Lyndon B. Johnson Reservoir - 2004 Survey Report

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Prepared by C. Craig Bonds and Stephan J. Magnelia
Inland Fisheries Division
District 2-C, San Marcos, Texas

This is the authors' summary from a 36-page report. For a copy of the complete report, use the download link in the sidebar.

Lyndon B. Johnson Reservoir was surveyed in 2004 using trap nets and boat electrofisher, and in 2005 using gill nets. Structural habitat, aquatic vegetation, and angler access surveys were conducted in 2004. This report summarizes the results of these surveys and contains a fisheries management plan for the reservoir based on those findings.

Reservoir Description

Lyndon B. Johnson Reservoir is a 6,449-acre impoundment of the Colorado and Llano Rivers in Burnet and Llano counties. It was constructed in 1951 by the Lower Colorado River Authority (LCRA) for purposes of hydro-electric and steam-electric power, flood control, and water conservation. Lyndon B. Johnson Reservoir has a drainage area of approximately 36,290 square miles and a shoreline length of about 200 miles. Residential and commercial properties border most of the shoreline area. Seventeen boat ramps provided excellent public boat access. Boat ramps were available in most areas of the reservoir. Most boat ramps offered courtesy docks, but none offered public fishing piers. The majority of the shoreline was privately owned, esulting in poor bank access.

Aquatic Vegetation

Lyndon B. Johnson Reservoir supported a diverse mix of aquatic vegetation species (page 9), but these species covered less than 5% of the reservoir surface acreage. Historically, submerged aquatic vegetation (milfoil, pondweed, and coontail) was common and sometimes problematic until the late 1960s when rooted vegetation disappeared (Butler 1983) and never fully recovered (Tibbs and Magnelia 1998). Near undisturbed shoreline areas waterwillow can be abundant, but shoreline development (in the form of bulkhead banks) has increased in recent years (1997 = 46 % of shoreline; 1999 = 55% of shoreline; 2004 = 60% of shoreline).

Evidence in the scientific literature shows a negative correlation between emergent and floating-leaf vegetation and the percentage of lakeshore development (Radomski and Goeman 2001). Increased conversion of vegetated shoreline to bulkhead habitat can lead to poor littoral fish assemblages (Trial et al. 2001). A cooperative native aquatic vegetation restoration project was initiated in 1999 by personnel from the Texas Parks and Wildlife Department (TPWD), the LCRA, the U.S. Army Corps of Engineers (USACOE) aquatic research laboratory in Lewisville, and the Lake LBJ Habitat Committee (made up of interested public). The objective of the project was to improve aquatic habitat which might result in increased standing stocks of cover seeking fishes (largemouth bass and sunfish) and higher angler catch rates of these fish. A variety of aquatic plant species (Bonds and Magnelia 2000) were planted inside multiple types of protective cages (to prevent herbivory on newly established plants) at several locations within the reservoir in summer of 2000. Expansion of submerged aquatic plants was insignificant. Limited success was achieved by uprooting existing shoreline emergents (e.g., waterwillow, and spatterdock) and replanting in areas lacking aquatic vegetation.

Eurasian watermilfoil expanded from only a trace presence in 2000 to occupy over 80 acres (1.3% surface acreage) in 2004. This plant was mainly relegated to the upper end of the reservoir below the confluence of the Llano and Colorado Rivers. In addition, 2 to 3 acres of hydrilla were discovered scattered over approximately 45 acres subsequent to the aquatic vegetation survey in 2004. These four patches of hydrilla were treated in April 2005 with Nautique (main lake areas) and Sonar (inside cove) herbicides in an attempt to prevent expansion of this exotic plant (Appendix E).

Water hyacinth, an exotic floating aquatic plant, was discovered growing in several isolated patches in the upper end of the reservoir in 2003. This plant offered no significant benefit as fisheries habitat, and had the potential for rapid expansion. These plants were both chemically (i.e., glyphosate herbicide) and mechanically (i.e., by hand) removed from the reservoir in 2004. These plants were still absent by Spring 2005.

