Article

Abstract

SUMMARYThe eel population of the Camargue lagoon system can be divided into two subunits 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. Key-words:European eel, Anguilla anguilla, silvering, ocular index, gonadosomatic index, growth pattern, age at metamorphosis, freshwater, brackish water. Bull. Fr. Pêche Piscic. (2003) L'ARGENTURE CHEZ DES ANGUILLES FEMELLES (ANGUILLA ANGUILLA)DE DEUX SOUS-POPULATIONS DU DELTA DU RHÔNE. RÉSUMÉLe peuplement anguillicole du système lagunaire camarguais peut être scindé en deux sous unités caractérisées par des structures de population et notamment des patrons de croissance différents en rapport avec deux biotopes distincts (eau douce et eau saumâtre). La sous population du Fumemorte (salinité : 0,9 g/l en moyenne) s'apparente à une population de type « eau douce » (81,6 % de femelles, âges maximum observés = 9 -11 ans, croissance faible (gain annuel de 46 mm)) tandis que celle de l'étang du Vaccarès (salinité : 9,7 g/l en moyenne sur la même période) est caractéristique d'une lagune (45,6 % de femelles, âges maximum 5 -6 ans, forte croissance (gain annuel de 90 mm)).Pour chaque sous-population, nous avons mesuré le Rapport Gonadosomatique (RGS) et l'Index Oculaire (IO) des anguilles femelles sur une péri...

PRELIMINARY INVESTIGATION ON MORPHOMETRIC AND BIOMETRIC CHARACTERISTICS OF FEMALE SILVER AND YELLOW, Anguilla anguilla, FROM EASTERN MEDITERRANEAN (GÖKSU DELTA/TURKEY)

“…As far as IG is concerned our findings with regard to yellow and silver eels from Göksu Delta further support Marchelidon et al (1999) and Acou et al (2003) who have reported a threshold value of ≥ 1.4% for gonado-somatic index characterizing the silvering stage.…”

“…Lefebvre et al (2003) have reported significant differences in size (length) of female silver eels even from two sites in the same delta (Rhone Delta). Likewise, Acou et al (2003) underline that the ovarian development which is characteristic of metamorphosis to silver stage occur at very variable body lengths varying between 400 and more than 750 mm. Nevertheless; mean body length of migrating female silver eels from Göksu Delta (72.90 ±4.88 cm) is consistent with classification made by Durif et al (2005) and Durif et al (2009).…”

“…For example; mean eye index of female yellow and silver eels from Grevelingen Lake have been reported as 7.14 ±1.31 and 10.17 ±1.04 respectively by (Ginneken et al, 2007). Acou et al (2003) have investigated morphometric characteristics of eel population from two sites (Fumemrte and Vaccares) in Rhone Delta and have found different values of mean eye index for two sub-populations ranging from 4.20 (Vaccares) to 6.59 (Fumemorte) for female yellow eels and from 8.0 (Vaccares) to 9.71 (Fumemorte) for female silver eels.…”

“…Several studies have reported morphologi-Journal abbreviation: J FisheriesSciences.com cal transformation indices like eye diameter and pectoral fin length as well as biometric characteristics such as gonad and liver mass shedding light on silvering process (Pankhurst, 1982;Beullens et al, 1997;Marchelidon et al, 1999;Acou et al, 2003;Durif et al, 2005;Ginneken et al, 2007;Durif et al, 2009;Yokouchi, et al, 2009). Some of these studies have further focused on proposing threshold values for ocular and gonadosomatic indices or have developed indices (Silver index) based on some easily measurable morphometric characteristics such as body length, body weight, pectoral fin length and eye diameter to identify migrating silver eels from external measurement (Durif et al, 2005;Durif et al, 2009).…”

“…France, Netherland, Poland and Ireland (Marchelidon, et al, 1999;Acou et al, 2003;Robak, 2005;Durif et al, 2005;Ginneken et al, 2007;Durif et al, 2009;Yokouchi, et al, 2009). Farrugio and Elie (2011) underline the spatial diversity of the European eel population and the fact that part of this population may be specific to Mediterranean.…”