Original paper

Size and number of male silver eels Anguilla anguilla (L.) in a Cantabrian river over two decades (1990-2011)

Iglesias, Tania; Lobón-Cerviá, Javier


Total density of eels and the size and number of male silver eels Anguilla anguilla (L.) were quantified between 1990-2011 at 15 sites spread along four Rio Esva tributaries (Asturias, north-western Spain). A severe decline of recruitment and of eel density in the Rio Esva estuary from 1990 onwards coincided with a decline in the in-stream density of eels in Rio Esva tributaries; from 2100 ind ha−1 in 1990 to only 400 ind ha−1 in 1999. Eel numbers began to rise around 2000 to attain 1800 ind ha−1 in 2005-2006 and 1225 ind ha−1 during 2007-2011 when recruitment was at a record historical low. During this period, seven silver females (range 52-71 cm) and 290 silver males (range 27-40 cm) were recorded. Longitudinal segregation ensures the occurrence of larger silver males at the upstream sites. Silver males in Rio Esva appear the smallest within the distributional range of the European eel. Their mean size remained constant over the years (i.e., 33.1 cm). However, variations around the mean size increased with increased total in-stream density. Temporal variation in silver males density appeared linked to the rise-and-fall pattern underpinning in-stream density. Current results combine with previous studies on the Rio Esva eels to offer evidence of compensatory density-dependence (probably mediated by temporally variable carrying capacity) and positive relationships between total in-stream density and density of silver males. This implies that at the lowest recruitment levels recorded during the last years, a relatively high abundance of silver males may still contribute to the reproductive component of the species population. However, marine migration is further questioned given a severe infestation of the Rio Esva eels by Anguillicoloides crassus.


silver male eelsanguilla anguillastream habitatdensitylong-term monitoring