Fish Community

• Prey species: Gizzard shad electrofishing catch-per-unit-effort (CPUE) in 2004 was 134.5/hour, which was similar to the average CPUE from the previous three surveys (1997 – 2000 mean = 138.7/hour). However, annual variability in production of young shad has occurred, evidenced by the wide range of index of vulnerability (IOV) values (range = 7 – 82). Threadfin shad were present in the reservoir in low densities and comprised a small quantity of the electrofishing catch (1997 = 4.0/hour; 1999 = 6.0/hour; 2000 = 2.0/hour; 2004 = 19.5/hour). Redbreast sunfish CPUE (63.5/hour) and size structure were fairly consistent with previous surveys (1997 – 2000 mean = 66.9/hour; Range = 32.0 – 117.3/hour). Redbreast sunfish were sampled up to 8 inches in length, providing a limited fishery. Bluegill electrofishing CPUE (218.5/hour) was moderate in 2004 and similar to previous surveys (1999 = 190.0/hour; 2000 = 154.0/hour). Bluegill PSD increased to 22 in 2004, compared to a range of 12 – 14 for years 1997 through 2000. Electrofishing catch rate of bluegill 7 inches and longer (2004 = 10.0/hour) has steadily increased across recent surveys (1997 = 0/hour; 1999 = 1.3/hour; 2000 = 3.3/hour). These data suggest that the size distribution of bluegill has shifted toward larger fish. Other sunfish species collected in 2004 were green sunfish (21.0/hour), redear sunfish (20.0/hour), longear sunfish (18.0/hour), and warmouth (6.0/hour). Other prey fishes collected were bullhead minnow (29.5/hour), inland silverside (12.0/hour), blacktail shiner (7.5/hour), blue tilapia (2.5/hour), and Rio Grande cichlid (1.0/hour).
• Catfishes: The gill netting catch rate for channel catfish in 2005 (3.5/net night) was the lowest reported since 1991 (3.4/net night). However, these rates were only slightly below the average for other surveys conducted within this time period (average CPUE 1994 – 2001 = 5.0/net night; Nsurveys = 6; Range = 4.7 – 5.3/net night). Size structure was consistently dominated by fish measuring 9 to 18 inches, but larger fish (21 – 24 inches) were routinely collected. Most relative weights ranged from 80 to 110, and typically increased with fish size. This pattern was similar among recent samples. One blue catfish measuring 24 inches was collected in the 2005 gill net survey (0.1/net night). Blue catfish were never stocked in this reservoir, but most likely immigrated from upstream reservoirs where stockings occurred. Blue catfish were collected in each of the past four gill net surveys, but never in large numbers (< 0.6/net night). Flathead catfish were collected in the 2005 gill netting survey (1.6/net night). Historical catch rates were consistent and all less than 1.4/net night.
• White bass: This reservoir supported a low density white bass population. The 2005 gill net catch rate (1.6/net night) was slightly lower than the average for the previous five surveys (2.6/net night), but within the range (1.0 – 5.7/net night) of prior sampling variability. White bass 8 to 16 inches were represented in the sample, and most relative weights were between 80 and 90. The experimental 12-inch length limit on white bass implemented in 1995 was returned to the statewide 10-inch length limit in 2003. The experimental length limit failed to meet the objectives of increasing white bass abundance and stabilizing fluctuations in year-class production.
• Black basses: Six individual largemouth bass, ranging from 5 to 7 inches in length, were collected during an electrofishing survey in 2002. No largemouth bass were collected in the 2004 electrofishing survey.
• White crappie: The 2004 trap net catch rate for white crappie was the lowest recorded for this reservoir (0.8/net night). The average catch rate for years 1989 through 2000 (N = 5 surveys) was 2.5/net night, and ranged between 1.0/net night (1994) and 4.5/net night (2000). Relative weight values for white crappie were typically between 90 and 100. Sample sizes of white crappie were generally too small to adequately describe growth.

Management Strategies

Based on current information, the reservoir should continue to be managed with existing regulations. Florida largemouth bass fingerling stockings in 2001 and 2002 failed to increase the genetic contribution of the Florida subspecies in a 2004 age-0 sample compared to a similar sample collected in 2000. Population characteristics of the largemouth bass population have been consistent across recent surveys. However, the introduction of hydrilla and the expansion of Eurasian watermilfoil have the potential to alter the population dynamics of bass and their prey. Electrofishing surveys should be conducted in 2006 and 2008 to monitor population characteristics of the largemouth bass, sunfishes, and shad species.

Survival and expansion of introduced native aquatic plants has mostly been disappointing, except for translocated emergent plants. Additional plantings of submerged species such as waterstargrass, American pondweed, and Vallisneria sp. should discontinue. Future efforts should be made to maintain protective cages surrounding surviving aquatic plants and translocating existing waterwillow and spatterdock to locations devoid of habitat. Hydrilla, water hyacinth, and Eurasian watermilfoil have the potential to rapidly spread beyond coverage observed in 2004. Aquatic vegetation surveys should be conducted annually until exotic aquatic plant growth has stabilized.

Performance Report as required by Federal Aid in Sport Fish Restoration Act Texas Federal Aid Project F-30-R-30 Statewide Freshwater Fisheries Monitoring and Management Program