The objectives of this project were to evaluate i) whether the gonad alterations of whitefish ( Coregonus lavaretus spp.) in Lake Thun represent abnormal morphological variations specific to this lake, and, if so, ii) whether the malformations are related to chemical exposure, in particular to exposure to endocrine-disrupting compounds (EDCs). Large-scale monitoring data revealed that, although whitefish in other lakes display some background variation of gonad morphology, the situation in Lake Thun, is unique because of the significantly higher prevalence of gonad malformations. The abnormal variations of whitefish gonad morphology include aplasias, compartmentations, fusions, and intersex. In the search for the factor(s) causing the gonad malformations, coregonids were exposed from fertilization up to maturity to Lake Thun water and plankton or to contaminants possibly being present in the lake, including trinitrotoluenes, and naphtalene sulfonates. Since these experiments are still ongoing, a conclusive answer cannot be given yet, but initial observations point to a role of the lake plankton. The possible presence of EDCs in Lake Thun was assessed using bioanalytics and biomarkers. The bioanalytical studies found estrogenic activities in concentrated plankton extracts of Lake Thun, however, estrogenic activities occurred also in plankton extracts of reference lakes. Bioassay-directed fractionation of the plankton samples points to degradation products of natural substances as a cause of the estrogenic activity. Examination of Lake Thun whitefish for EDC biomarkers such as vitellogenin, sex steroid levels or intersex frequency yielded no indications of exposure to EDCs, neither in fish with normal nor in fish with abnormal gonad morphology. Long-term laboratory exposure of developing coregonids to the prototype estrogenic compound, 17β -estradiol, resulted in an increased frequency of intersex gonads, but did not induce the other gonad malformations typical for Lake Thun coregonids. In summing up, the currently available evidence does not support an EDC or chemical etiology of the gonad malformations, however, this preliminary conclusion needs to be substantiated in the ongoing investigations. The project also highlights the need for more detailed knowledge of natural variation in wildlife populations to be able to recognize anthropogenically caused variation.

In Lake Thun, Switzerland, a significant number of whitefish Coregonus lavaretus were found to exhibit abnormal gonadal morphology. To determine the time at which the morphological deviations from a normal gonadal arrangement develop, we conducted experiments designed to: (1) examine whether the morphologic gonadal variations develop during or after gonadal differentiation, and (2) characterise the entire ontogenetic gonadal development in whitefish from hatching until maturity. For these experiments, whitefish were reared under controlled conditions with distinct water sources and temperature regimes (Lake Thun water: 4 to 22°C; spring water: 8 to 9°C). Gonadal development in the spring water group was clearly delayed with respect to age, but similar to the Lake Thun water group with respect to degree-days in Celsius (°D). Undifferentiated gonads were first seen at 65 d post-hatch and 491°D. Ovarian differentiation (starting from 1734 to 1820°D) preceded testicular differentiation (starting from 1989 to 3673°D). The first fish with mature germ cells were recognised at 8163 to 8356°D. Morphological gonadal deviations became evident during gonadal differentiation. Constrictions and asymmetries developed for the first time in the first (0+) or second (1+) year-of-life, at a body length of 13 to 18 cm. Aplasia and compartmentation were recorded in the third (2+) year-of-life at a body length of 16 to 18 cm. Intersex fish occurred at a frequency of 8 to 9% during gonadal sex differentiation. Because no evidence for the presence of exogenic endocrine active substances was revealed, we suppose that there is a naturally increased mosaic intersex condition in whitefish during the ontogenetic gonadal differentiation process.

A high prevalence of gonad morphological variations has been observed in whitefish Coregonus lavaretus from Lake Thun (Switzerland). To clarify the role of endocrine disruption as a possible cause of the gonad alterations, whitefish were reared in a long-term laboratory experiment under exposure to 17β-estradiol (E2). Fish were fed from first-feeding until 3 yr of age at a daily rate of 0 (control), 0.5 or 50 µg E2 kg -1 fish. E2 exposure resulted in a time-and concentration-dependent increase of prevalence and intensity of intersex gonads, i.e. gonads that macroscopically appeared as either testis or ovary but microscopically contained both male and female germ cells. Four types of intersex could be distinguished: Types 1 and 2 were composed of mainly male tissue, with Type 1 containing single oocytes and Type 2 displaying an ovary-like lamellar structure of the tissue. In Type 3, an increased percentage of the tissue was occupied by female germ cells, while in Type 4, the majority of the gonad tissue consisted of female germ cells. Chronic E2 exposure additionally resulted in a concentration-dependent shift of the sex ratio towards females, a reduced condition factor, retarded gonad growth together with delayed maturation of germ cells, and elevated levels of hepatic vitellogenin mRNA. However, Lake Thun-typical alterations of gonad morphology were not induced by chronic E2 exposure. The results provide evidence that estrogen-active compounds unlikely play a role in the etiology of gonad malformations in Lake Thun whitefish. KEY WORDS: Estrogen · Whitefish · Coregous lavaretus · Gonad morphology · Sex differentiation · Intersex · Vitellogenin · Endocrine disruption Resale or republication not permitted without written consent of the publisherDis Aquat Org 84: [43][44][45][46][47][48][49][50][51][52][53][54][55][56] 2009 drinking water for ~400 000 inhabitants of the area, a possible impact of environmental pollution is of particular public concern. In addition, whitefish is a frequently eaten fish and chemical contamination could therefore pose a risk to consumers. Relevant to human health and the health and development of the coregonid populations, it is thus important to clarify the causes of the gonad alterations.Variations in gonad morphology of fish have been reported in a number of studies worldwide (Kinnison et al. 2000, Blazer 2002, Mikaelian et al. 2002. In fact, the morphology and differentiation of the gonads of fish are known to be susceptible to a variety of environmental factors such as temperature (Patino et al. 1996, Baroiller et al. 1999, parasites (Wiklund et al. 1996, Jobling & Tyler 2003 and chemical substances (Kime 1995, Jobling et al. 1998. One environmental condition that has repeatedly been found to cause gonad alterations in fish is exposure to endocrine-disrupting compounds (EDCs) (Segner et al. 2003, Matozzo et al. 2007). Among the endocrine-active substances, chemicals that act as estrogen-receptor ligands and activate estrogen signaling pathways have received the mos...