U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006-3131

Version 1.1, October 2007

Prepared in cooperation with the U.S. Fish and Wildlife Service and National Park Service

Investigations of the Effects of Synthetic Chemicals on the 

Endocrine System of Common Carp in Lake Mead, Nevada 

and Arizona

By Michael R. Rosen, Steven L. Goodbred, Reynaldo Patiño, Thomas J. Leiker, U.S. Geological Survey, and 

Erik Orsak, U.S. Fish and Wildlife Service

Introduction

Lake Mead is the largest reservoir by volume in the United 

States and was created by the construction of the 221-meter 

high Hoover Dam in 1935 at Black Canyon on the lower 

Colorado River between Nevada and Arizona (fig. 1). Inflows 

of water into the lake include three rivers, Colorado, Virgin, 

and Muddy; as well as Las Vegas Wash, which is now perennial 

because of discharges from municipal wastewater treatment 

plants (Covay and Leiker, 1998) and urban stormwater runoff. 

As the population within the Las Vegas Valley began to increase 

in the 1940s, the treated effluent volume also has increased and 

in 1993 it constituted about 96 percent of the annual discharge 

of Las Vegas Wash (Bevans and others, 1996). The mean flow 

of Las Vegas Wash into Las Vegas Bay from 1992 to 1998 was 

about 490,000 m3

/d (Preissler and others, 1999) and in 2001 

increased to 606,000 m3/d (U.S. Bureau of Reclamation, 2001). 

The nutrient concentration in most areas of the lake is low, 

but wastewater discharged into Las Vegas Bay has caused an 

increased level of nutrients and primary productivity (aquatic 

plant and algal production) in this area of the lake (LaBounty 

and Horn, 1997). A byproduct of this increase in productivity 

has been the establishment of an important recreational fishery 

in Las Vegas Bay. However, concentrations of chlorophyll a

(a measure of algal biomass) have also increased (LaBounty 

and Horn, 1997). In the spring of 2001, parts of Lake Mead 

experienced massive algal blooms.

In addition to nutrient loading by wastewater, the presence 

of numerous synthetic chemicals in water, bottom sediments, 

and in fish tissue also has been reported (Bevans and others, 

1996). Synthetic chemicals discharging into Las Vegas Bay 

and Lake Mead (fig. 1) originate from several sources that 

include surplus residential-irrigation water runoff, stormwater 

runoff, subsurface inflow, and tertiary treated sewage effluent 

discharging from three sewage-treatment plants. Chemicals 

detected in Las Vegas Wash and Bay environments include 

polycyclic aromatic hydrocarbons (PAHs), polychlorinated 

biphenyls (PCBs), organochlorine pesticides (including DDT 

and DDE), and “emerging contaminants” such as fragrances/

musks, flame retardants, triclosan and its breakdown products, 

Figure 1. Location of sampling sites in published studies and proposed sites of synthetic 

chemicals and endocrine disruption in Lake Mead.
personal care products, and pharmaceuticals (Bevans and 

others, 1996; Boyd and Furlong, 2002; Goodbred and others, 

2007). Many of these compounds are able to interact with the 

endocrine system of animals and potentially cause reproductive 

impacts.

The National Park Service (NPS) manages Lake Mead 

National Recreation Area (LMNRA) with about 8 million 

yearly visitors including 500,000 anglers drawn to its worldclass recreational fishery. The U.S. Fish and Wildlife Service 

(FWS) provides management for the federally designated, 

endangered razorback sucker (Xyrauchen texanus) and for 

more than 180 species of migratory birds that utilize LMNRA 

surface waters. These multiple uses of surface water in the area 

demonstrate their vital importance to the LMNRA as well as the 

need to maintain the quality of water at levels that are adequate 

for these uses.

