Lake Mead Science Symposium Concurrent Sessions

Tuesday Afternoon Emerging Issues Abstracts Limnology and Water Quality Abstracts

Wednesday Morning Aquatic Biota and Fisheries Abstracts Riparian and Shoreline Resources Abstracts

Wednesday Afternoon Environmental Contaminants Lake Management

AQUATIC BIOTA AND FISHERIES
Session Chairs:
Jon Sjöberg and Erik Orsak
Wednesday, January 14, 2009 * 8:30 am to 11:45 am * Ballroom

8:35 – 8:55 am
Lake Mead Razorback Sucker Recruitment:  An Informative Anomaly Regarding Continued, Natural, Wild Razorback Sucker Recruitment Despite Nonnative Fish Presence
Kegerries, Ron B.; Albrecht, Brandon; and Holden, Paul B., BIO-WEST, Inc.

An ongoing razorback sucker (Xyrauchen texanus) research project on Lake Mead, Arizona and Nevada has been funded by the Southern Nevada Water Authority and the U.S. Bureau of Reclamation for 12 years. This study continues to document the continued presence of wild razorback sucker recruitment in the form of young, sexually immature individuals. This recruitment denotes the Lake Mead razorback sucker population as an anomaly in terms of razorback sucker persistence throughout the Colorado River drainage, despite similar non-native fish composition and densities as other locations. Fin ray aging data and back-calculation techniques have indicated that recruitment of razorback sucker on Lake Mead has occurred nearly every year. It was once thought that high lake elevations coincided with high recruitment. However, beginning with the 2007 spawning period we have captured individuals spawned during low-water years.

We believe that cover, both vegetative and in the form of turbidity, provides protection and food resources for larval and juvenile razorback sucker, thereby enabling them to avoid predation by nonnative sport fish present in the system. Interestingly, it appears as though turbidity may be even more important than we have typically considered. As monitoring efforts on Lake Mead continue we hope to further document wild recruitment, and perhaps begin to understand more fully how to enable this unique trend in other locations. We suggest that Lake Mead provides a view of what naturally recruiting razorback sucker populations look like in the real world of nonnative predators.  

9:00 – 9:20 am
The Development of the Lake Mohave Striped Bass (Morone saxatilis) Fishery and its Impact on the Threadfin Shad (Dorosoma petenense) Population and the Stocked Rainbow Trout (Oncorhynchus mykiss) Fishery
Burrell, Michael D., Nevada Department of Wildlife

In 1980 Lake Mohave sport fishing was supported by a stocked rainbow trout fishery and a self reproducing population of largemouth bass (Micropterus salmoides) and channel catfish (Ictalurus punctatus). There was a large population of threadfin shad. Striped bass were never intentionally stocked. They are believed to have entered the lake through angler introductions or more likely by passing through or over Hoover Dam as eggs, larvae, or possibly older life stages. They were first documented in Lake Mohave in the early 1980's. Reproduction was documented by the mid 1980's. By 1990 they had become the primary sport fish. The striped bass population is comprised primarily of fish less than 50 cm. The availability of medium size forage is limited. The result is poor recruitment of fish greater than 50 cm. A small percentage, however, does pass through the forage bottleneck and can grow to exceptional size by utilizing stocked trout and carp as forage. As the striped bass population grew they quickly eliminated threadfin shad. Unlike Lake Mead, Lake Mohave lacks the turbid inflows that provide refuge areas where a core population of shad can be maintained. By 1996 threadfin shad were essentially gone. As the number of large striped bass increased in the mid 1980's the stocked rainbow trout fishery declined quickly. Predation by striped bass and an angler preference change from rainbow trout to striped bass resulted in declining harvest numbers. Stocking procedures evolved to adapt to the deleterious effects of the growing striped bass population.

9:25 – 9:45 am
The History of Threadfin Shad in Lake Mead and Their Importance to the Sport Fishery
Beckstrand, Mark, Nevada Department of Wildlife

The threadfin shad (dorosoma petenense) is a member of the herring family (Clupeidae). Threadfin shad are native to the southeastern United States but have been widely introduced to waters outside their native range. They have become a popular species for use in creating a forage base for larger game fish. Lake Mead supported an excellent warm water fishery after its formation in 1935. But by 1941, fishermen of the area became concerned when many of the largemouth bass were found to be thin and in poor condition. This poor condition was thought to result from a lack of adequate forage species. Threadfin shad were first released into Lake Mead in 1954 and by 1956 had become established throughout the reservoir. Soon after the successful introduction of threadfin shad to Lake Mead, all species of game fish were found to be utilizing shad as forage, which resulted in better body condition and growth for those species. Today, threadfin shad are the principle forage species for game fish in Lake Mead. But, recent introductions of gizzard shad (Dorosoma cepedianum) and the quagga mussel (Dresseina bugensis) to Lake Mead pose potential threats to this important species and, in turn, have a negative impact on the whole sport fishery of Lake Mead.

