Age structure of eel, Anguilla anguilla (L.), populations in the River Severn, England, and the River Dee, Wales

Abstract. Sampling was carried out at 93 sites on the River Severn, England and at 8 sites on the River Dee, Wales. The age structure of the population shifted gradually from the younger to the older age-classes with distance upstream. This was related to the time required for the eels to reach these regions and to the increase in the proportion of females.
In the Severn, eels can migrate (unaided by tidal transport) at rates of between 20 and 30 km year⁻¹, while in the Dee, passage upstream is between 10 and 20 km year⁻¹. It is suggested that this slower rate is related to a more arduous migration, resulting from the steeper gradient of the Dee, In the lower reaches of the Severn, eels aged from 7 to 8 years were generally the most abundant, while in the middle and upper reaches, eels aged 13 years occurred most frequently and in the greatest numbers.
The age structures of the populations were classified using twinspan into three main groups and related to distance, upstream of the region of flow reversal, using discriminant analysis. The analysis demonstrated that age structure was related to the distance upstream of the region of flow reversal and that 60–76% of the twinspan site groupings could be classified in terms of distance.