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\ qq 3 oeYR-/r6j et- aJ cr\.rIH '.J\Jf:~vironmenlal Toxicology and Chemistry. Vol. 13. pp. 101-107. 1994 Printed in the USA. Pergamon Press Ltd. "lIP 077CJY ~ Copyright @ 1993 SET AC TOXICITY OF CARBARYL AND MALATHION TO TWO FEDERALLY ENDANGERED FISHES, AS ESTIMATED BY REGRESSION AND ANOVA ~ DANIEL W. BEYERS, *t THOMAS J. KEEFE+ and CLARENCE A. CARLSONt tLarval Fish Laboratory, Department of Fishery and Wildlife Biology and tDepartment of Environmental Health, Colorado State University, Fort Collins, Colorado 80523 (Received 2 November 1992; Accepted 19 April 1993) Abstract- Toxicity of technical carbaryl, Sevin@-4-0il, and technical malathion to federally endangered Colorado squaw fish (Ptychocheilus lucius) and bony tail (Gila elegans) was estimated by 4-d renewal-acute and 32-d early life-stage (ELS) tests. Me- dian lethal concentrations were estimated by probit analysis. Effect concentrations for ELS tests were estimated by ANOVA and an alternative procedure using a linear-plateau regression model. Linear-plateau regression estimated a threshold concen- tration above which toxic effects began to occur. Median lethal concentrations and 950/0 confidence limits (in parentheses) for carbaryl, Sevin-4-0il, and malathion were 1.31 (1.23, 1.40), 3.18 (2.87, 3.52), and 9.14 mg/L (8.36, 10.0) for Colorado squaw- fish and 2.02 (1.78, 2.25), 3.31 (3.06, 3.55), and 15.3 mg/L (14.4, 16.4) for bony tail. NOECs for Colorado squawfish were 445 /lg/L carbaryl and 1,680 /lg/L malathion. The NOECs for bony tail were 650 /lg/L carbaryl and 990 /lg/L malathion. Thresh- old concentrations for Colorado squawfish in ELS tests were 364 /lg/L carbaryl and 455 /lg/L malathion. Threshold concen- trations for bony tail were 217 /lg/L carbaryl and 521 /lg/L malathion. Estimates of effect concentrations from linear-plateau regression were consistently lower than those estimated by hypothesis testing. Linear-plateau regression models accounted for a significant amount of total variation and adequately described the observed concentration-response relation. Keywords - Endangered species Malathion Carbaryl INTRODUCTION The insecticides carbaryl and malathion are used to control grasshopper infestations on rangeland and cropland through- out the western United States. Both chemicals exert their toxic effects by inhibiting the enzyme acetylcholinesterase (AChE), thereby causing disruption of the central nervous system. Insecticides used to control grasshopper infestations may pose a particular threat to fishes because although no- spray buffer zones are observed around aquatic habitats, pes- ticide may be deposited by accidental drift or mobilized from upland areas by runoff [1]. The Colorado squawfish (Ptychocheilus lucius) and bony- tail (Gila elegans) are large minnows that historically oc- curred throughout the Colorado River Basin [2J. Populations of both species have declined as a result of construction of reservoirs and other management practices in the basin [3J. In response to the rapid decline and threat of extinction of these fishes, the U.S. Fish and Wildlife Service authorized listing Colorado squawfish and bony tail as federally endan- gered species in 1967 and 1980, respectively. Although the scarcity and federally endangered status of these fishes have made study of their habits in the wild difficult, some infor- mation on life history and behavior has been collected [4,5J. During the first several months of life (June-October), young Colorado squawfish and bonytail occupy shallow, low-veloc- ity, near-shore habitats. Because these habitats have low rates of water exchange, pesticides deposited in them may be present long enough for toxic effects to occur. Grasshopper I , 'To whom correspondence may be addressed. Contribution 55, Larval Fish Laboratory, Colorado State University. Threshold concentration Regression control programs and life histories of Colorado squaw fish and bonytail overlap such that potentially sensitive life stages (<30 d old) are at risk of exposure. Toxicity of carbaryl and malathion to fish and aquatic in- vertebrates has been thoroughly studied [6J, but none of the previously tested fishes is closely related to the native fishes of the Colorado River Basin. Because of uncertainty of pre- dicting the sensitivity of Colorado squaw fish and bonytail to carbaryl and malathion, we estimated toxicity of these chem- icals using prescribed methods for 4-d acute [7J and 32-d early life-stage (ELS) toxicity tests [8]. Data from toxicity tests were analyzed by ANOVA hypothesis testing and an alter- native procedure using a linear-plateau regression model. Ste- phan and Rogers [9J described the computational and conceptual advantages of using regression analysis for con- centration-response data, We present comparisons of the ef- fect-concentration estimates obtained by the two alternative statistical methods. The purpose of this study was to estimate the toxicity of technical carbaryl, Sevin@-4-0il (a formula- tion of carbaryl), and technical malathion to Colorado squawfish and bony tail. The objectives were to (a) estimate 4-d median lethal concentrations, (b) estimate and compare effect concentrations for survival and growth, and (c) com- pare estimates of effect concentrations obtained by hypoth- esis testing and regression analysis. MATERIALS AND METHODS Experimental animals Fertilized eggs of Colorado squawfish and bony tail were obtained from Dexter National Fish Hatchery (Dexter, NM). Hatchery researchers artificially induced spawning of Col- orado squaw fish [1OJ and bony tail [11], and fertilized eggs 101