Evaluating Silvering Stages in European Eels: A Study on Biological and Morphometric Variations in the Asi River, Türkiye

The European eel (Anguilla anguilla) undergoes significant morphological and physiological changes during its transition from the yellow to the silver stage, which are critical for its long-distance spawning migration. This study aimed to investigate these changes in European eels from the Asi River, located in Hatay, Türkiye, during their silvering process. A total of 96 eels were sampled in February 2019, and various morphometric measurements, including total length, body weight, eye dimensions and height, and pectoral fin lengths, were taken. Liver and gonad weights were also measured to assess the hepatosomatic index (HSI). The length–weight relationship for silver eels was described by the equation, W = 0.0072 × L2.732, with silver-stage eels showing a higher growth rate compared to yellow-stage eels, which had a relationship of W = 0.0184 × L2.397. The average total length of silver eels (431.2 ± 16.7 mm) was significantly greater than that of yellow eels (382.4 ± 11.9 mm). Additionally, pectoral fin length was significantly longer in silver eels (20.8 ± 1.1 mm) compared to yellow eels (14.8 ± 0.9 mm). The hepatosomatic index (HSI) for silver eels was also found to be higher than for yellow eels, indicating increased liver size as an adaptation for energy storage during migration. Eye height, a key indicator of silvering, showed a substantial increase during the transition, with silver-stage eels having an average eye height of 5.3 ± 0.2 mm compared to 4.2 ± 0.1 mm in yellow-stage eels.