Anguilla anguilla, a catadromous species, is believed to undergo significant morphological and physiological changes in the final stage of its freshwater life. These changes are closely associated with gonad development and enable adaptation for long migration. This study examined 89 eels from the Gediz Delta, caught by commercial fishermen between 2020-2021. The eels' total lengths and weights were measured in the laboratory before they were dissected to examine gonad development. The total length and weight values of the specimens ranged from 19.2-72.5 cm (average: 46.77 cm) and 10.1-1002.4 g (average: 254.69 g) respectively. The ratio of female to male eels was 1:1.28. The chi-square test revealed no significant difference between female and male individuals (χ² calculation: 0.625 < χ² table: 3.841). The Gonadosomatic Index (GSI) value of the species was found to be between 0.22-0.47, peaking in the fall (0.47). Upon histological examination of the females' ovaries, two phases were identified - immature and maturing. Oocytes in the ovary consisted of pre-vitellogenic and early vitellogenic phases. The pre-vitellogenic phase featured a large nucleus situated in the centre of dense cytoplasm. The diameter of oocytes in this stage was measured between 90-200 µm. The early vitellogenic stage was characterized by larger oocytes located primarily in the peripheral zone of the ovarian stroma, containing a high amount of adipose tissue. The oocytes in this stage had a diameter ranging from 210-280 µm.
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