Granger 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 32-page report. For a copy of the complete report, use the download link in the sidebar.

Granger 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. An angler creel survey was conducted in spring (March – May) 2005. This report summarizes the results of these surveys and contains a fisheries 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. The reservoir is located approximately 40 miles northeast of Austin, Texas, within the Brazos River drainage. It was constructed in 1980 by the U. S. Army Corps of Engineers (USACOE) 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 40 miles.

A significant portion of the reservoir is bordered by the Granger Wildlife Management Area. The USACOE controls four parks. Each park provided camping and a boat ramp. Bank access was good within the park boundaries and Wilson Fox park on the south shore contained a fishing pier accessible to physically challenged persons.

Aquatic Vegetation

Turbid water limited the presence of submerged aquatic plants. Water clarity, expressed as secchi depth, was typically less than 1 foot, but clarity generally increased toward the dam. No aquatic plants were observed growing in Granger Reservoir prior to 2003. In 2003, hydrilla was discovered within the Wilson Fox Park boat ramp cove. The USACOE conducted a herbicide treatment in 2003, and no hydrilla was observed in 2004. However, the 2004 aquatic vegetation survey documented several isolated patches of the exotic, floating aquatic plant water hyacinth in the upper San Gabriel arm of the reservoir. Total coverage of these colonies was 0.3 acres. Following heavy rains and high reservoir inflows, no hyacinth plants were observed during survey trips in Spring 2005.

Spring 2005 Creel Survey

Pole and line anglers expended an estimated 36,542 (RSE = 13.2) hours (9.1 hours/acre) of fishing during daylight hours from March through May 2005. Total angler trip expenditures were estimated at $172,222 (RSE = 41.4). Most (61.8%) anglers traveled less than 30 miles, and 92.7% traveled less than 50 miles to fish this reservoir. 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%).