Previous Studies of Endocrine Disruption

A pilot study of Lake Mead was conducted in 1995 

by the U.S. Geological Survey (USGS) and other Federal 

agencies including NPS and FWS. The purpose of this study 

was to investigate the potential for endocrine disruption in fish 

from the lake. Male and female common carp (Cyprinus carpio) 

were sampled in Las Vegas Wash, Las Vegas Bay and Callville 

Bay (fig. 1) once during the spawning season and analyzed 

for several endocrine and reproductive biomarkers. The study 

provided evidence suggesting that common carp from Las 

Vegas Wash and Bay exhibit endocrine disruption compared to 

fish from Callville Bay (Bevans and others, 1996). One notable 

finding was that male fish from Las Vegas Bay had significantly 

lower levels of the sex steroid hormone 11-ketotestosterone 

(11KT), a major androgen responsible for testicular function 

and sperm production in fishes (Schulz and Miura, 2002). 

Some evidence of endocrine disruption also was reported for 

female carp from Las Vegas Wash and Bay, such as altered 

ratios of sex hormones (Bevans and others, 1996).

The study of Bevans and others (1996) also provided 

evidence of a higher number and higher concentrations of 

environmental contaminants in lake sediment, water, and fish 

tissue from Las Vegas Bay and Wash relative to Callville Bay. 

The contaminants found included organochlorines (e.g., DDT 

and DDE), PAHs, furans, phthalates, phenols, and PCBs. Unlike 

Las Vegas Wash and Bay, Callville Bay has no point source 

discharges. Thus, the likely sources of synthetic chemicals in 

Las Vegas Wash and Bay are wastewater discharges and urban 

runoff flowing into Las Vegas Wash.

The report of Bevans and others (1996) raised concerns 

about the general quality of the aquatic habitat in the Las 

Vegas Wash and Bay regions of the lake and prompted a 

more thorough study. A follow-up collaborative study by the 

Department of the Interior (USGS; FWS; NPS; Bureau of 

Reclamation; University of Nevada, Las Vegas; University 

of Florida, Gainesville; Texas Tech University; and Nevada 

Division of Wildlife) was conducted in 1999–2000. The 

purpose of this study was to determine if the earlier evidence 

of endocrine disruption could be confirmed during a oneyear, full reproductive cycle (May 1999 to May 2000); and to 

What is the endocrine system?

The endocrine system is composed of glands and 

tissues including the hypothalamus, pituitary, thyroid, 

adrenal, thymus, pancreas, ovaries, testes and others. These 

glands produce and release hormones into the bloodstream 

that travel to different parts of the body to control activities 

such as development, growth, reproduction and behavior. 

In some animal species, hormones also determine whether 

individuals become male or female during their early 

development.

What is an endocrine disruptor?

An endocrine disruptor is a natural or synthetic 

chemical that when taken up by an organism mimics, 

modifies or blocks the actions of hormones and disrupts 

normal physiology. Endocrine disruption occurs through 

alterations in the production or metabolism of hormones 

or their delivery in the bloodstream, or through direct 

actions on the tissues regulated by hormones. Synthetic 

chemicals known to cause endocrine disruption include 

diethylstilbestrol, ethynylestradiol, dioxins, PCBs, DDT/

DDE, perchlorate, flame retardants, some heavy metals, 

and others. A great deal of attention has been placed on 

endocrine disruptors with estrogenic (feminizing) activity.

Why is knowledge of endocrine disruption important?

At levels observed in the environment, many synthetic 

chemicals cause endocrine disruption in laboratory 

animals. Some field studies also have implicated endocrine 

disruption as a factor contributing to the impaired health of 

fish and wildlife populations. 

For further information see National Institute of 

Environmental Health Sciences endocrine fact sheet 

http://www.niehs.nih.gov/oc/factsheets/pdf/endocrine.pdf

Hydrologist deploying passive samplers to 

sample organic compounds in Lake Mead.
further characterize environmental contaminants in Lake Mead 

including emerging compounds such as pharmaceuticals and 

personal care products. Data from this new study would also 

serve as a baseline to monitor the reproductive condition of 

fish in Lake Mead and as reference point for other research 

and monitoring studies of fish reproduction. The study 

compared observations made in Las Vegas Bay against those 

in a reference site, Overton Arm. Results for a number of 

biomarkers of reproductive condition were described by Patiño 

and others (2003). Male carp from Las Vegas Bay had smaller 

testes (gonadosomatic index) and higher levels of testicular 

macrophage aggregates (biomarkers of contaminant exposure) 

Common carp held in livewell before processing.

