 To whom correspondence should be addressed at present address: National Institute of Public Health and the Environment, P.O. Box 1, 3720 BA Bilthoven, The Netherlands. Fax: #31302744401. E-mail: erwin. roex@rivm.nl. Ecotoxicology and Environmental Safety 48, 196 } 201 (2001) Environmental Research, Section B doi:10.1006/eesa.2000.2029, available online at http://www.idealibrary.com on Reproductive Impairment in the Zebrafish, Danio rerio, upon Chronic Exposure to 1,2,3-Trichlorobenzene Erwin W. M. Roex, Marie K lla Giovannangelo, and Cornelis A. M. van Gestel Vrije Universiteit, Institute of Ecological Science, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands Received March 30, 2000 Most organic pollutants are supposed to act via the mecha- nism of nonpolar narcosis upon acute exposure. Because the chronic e4ects of these compounds are still relatively unknown, in this study a chronic toxicity experiment was performed with zebra5sh, Danio rerio, exposed to 1,2,3-trichlorobenzene (123TCB), a nonpolar narcotic. Fish were exposed in a 6ow- through system for 68 and 147 days. Parameters measured are survival, growth, reproduction, and glycogen and protein content. The only parameter which was in6uenced was the number of eggs produced per female, resulting in an EC 50 of 40 g/ L. Using this value and acute toxicity data for 123TCB, an acute to chronic ratio (ACR) of 80 was calculated, which is larger than ACRs for other species exposed to nonpolar narcotics. This 5nding might indicate that compounds acting by nonpolar narcosis in acute tests can have completely di4erent e4ects upon chronic exposure.  2001 Academic Press Key Words: reproduction; zebra5sh; nonpolar narcotic; chronic. INTRODUCTION Environmental risk assessment of chemicals is mostly based on standardized toxicity tests, usually lasting for only a few days and performed with a limited number of selected organisms. The results of these standardized tests are ex- trapolated to safety limits by using "xed factors (Okkerman et al., 1991). These extrapolation factors depend on the amount and the quality of the data available, but do not take into account the mode of action of the compound, the life history, or the physiology of the test species. Four classes of chemicals can be distinguished by their mode of action (Verhaar et al., 1992): (1) nonpolar narcotics, (2) polar narcotics, (3) reactive chemicals, and (4) speci"cally acting chemicals. This classi"cation is based on results of acute toxicity tests. Because mortality is the most important parameter in these tests, relatively high concentrations must be used. In the environment, however, organisms are usually exposed to sublethal concentrations of toxicants for a long time. Because of processes like bioactivation and induction of metabolizing enzymes, other modes of action can become active upon chronic exposure, which do not appear in acute tests. Kenaga (1982) stated that the formation of more toxic metabolites can result in large di!erences between acute and chronic e!ect concentrations for certain compounds. These di!erences may exceed the extrapolation factors, which are used in risk assessment. Furthermore, important life-cycle parameters of the test organism may be a!ected after chronic exposure, such as growth, development, and reproduction. These parameters are usually not taken into account in acute tests. Small adverse e!ects on di!erent life-history parameters could seriously damage the stability of a popula- tion in the "eld. Chronic life-cycle toxicity tests, in which these parameters were included, would be more ecologically relevant than the standardized short-term toxicity tests (Van Straalen and Kammenga, 1998). Because of time and cost-consuming aspects, mostly small invert- ebrate species are chosen as test organisms in chronic tests. Only few toxicity tests have been performed which investi- gate the chronic e!ects of compounds on aquatic verte- brates, like "sh species, and the compounds tested in chronic "sh toxicity tests mostly belong to the classes of polar narcotics and speci"cally acting chemicals. Although the data are rather scarce, "sh species demonstrate the largest di!erences between acute and chronic toxicity, inde- pendent on the mode of action upon acute exposure (Roex et al., 2000). However, most organic chemicals are supposed to act via the mechanism of nonpolar narcosis (Veith et al., 1983). This mechanism has drawn a lot of attention and a lot of e!ort has been put into modeling acute toxicity, for example, by means of QSARs (Ko K nemann, 1981; Veith et al., 1983; Klopman et al., 1999). Chronic e!ect levels of compounds 196 0147-6513/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.