Silvering of females eels (Anguilla anguilla) in two sub-populations of the Rhône Delta

• Main Authors: A. Acou, F. Lefebvre, P. Contournet, G. Poizat, J. Panfili, A. J. Crivelli
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2003
• Subjects: Bull. Fr. Pêche Piscic; Anguilla anguilla
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.489.4623; http://www.onema.fr/BFPP/bfpp/Article/368/368p055.pdf

The eel population of the Camargue lagoon system can be divided into two sub-units living in two distinct biotopes (freshwater and brackish water) and characterised by different population structures and especially growth patterns. The sub-population living in the Fumemorte canal (average salinity: 0.9 g/l) is a freshwater population type (81.6 % females, maximum ages observed = 9- 11 years, slow growth (annual gain 46 mm)) whereas that of the Vaccarès lagoon (average salinity: 9.7 g/l over the same period) is typical of a lagoon (45.6 % females, maximum ages 5 – 6 years, fast growth (annual gain 90 mm)). In each sub-population, Gonadosomatic Index (GSI) and OI (Ocular Index) were measured on female eels over a sampling period that started in January 1997 and ended in December 2000. In both types of habitat, and in agreement with previous studies, we observed a threshold value of GSI ≥ 1.4 % characterising the silver stage (MARCHELIDON et al., 1999). The increase in eye area was a progressive phenomenon, initiated in the yellow stage and was significantly correlated (p < 0.001) with the GSI. As with the GSI, OI increased in silver eels, the threshold value of ≥ 8.0 discriminating silver eels. We will show that silvering follows different patterns between the two habitats. Age at metamorphosis in female eels take place very early in the Vaccarès lagoon (83.3 % of females metamorphose between 4 and 5 years) compared to those in the Fumemorte drainage canal (94 % of females metamorphose between 7 and 9 years). We suggest that the growth potential provided by Vaccarès lagoon accounts for this difference in age at metamorphosis. Although, such a turnover cannot explain alone the different proportions of female silver eels observed in the Fumemorte drainage canal (23%) and Vaccarès lagoon (2.4%). Influence of population’s parameters like sex-ratio, population structure and abundance on silvering process, is discussed.