10:05 – 10:25 am
Value of Original Data Sets for Interpreting Ecological Trends for Lake Mead
Burke, Thomas A., U.S. Bureau of Reclamation

The recent invasion of Lake Mead by the exotic quagga mussel has spawned research and monitoring actions to assess the ecological impact of this new species. The first step is to acquire and review the existing baseline data about the physical, chemical and biological conditions of the Lake Mead prior to this invasion. As the outcome of the assessment will be influenced by the interpretation of these historical data, it is paramount that researchers review the original data or data-sets, and not the summarized data, in conducting their analyses. Case studies are presented for two Lake Mead data sets wherein investigators failed to review the original data, resulting in erroneous conclusions. The first case compared limnetic zooplankton densities of Lake Mead across a ten year period (1971-1980), concluding that a six-fold decline had occurred. It was later found that the original 1971 data-set presented numbers of zooplankton per sample, not per liter and that the sampler used was a six-liter Van Dorn bottle, hence no six fold change in the community. In the second case, sport fish creel data for Lake Mead, showed a harvest of over 890,000 largemouth bass in 1963, which declined to 360,000 by 1974. Coincidentally, Lake Powell began filling in 1963. These harvest data were instrumental in acquiring funding for a five-year black bass study. Investigations after the study was completed found the original 1963 estimate to be in error. Due to funding limits, creel methods and analyses were modified in 1963, roughly doubling the harvest estimate. In each case no malice was intended. The authors of the original data had not misrepresented it. However, the investigators who later reached back in time, extracted only the data summaries. Had an adequate review been conducted on the original data-set, these original data would not have been misrepresented.

10:30 – 10:50 am
Threadfin Shad and Invasive Quagga Mussels in Lake Mead, Nevada
Loomis, Eric; Wong, David; and Gerstenberger, Shawn, University of Nevada, Las Vegas

The recent introduction of the invasive bivalve quagga mussel (Dreissena bugensis) has the potential to impact trophic food webs and the aquatic environment. Shad are the primary forage fish for striped bass, largemouth bass, and channel catfish and may exert great influences over this system. It’s unclear what the effects of quagga mussels will have on the fishery. The lake has already seen massive changes in zooplankton abundance and distribution. Larval and adult threadfin shad (Dorosoma petenense), which utilize zooplankton as a critical food source, have been collected in Las Vegas Bay and Overton Arm, Lake Mead through joint cooperation with the Nevada Department of Wildlife and funded through the National Park Service. This study is to determine feeding habits and health status (k-factor), as well as diets of shad since the mussel invasion. As quagga mussels proliferate throughout Lake Mead, the food chain could be altered in a manner that threatens the sport fishery by reducing the most important forage fish in the lake—threadfin shad. The abundance of shad in 2008 is one of the two lowest data points in an 18-year record, though the overall trend is not clear. This study will serve as a baseline of projected long-term monitoring programs through various inter-agency involvement related to the recent invasion of quagga mussels to the southwest.

10:55 – 11:15 am
Response of the Algal Community in Boulder Basin, Lake Mead to the Introduction of Quagga Mussels and Reduced Water Levels
St. Amand, Ann, L.1; Roefer, Peggy2; LaBounty, James, F.2; Bolt, Dustin, M.1 (1) PhycoTech, Inc. and (2) Southern Nevada Water Authority

Algal data were analyzed at several surface stations in Boulder Basin, Lake Mead from 2002-2008. Stations ranged from the Las Vegas Bay to just upstream of the Hoover Dam and included a variety sites, including the intake for the drinking water treatment plant at depth. The volume of the reservoir has been declining in recent years, although water treatment efforts have substantially decreased input nutrients, especially phosphorus (1994 and 2006 forward). Quagga mussels (Dreissena rostriformis bugensis) were first detected in Lake Mead in January 2007, but archival samples indicated that the mussels had been present at low levels since at least November 2006. Quagga and zebra mussels have been associated with a shift towards toxin producing Cyanophytes in other systems, including the Great Lakes, and have also been associated with ecosystem changes relating to zooplankton food supply. Both raw data (density, total biovolume, total area, total volume, GALD) and water quality indices and ratios were analyzed correlating results to several key environmental variables (water level, surface temperature, salinity, nutrient levels),  and the invasion of quagga mussels. In addition to total community indicators, several species/genera were also used in the analysis including Pyramichlamys cordiformis (Tetraselmis), Certium hirundinella, Cylindrospermopsis raciborskii, Aphanizomenon flos-aquae, Microcystis aeruginosa, Cyclotella comta var. bodanica, Cyclotella ocelllata, and several key species of picoplankton that have become increasingly important since 2005 (Cyanogranis ferruginea and Cyanocatena plantonica). Both time series analysis and multidimensional scaling analysis (Primer 6) were used to explore ecological relationships among algal indicators and environmental variables.

11:20 – 11:40 am
The Contemporary Food Web Structure of Two Bays in Lake Mead      
Umek, John1; Chandra, Sudeep1; Rosen, Michael2; Goodbred, Steven2; Orsak, Erik3 (1) University of Nevada, Reno (2) U.S. Geological Survey (3) U.S. Fish and Wildlife Service

Lake Mead is the largest man-made lake in the United States and supports a robust recreational fishery. There is little contemporary information, however, regarding the ecology and food-web structure of the lake. This study provides a contemporary snapshot of the zooplankton and benthic invertebrate community and the lake's food-web structure in two bays: Overton Arm and Las Vegas Bay.  Special emphasis is placed on understanding the biomass of primary and secondary producers. Benthic invertebrate biomass was greater in Overton Arm than Las Vegas Bay with variability over two seasons. Zooplankton biomass was variable in time and space across the bays. The food-web structure determined from stable isotope measurements (carbon and nitrogen) and stomach-content data suggests that the bottom habitats drive both Las Vegas Bay and Overton Arm food webs, and shad do not contribute to the large game, striped bass (Morone saxatilis) and large mouth bass (Micropterus salmoides), energetics. Carp, a model surrogate species used to assess exposure of emerging contaminants obtain most of their energy from the benthic invertebrate community, not from the bottom algae or plants in both bays.

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