Specially designed electroshocking boat for sampling fish in 

Lake Mead.

Biologist dissecting otoliths for aging common carp.

Janet Kirsch, Bureau of Reclamation

throughout the year (Patiño and others, 2003). However, 

an assessment of female fish indicated that differences in 

reproductive biomarkers between Las Vegas Bay and Overton 

Arm could be explained by differences in water temperature 

profiles between the two sites through the annual cycle of 

seasons. This observation with female fish highlighted the need 

for seasonal sampling designs in field studies of endocrine 

disruption. Patiño and others (2003) concluded that, in 1999–

2000, the aquatic environment in Las Vegas Bay specifically 

impacted the reproductive condition of male fish.

What environmental factors or synthetic compounds were 

responsible for altering the reproductive condition of male fish 

in the studies of Bevans and others (1996) and Patiño and others 

(2003)? Although the answer to this question is uncertain, 

results of a study by Covay and Beck (2001) indicated the 

presence of twice as many organic contaminants in sediment 

from Las Vegas Bay compared to Overton Arm (samples for 

this study were collected in 1998). They also found that the 

concentrations of pesticides and PCBs in sediment were over 

one order of magnitude higher at Las Vegas Bay. In addition, 

dioxins and furans were present in sediment of Las Vegas Bay 

but not Overton Arm (Covay and Beck, 2001). Goodbred and 

others (2007) recently reported the presence of degradation 

products of triclosan, a commonly used antimicrobial 

compound, in the same male fish from Las Vegas Bay that 

were analyzed by Patiño and others (2003) but not in male 

fish from the reference site in Overton Arm. Water samples 

collected from Las Vegas Wash and Bay in 1997 contained 

feminizing compounds such as 17β-estradiol (natural estrogen) 

and 17α-ethynylestradiol (a synthetic estrogen used in oral 

contraceptives), but samples from references sites (Callville 

Bay) tested negative for these compounds (Snyder and others, 

2001). Bevans and others (1996) had previously reported the 

presence of several other synthetic estrogens (xenoestrogens) 

in Las Vegas Wash and Bay during 1995. Although the specific 

compound(s) that are causing the altered reproductive condition 

in male fish from Las Vegas Bay can not be identified yet, 

ongoing studies of Lake Mead are being conducted by the 

Federal government to help answer this question. Most of 

the evidence from laboratory and field studies suggests that 

xenoestrogens are a primary cause of feminization of male fish 

(Sumpter and Johnson, 2005).

Ongoing Studies of Endocrine Disruption 

Since the previous studies of Lake Mead were completed, 

the southwest region of the United States has been experiencing 

a drought that has caused current lake levels to be at historic 

lows. This change in limnological conditions could potentially 

exacerbate or change the patterns of contaminant distribution 

and endocrine disruption in Las Vegas Bay and other sites of the 

lake. Moreover, wastewater dischargers plan to divert a majority 

of their wastewater from Las Vegas Wash and discharge it 

through a submerged pipeline in the Boulder Basin (fig. 1). 

The pipeline will include a discharging diffuser designed to 

provide the capability to discharge wastewater at a number 

of depths, including depths below the thermocline and photic 

zone. This change might result in wastewater being introduced 

for the first time into new parts of the LMNRA. New studies 
are now in place to (1) assess the potential effects of droughtinduced alterations in the hydrology of the lake, and (2) provide 

baseline information to monitor changes in contaminant 

distribution and potential for endocrine disruption that may 

occur due to the redistribution of wastewater inflow sites in the 

Lake Mead and Colorado River ecosystems. These new studies 

are wider in geographic and scientific scope and address not 

only endocrine disruption and the presence and distribution of 

chemical contaminants, but also contaminant sediment flux and 

microbiology, food web dynamics, and wastewater treatment 

effects. Such comprehensive studies have never been conducted 

within the LMNRA.

References

Bevans, H.E., Goodbred, S.L., Miesner, J.F., Watkins, S.A., Gross, 

T.S., Denslow, N.D., and Schoeb, T., 1996, Synthetic organic 

compounds and carp endocrinology and histology in Las Vegas 

Wash and Las Vegas and Callville Bays of Lake Mead, Nevada, 

1992 and 1995: U.S. Geological Survey, Water-Resources 

Investigations Report 96–4266, 12 p.