Fish Community

• Prey species: Gizzard shad electrofishing catch-per-unit-effort (CPUE) in 2004 was 219.0/hour, and appeared to be dominated by age-0 fish. No gizzard shad were aged, but fish measuring less than 5 inches were assumed to be age-0. Annual variability in production of young shad has occurred, but catch rates of older shad (> 7 inches) have typically been less than 10/hour. The index of vulnerability (IOV) for the gizzard shad sample was 100, which indicates that all gizzard shad were less than 8 inches in length, making them susceptible to predation by most predators. The IOV values have remained similar between surveys, ranging from 95 (2000) to 100 (2004), indicating that the gizzard shad population was dominated by young fish. Threadfin shad were collected at the rate of 21.0/hour in 2004 compared to 2000 (44.7/hour) and 1997 (23.3/hour). Bluegill electrofishing CPUE (40.0/hour) was higher than previous surveys (1997 =12.7/hour; 2000 = 0/hour), but still could be described as low. Longear sunfish CPUE (28.0/hour) and size structure were also consistent with previous surveys. No redbreast sunfish were collected in 2004, but were sampled in past surveys (2000 = 2.0/hour). Warmouth were also collected in low numbers (8.0/hour). Sunfish recruitment could be negatively impacted by abundant gizzard shad through inter-specific competition for zooplankton at larval stages (Noble 1981), or because of lack of suitable habitat.
• Catfishes: Blue catfish were stocked in 1995 and 1996 to take advantage of abundant shad and to provide additional angling opportunities. Gill nets collected few of these fish in 1997 (1.8/net night), 2001 (0.2/net night), and 2003 (0.3/net night). However in 2005, blue catfish became the dominant catfish species collected (3.0/net night). Recruitment of young blue catfish to adult sizes was evidenced by the expanded size distribution. The blue catfish expansion has occurred concurrent with a decline in channel catfish numbers. The gill netting catch rate for channel catfish in 2005 (0.9/net night) was the lowest recorded for this reservoir. Catch rates have steadily declined since 1997 (1997 = 9.0/net night; 2001= 2.4/net night; 2003 = 1.8/net night). No evidence exists to link the blue catfish expansion to the channel catfish decline. However, this relationship is worth noting. Flathead catfish were collected in the 2005 gill netting survey in low numbers (0.4/net night). Historical catch rates have varied but were typically less than 1.0/net night. Flathead catfish were the least abundant catfish species present, but gave anglers the opportunity to catch large fish. In the Spring (March – May) 2005 creel survey, 16.8% (1.5 hours/acre) of pole-and-line angler effort was directed for catfishes. Anglers specifically targeting blue catfish comprised the highest percentage (31.4%) of that effort. Anglers targeting channel catfish and flathead catfish were too few to estimate meaningful catch statistics. The estimated catch rate for anglers targeting blue catfish was 0.26 fish/hour (RSE = 22.7). Of the estimated 1,365 (RSE = 184.3) blue catfish caught, 78.3% were harvested. Ninety-four percent of legal-sized blue catfish caught were harvested. Size distribution of blue catfish measured during creel survey interviews ranged from 14 to 32 inches. Passive gear (i.e., trot lines and jug lines) anglers were not enumerated or interviewed during the creel survey. However, anecdotal reports suggested that passive gear angling for catfishes was a popular activity on this reservoir.
• White bass: White bass gill net catch rate (2.9/net night) was slightly higher than recent surveys. Since 1991, white bass gill net catch rates have been consistent, averaging 1.3/net night and ranging from 0.8 – 2.0/net night. In the two most recent surveys, white bass size distribution ranged from 6 to 16 inches. White bass anglers comprised 5.0% (0.5 hours/acre) of the total directed angling effort during the Spring 2005 creel survey. Angler catch rate for white bass was high (3.3 fish/hour; RSE = 60.7). Of the 11,321 (RSE = 38.1) white bass estimated harvested during the creel survey period, 30.8% were harvested. Of the legal-sized white bass caught, 82.2% were harvested by anglers. Harvested white bass observed during creel interviews measured between 10 and 16 inches in length.
• Largemouth bass: Largemouth bass electrofishing catch rate (7.0/hour) in 2004 was similar to the low catch rate in 2000 (4.0/hour). Largemouth bass relative abundance declined since the 1980s (1989 = 42.7/hour, 1991 = 30.7/hour, 1994 = 28.7/hour, 1997 = 15.3/hour). As in recent surveys, the few adult largemouth bass sampled exhibited exceptional body condition which is indicative of a low-abundant population with access to a surplus of prey. Possible reasons for low density are lack of suitable spawning and/or juvenile habitat, or indirect negative impacts of an abundant gizzard shad population depressing sunfish prey numbers (Kirk and Davies 1985). Low sunfish production could negatively impact age-0 largemouth bass recruitment in situations when most young-of-the-year largemouth bass were not large enough to prey on fast growing age-0 gizzard shad (Allen et al. 1999). Anglers spent 0.2 hours/acre fishing for largemouth bass at Granger Reservoir during the Spring 2005 creel survey. Only 2.5% of all anglers targeted largemouth bass. Bass angling popularity among anglers ranked fourth behind white crappie, catfishes, and white bass. Too few angler interviews precluded the estimation of meaningful catch statistics for this species.
• Crappie: Granger Reservoir supported a good white crappie population. Trap net catch rates during 2004 were 13.3 fish per net night, which was near the upper range for previous years (Range 1991 – 2003 = 2.8 – 15.3/net; Mean = 8.0; SE = 1.4). Evidenced by age-0 catch rates, white crappie produced strong year classes in 1997, 2001, and 2003. Strong year classes in 2001 and 2003 were partly responsible for higher than average trap net catch rates from 2001 through 2004 (> 10.0/net). White crappie size structure was good with a variety of sizes (Range = 2 – 14 inches) represented in samples. Growth of white crappie in Granger Reservoir was good. Average age at 10 inches was 1.8 years (N = 15; Range = 1 – 3 years). Granger Reservoir white crappie were heavier on average (mean Wr = 105.9; N = 140; SD = 7.4) than crappie of similar lengths in most reservoirs as evidenced by relative weights greater than 100. This indicated that prey resources were not in short supply. White crappie were the most sought after species by anglers interviewed in the Spring 2005 creel survey (61.5% total effort; 5.6 hours/acre). Bank anglers expended similar effort (2.7 hours/acre) compared to boat anglers (2.9 hours/acre), and experienced similar catch rates (CPUEbank = 1.3 fish/hour; CPUEboat = 1.4 fish/hour). White crappie anglers caught an estimated 55,057 (RSE = 22.0) white crappie, of which 41.8% were harvested. Of legal-sized white crappie caught by anglers, 96.2% were harvested. The majority of white crappie harvested were between 10 and 14 inches. Two percent of harvested white crappie measured during the creel survey were less than the minimum size limit (i.e., 10 inches).

Management Strategies

Based on current information, the reservoir should continue to be managed with existing regulations. White crappie anglers accounted for the majority of fishing effort during spring months. Anecdotal angler reports attest to the popularity of this species throughout the year. Year class production and relative abundance have fluctuated in past years. Trap net surveys
should be conducted annually to better monitor the population dynamics of this species. Bank anglers were equally successful at catching white crappie during spring months compared with boat anglers. The shallow/near-shore movements of spawning white crappie allowed these fish to be accessible to bank anglers during this time. Public bank access was good inside park boundaries and on a fishing pier. Many anglers without access to a boat may not be aware of bank fishing opportunities on this reservoir. Bank fishing opportunities should be communicated to anglers through appropriate media outlets.

The blue catfish population has established a self-sustaining population. Additional blue catfish stockings are not warranted. The channel catfish population continued to decline following the stocking of blue catfish. Anglers may be able to improve success by altering angling techniques to target blue catfish. This strategy should be communicated to local anglers through appropriate media outlets.

An herbicide treatment eliminated hydrilla from the Wilson Fox boat ramp cove in 2003. High reservoir inflows eliminated newly discovered (Summer 2004) water-hyacinth plants from the upper San Gabriel arm of the reservoir by Spring 2005. These exotic aquatic plants have the potential to rapidly spread and inhibit bank fishing access. Annual surveys should be conducted to monitor for the presence of these two aquatic plants. If hydrilla or water hyacinth plants return, treatment options should be coordinated with the USACOE.

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