We present the results of a survey of anguillicolosis in the Rhône River delta. From January 1997 to December 2000, a total of 13 319 eels (Anguilla anguilla from elver to silver phase) were examined, in which we found 22 227 swimbladder nematodes (Anguillicola crassus adults and preadults). A generalised linear model (GLM) framework was used to explore the relative contribution of various factors to the occurrence, intensity and abundance of the parasite. We reveal a major influence of the month of sampling, and we document the existence of a seasonal pattern with regular peaks in early summer and late winter. In contrast, the year of sampling is of secondary importance, and no particular trend in the development of the infection can be detected. More than a decade after the first record of A. crassus in the Rhône River delta, anguillicolosis has thus attained a constant infection rate of nearly 50%, with a mean number of 3 or 4 macroscopic lumen worms per infected eel. The eel length strongly influences the intensity and the abundance of the nematode, but has little if any effect on the probability of being infected. There exists a linear relationship between eel size and the number of parasites, but not between eel size and prevalence. We observe a decrease in the proportion of infected individuals among elver eels. We discuss this result in relation to the possible mortality of heavily infected individuals and/or a change in the eels' alimentary diet.

We have investigated the spread of the infection by Anguillicola crassus among the silver phase of the European eel Anguilla anguilla in the Rhône delta. We reported values of prevalence, mean intensity and abundance in 4 habitats and we revealed negative relationships between these parasitic parameters and values of salinity (prevalence from 52 % in brackish waters to 77 % in fresh waters). We have also assessed the health state of the infected organ, i.e. the swimbladder. This may be a way to check the parasitic history of individuals throughout their continental phase. In silver eels free from parasites, the proportion of past infected individuals was ranged between 40 % and 78 %. When adding individuals showing worms at the autopsy with those showing signs of past infection(s), we highlighted a great proportion of silver eels really affected by anguillicolosis (from 71 % to 95 %, with a negative relationship in respect to salinity values). So, considering the spread of the infection, and its potential impairments on body condition, gas exchanges, hydrostatic abilities, etc., one may legitimately question about the proportion of silver eels that may get back to the Sargasso sea and reproduce. -97 -Bull. Fr. Pêche Piscic. (2003) 368 : 97-108 Article available at http://www.kmae-journal.org or http://dx.doi.org/10.1051/kmae:2003039 concernée par l'anguillicolose (entre 71 % et 95 %, avec une relation négative par rapport aux valeurs de salinité). Dès lors, considérant l'étendue de l'infection et ses impacts potentiels sur la condition somatique, les échanges gazeux, la régulation hydrostatique, etc., on peut légitimement s'interroger sur la proportion d'anguilles argentées qui pourront effectivement rejoindre la mer des Sargasses et se reproduire. Key-words:

In 1993, a 100-y return-time flood brought about major hydrological changes in the Vaccarès Lagoon (Rhône Delta, France). Four years after the floods, Gobius niger had disappeared from the lagoon. Only the fall in salinity can explain this change. Reproductive traits in 1993 and 1994 were compared to determine the choices this species takes to improve its fitness in such unfavourable conditions. Evidence suggesting that G. niger is not a sedentary lagoon species is also discussed.