last two surveys and the body condition of all stock-size fish was high. No flathead catfish were caught in 2021; catch rates have been very low in previous surveys.
• White Bass: Total catch rate for White Bass was considerably higher than in the last two surveys. A large proportion of fish were of harvestable size (i.e.,10 inches), with the largest fish up to 15 inches in length.
• Largemouth Bass: Largemouth Bass were present in very low numbers, as has been the case in previous surveys. Only one harvestable-size fish was caught.
• White Crappie: White Crappie abundance had declined compared to the previous two surveys. Nevertheless, the catch rate was similar to historical levels. Body condition was high for most fish.

Management Strategies

• Based on current information, the reservoir should continue to be managed with existing regulations.
• White Crappie is the most sought-after species at this reservoir; therefore, trap net surveys will be conducted biennially to monitor the population dynamics of this species.
• Conduct general monitoring surveys with electrofishing and gill nets in 2024-2025.
• Access, shoreline structural habitat, and vegetation surveys will be conducted in 2024.
• Inform the public about the negative impacts of aquatic invasive species.

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