Life-History Characteristics of the European Eel Anguilla anguilla in the Imsa River, Norway

Each summer between June and September, elvers (6.4–9.0 cm total length) and a small number of yellow (12–26 cm) European eels ascend the Imsa River. In fresh water, the mean annual length increment from the elver stage to age 8 years was 6.2 cm. Males grew more slowly than females in the corresponding year classes. The male:female ratio was approximately 1:20. Male and female yellow European eels transformed into silver eels at lengths of about 40 cm and 62 cm, respectively. Rapidly growing eels of both sexes transformed into silver eels at younger ages than more slowly growing individuals. Length was more important than age for the onset of transformation. Length-weight relationships indicated that growth was allometric and that silver eels were heavier than yellow eels of the same length. Gonad weight of the females increased exponentially with body length, whereas the mean gonadosomatic index, gonad weight/(total weight - gonad weight), did not vary with length.