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Perchlorate anion (ClOsub4/subsup-/sup)is a contaminant found in groundwater and surface waters in locationsthroughout the United States, including Lake Mead and the Colorado River downstream of LakeMead. Lake Mead is a large reservoir formed by impoundment of the Colorado River behindHoover Dam. The reservoir serves as a source of domestic and agricultural water for more than22 million users and is home to the razorback sucker, an endangered species of fish.Perchlorate enters Lake Mead via contaminated groundwater runoff into the Las Vegas Wash,which enters Lake Mead at Las Vegas Bay. The relatively high concentrations ofperchlorate found in the Las Vegas Wash (average 600-800 ppb in 2003) are diluted in the LasVegas Bay. Results of previous studies indicate disruption of the reproductive systems of common carp(Cyprinus carpio) collected from the Las Vegas Bay of Lake Mead, but a cause has not beenidentified. Because thyroid and reproductive status and function are related in fish, thecurrent study was designed to determine whether thyroid and reproductive function are altered incarp exposed to perchlorate in Lake Mead. Adult male and female carp were collected in October2001 from the Las Vegas Bay and from two reference sites in Lake Mead. Endpoints examinedincluded circulating thyroid hormone (thyroxine, or T4, and triiodothyronine, or T3) and sexhormone concentrations, gonadosomatic index (GSI = (gonad weight / body weight) *100)),gonad histology, and thyroid histology. Sex hormones that were measured included the estrogen17-estradiol (E2) and the androgens testosterone (T) and 11-ketotestosterone (11-KT).Data currently available for male fish show no statistically significant effects of the Wash influenton GSI or sex steroids, but thyroid hormone levels appear to be less in male carp caught in theLas Vegas Bay than in those caught at Overton Beach. Mean T4 was less by approximately 2-fold in males caught from Las Vegas Bay than in those caught from Overton Beach. T3 wasslightly less in males in more advanced stages of reproductive development caught from LasVegas Bay than in those caught from Overton Beach. Testis histology suggests that reproductivedevelopment might be delayed in male carp captured from Las Vegas Bay, but any effects werenot sufficiently severe to affect GSI. The significance of these findings will not be known untilhistological examinations of thyroid samples from these fish are completed and the findings areput into perspective in relation to activity of the reproductive and thyroid systems. Data currentlyavailable for females do not suggest endocrine disruption, but examination of thyroid histologyand gonad histology continue. Includes 10 references, extended abstract only.

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Edition: Vol. - No. Published: 11/15/2004 Number of Pages: 3File Size: 1 file , 210 KB