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The 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.

L'âge individuel de l'anguille en milieu continental est toujours un paramètre difficile à évaluer. Actuellement, seule l'otolithométrie est utilisée. Une description rapide des techniques de préparation et d'observation des otolithes, de leurs avantages et de leurs inconvénients, est donnée : observation in toto, ponçage, lames minces, brûlage, coloration, microscopie électronique à balayage, analyse chimique et analyse d'images. Elles sont axées sur l'interprétation des marques de croissance saisonnières. Malgré la diversité de ces méthodes, le choix d'une méthode fiable et universelle est difficile. De plus, les applications doivent être examinées et validées en fonction de chaque population de cette espèce dont la plasticité adaptative est remarquable. Les expériences de validation des estimations de l'âge (connaissance exacte des rythmes de dépôts de marques identifiées) à partir des otolithes sont peu nombreuses dans la littérature. Deux expériences de ce type sont présentées pour des populations méditerranéennes : une validation directe, par marquage individuel à la tétracycline dans un étang naturel de Camargue, et une validation indirecte, par observation de l'apparition marginale des marques de croissance au cours du temps pour des populations du Languedoc (étang saumâtre) et de Camargue (canal dulçaquicole). Ces expériences ont permis de déterminer précisément la séquence de formation des marques de croissance sur les otolithes ainsi que leur identification en fonction du mode de préparation. Une zone opaque large se forme principalement au printemps, une zone hyaline large se dépose pendant les mois estivaux, et une «ligne d'arrêt de croissance» chromophile correspond à la période hivernale. Ces marques sont donc utilisables pour estimer l'âge des anguilles en région méditerranéenne. Il existe cependant une très grande variabilité des structures de croissance, des rythmes de dépôt et de la morphométrie des otolithes. Cette variabilité est présente au niveau des individus de la même population mais aussi entre les populations. Ces résultats aboutissent à une notion d'écotype pour les otolithes d'anguille : l'écotype est en relation avec le milieu colonisé par l'individu et semble être fonction de la salinité et probablement du niveau trophique ; il retrace aussi l'histoire individuelle. Ces variabilités interviennent directement sur la croissance des individus. Les croissances individuelles dans les milieux méditerranéens saumâtre et dulçaquicole ont été mesurées et comparées entre elles et avec la littérature. La croissance en milieu saumâtre est nettement plus importante que celle en milieu d'eau douce.

Age and growth of Rhizoprionodon acutus were estimated from vertebrae age bands. From December 2009 to November 2010, 423 R. acutus between 37 and 112 cm total length ( LT) were sampled along the Senegalese coast. Marginal increment ratio was used to check annual band deposition. Three growth models were adjusted to the length at age and compared using Akaike's information criterion. The Gompertz growth model with estimated size at birth appeared to be the best and resulted in growth parameters of L∞ = 139·55 ( LT) and K = 0·17 year−1 for females and L∞ = 126·52 ( LT) and K = 0·18 year−1 for males. The largest female and male examined were 8 and 9 years old, but the majority was between 1 and 3 years old. Ages at maturity estimated were 5·8 and 4·8 years for females and males, respectively. These results suggest that R. acutus is a slow-growing species, which render the species particularly vulnerable to heavy fishery exploitation. The growth parameters estimated in this study are crucial for stock assessments and for demographic analyses to evaluate the sustainability of commercial harvests. [ABSTRACT FROM AUTHOR]

Potential trophic competition between two sympatric mullet species, Mugil cephalus and Mugil curema, was explored in the hypersaline estuary of the Saloum Delta (Senegal) using δ13C and δ15N composition of muscle tissues. Between species, δ15N compositions were similar, suggesting a similar trophic level, while the difference in δ13C compositions indicated that these species did not feed from exactly the same basal production sources or at least not in the same proportions. This result provides the first evidence of isotopic niche segregation between two limno-benthophageous species belonging to the geographically widespread, and often locally abundant, Mugilidae family. [ABSTRACT FROM AUTHOR]

Despite the considerable commercial value of the milk sharkRhizoprionodon acutus(Rüppell 1837) along the Senegal coast, there are few data on its biology. Milk sharks examined in this study were caught by small-scale fisheries on the Senegalese coast from May 2009 to February 2011 at eight landing locations. Landings were higher during the warm season (May–October) (71.5%), and lower during the cold season (November–April) (28.5%). The largest landings were recorded in the central area (36.9%) and the smallest in the coastal area of the Casamance (South-West Senegal) (0.6%). The observed size of individuals ranged from 31 to 113 cm (total length, TL), and the total weight from 150 to 8 500 g. Landings mostly included small individuals with a modal size of 50 cm TL for males and 60 cm TL for females. A reduction in the size range of milk sharks in the study area was consistent with those studied elsewhere. Females were more abundant than males (sex ratio 1.65F:1M). Mature individuals were dominant at the northern landing sites whereas immature individuals were more abundant in the southern regions. Our observations suggest a possible seasonal migration towards the north for reproductive purposes. The size at first sexual maturity was 92 cm TL for females and 82 cm TL for males. The gonadosomatic index, nidosomatic index and hepatosomatic index peaked from May to August, indicating that the reproductive period is during the warm season. Milk sharks off the coast of Senegal appear to have an annual reproductive cycle. Ovarian fecundity varied from 2 to 8 follicles and uterine fecundity from 2 to 7 embryos. Parturition occurred between May and June. Moreover, the presence of mature spermatozoa in the oviducal glands during the warm season supports the notion that mating takes place during that period. [ABSTRACT FROM AUTHOR]