Boyd, R.A., and Furlong, E.T., 2002, Human-Health 

Pharmaceutical Compounds in Lake Mead, Nevada and Arizona, 

and Las Vegas Wash, Nevada, October 2000–August 2001: U.S. 

Geological Survey Open-File Report 02-385.

Covay, K.J., and Leiker, T.J., 1998, Synthetic organic compounds 

in water and bottom sediment from streams, detention basins, 

and sewage-treatment plant outfalls in Las Vegas Valley, Nevada, 

1997: U.S. Geological Survey Open-File Report 98–633, 15 p.

Covay, K.J., and Beck, D.A., 2001, Sediment-deposition rates and 

organic compounds in bottom sediment at four sites in Lake 

Mead, Nevada, May 1998: U.S. Geological Survey Open-File 

Report 01–282, 34 p.

Goodbred, S.L., Leiker, T.J., Patiño, R., Jenkins, J.A., Denslow, 

N.D., Orsak, E., and Rosen, M.R., 2007, Organic chemical 

concentrations and reproductive biomarkers in common carp 

(Cyprinus carpio) collected from two areas in Lake Mead, 

Nevada, May 1999 through May 2000: U.S. Geological Survey 

Data Series 286, 18 p.

LaBounty, J.F., and Horn, M.J., 1997, The influence of drainage 

from Las Vegas Valley on the limnology of Boulder Basin, 

Lake Mead, Arizona-Nevada: Journal of Lakes and Reservoir 

Management, v. 13, p. 95–108.

Patiño, R., Goodbred, S.L., Draugelis-Dale, R., Barry, C.E., Foott, 

J.S., Wainscott, M.R., Gross, T.S., and Covay, K.J., 2003, 

Morphometric and histopathological parameters of gonadal 

development in adult common carp from contaminated and 

reference sites in Lake Mead, Nevada: Journal of Aquatic 

Animal Health, v. 15. p. 55-68.

Preissler, A.M., Roach, G.A., Thomas, K.A., and Wilson, J.W., 

1999, Water Resources Data, Nevada Water Year 1998: U.S. 

Geological Survey Water Data Report NV-98-1.

Schulz, R.W., and Miura, T., 2002, Spermatogenesis and its 

endocrine regulation: Fish Physiology and Biochemistry, 

v. 26, p. 43–56.

Snyder, S.A., Villeneuve, D.L., Snyder, E.M., and Giesy, J.P., 2001, 

Identification and quantification of estrogen receptor agonists in 

wastewater effluents: Environmental Science and Technology, 

v. 35, p. 3620–3625.

Current Knowledge of Endocrine Disruption

•	 Studies conducted in 1995 and 1999–2000 showed that male carp from Las Vegas Bay have low blood levels of androgen and 

smaller testes compared to male fish from reference sites.

•	 The same studies and others also showed the presence of higher levels of synthetic chemicals in water, sediment and fish from 

Las Vegas Bay compared to reference sites.

•	 Commonly used products known as emerging contaminants, such as triclosan (an antimicrobial drug), are being accumulated in 

fish from Las Vegas Bay.

•	 Some of the chemicals present in Las Vegas Bay have been shown by laboratory studies to cause endocrine disruption in male 

fish.

Collaboration within the Lake Mead Project 

The results obtained from the ongoing projects are expected 

to greatly expand baseline chemical and biological data 

that will assist in management decisions and future 

monitoring efforts. The studies are being led by the USGS 

in partnership with the U.S. Fish and Wildlife Service; 

National Park Service; Bureau of Reclamation; University 

of Nevada, Las Vegas; University of Nevada, Reno, Desert 

Research Institute; and Texas Tech University.

For further information about this study, contact

Michael R. Rosen, Research Hydrologist

USGS Nevada Water Science Center

2730 North Deer Run Road

Carson City, NV 89701

Telephone: 775-887-7683

Fax: 775-887-7629

Email: mrosen@usgs.gov