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The occurrence of a large number of whitefish with morphologically altered gonads in Lake Thun (Switzerland) has been reported by commercial fishermen since the year 2000. Macroscopic and microscopic examination of gonads revealed that 35% (281 out of 808) of whitefish from Lake Thun were affected. Frequency of gonadal abnormalities varied significantly with sex, age and ecotype of whitefish. Apparent females (26% [106/408]) showed significantly less gonadal alterations than apparent males (40% [151/225]). Fish with deformed gonads were observed in all age classes from 1 to 6 yr, but were most frequent in the year classes 3+ to 5+. The gonadal alterations showed different frequencies among the 3 ecotypes of whitefish living in the lake: 1 ecotype was less affected (26%) than the other 2 forms (41% and 32%, respectively). The gonadal alterations included adhesions/fusions to the peritoneal wall and the lateral trunk musculature (overall: 5%; in females: 5%; in males: 5%), asymmetry (4%; 6%; 4%), atrophy (4%; 6%; 1%), compartmentations (11%; 4%; 18%), constrictions (3%; 1%; 7%) and hermaphroditism (1.1% of sampled fish, and 10 additional specimens collected by fishermen during the filleting process). In some cases, more than one alteration was observed in the same fish. Hermaphroditism manifested as 3 different types: (1) the lobular type, in which a consecutive sequence of testicular and ovarian lobes are found on the same gonad strand; (2) discrete gonads within one fish, with one gonad strand being ovarian tissue and the other being testicular tissue; and (3) the mosaic type, in which oocytes lie in tissue with normal testicular morphology, or spermatids or sperm are present in ovarian tissue. Several features of the gonadal alterations described in this study of whitefish from Lake Thun have also been reported for whitefish from other areas. However, the Lake Thun situation appears to be unique because of the high prevalence of fish affected and the broad variation of abnormal features. The causes of the frequent appearance of these gonadal alterations in whitefish from Lake Thun remain unclear.

Frequent and unexplained gonadal abnormalities in whitefish (central alpine Coregonus sp.) from an alpine oligotrophic lake in Switzerland