Quantifying the fitness cost that parasites impose on wild hosts is a challenging task, because the epidemiological history of field-sampled hosts is often unknown. In this study, we used an internal marker of the parasite pressure on individual hosts to evaluate the costs of parasitism with respect to host body condition, size increase and reproductive potential of field-collected animals for which we also determined individual age. In our investigated system, the European eel Anguilla anguilla and the parasitic invader Anguillicoloides crassus, high virulence and severe impacts are expected because the host lacks an adaptive immune response. We demonstrated a nonlinear relationship between the severity of damage to the affected organ (i.e. the swimbladder, our internal marker) and parasite abundance and biomass, thus showing that the use of classical epidemiological parameters was not relevant here. Surprisingly, we found that the most severely affected eels (with damaged swimbladder) had greater body length and mass (þ11% and þ41%, respectively), than unaffected eels of same age. We discuss mechanisms that could explain this finding and other counterintuitive results in this host -parasite system, and highlight the likely importance of host panmixia in generating great inter-individual variability in growth potential and infection risk. Under that scenario, the most active foragers would not only have the greatest size increase, but also the highest probability of becoming repeatedly infected-via trophic parasite transmission-during their continental life.

Variations in the liver and spleen masses of the eel Anguilla anguilla were analysed in relation to the parasite load of Anguillicola crassus at autopsy (current infection by swimbladder lumen worms) and in relation to the severity of damage observed in the swimbladder (a way of assessing the intensity of past infections). None of these measures of parasite pressure were shown to account for variation in the relative liver mass, either when controlling for somatic mass or eel age. In marked contrast, a significant increase in spleen size was revealed in eels harbouring many lumen worms and also in eels with severe damage in the swimbladder. Splenic enlargement was nearly two-fold higher among severely affected eels (harbouring more than seven lumen parasites and showing severe damage in the swimbladder) than among infectionfree eels (no lumen parasites and no pathological signs in the swimbladder). Several possible hypotheses are reviewed before arguing for an adaptive host response involving the haematological and immunological functions of the spleen. Indeed, among eels with no pathological signs in the swimbladder, the relative spleen mass was positively associated with the mass of lumen parasites, which suggests a hyper-synthesis of blood cells by the spleen in response to the bloodsucking activity of lumen worms. Nevertheless, among eels with no lumen parasites at autopsy, there was still an increase in spleen size in relation to the severity of the swimbladder damage, which also suggests a hyper-synthesis of splenic immune cells (lymphocytes and macrophages) in reaction to damaged tissues and particularly to larvae in the swimbladder wall. # 2004 The Fisheries Society of the British Isles

International audienceChanges in the numbers and size-class structure of European silver eels, Anguilla anguilla, in the River Fre'mur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996

Maturing sub‐adults of two species of anguillid eels (a female Anguilla bicolor bicolor and a male Anguilla marmorata) were collected for the first time at Réunion Island, western Indian Ocean. Both were silver eels, i.e. maturing eels at the onset of their spawning migration, and characterized by advanced sexual maturation that has been only observed in Anguilla dieffenbachii from New Zealand.

Thirty migrating silver eels Anguilla anguilla were collected in a river system where algal blooms occurred yearly. Fifty per cent of eel livers were contaminated by microcystin-LR (mean AE S.D. toxin level: 28Á1 AE 22Á4 ng g À1 ). Contaminated silver (v. healthy) eels had lower fish condition. Consequences of this impact for the breeding potential of these migrating eels are discussed.

RÉSUMÉDes prélèvements d'anguilles (Anguilla anguilla) femelles ont été effectués dans les bassins versants de la Vilaine (n = 569) et de la Loire (n = 221), en zone fluviale et dans les deux estuaires. Les otolithes ont été poncés suivant le plan sagittal et le patron d'agencement des lignes d'arrêt de croissance (LAC) révélées par coloration au bleu de toluidine a été étudié avec une attention particulière pour l'origine fluviale ou estuarienne des échantillons. Pour une même classe de taille des poissons, les otolithes des anguilles fluviales présentaient jusqu'à deux fois plus de lignes d'arrêt de croissance que ceux des anguilles estuariennes. L'observation de la microstructure des LAC en microscopie électronique à balayage a conduit à distinguer deux catégories de LAC : des LAC composites et larges (> 10 um), préalablement validées comme les annuli saisonniers, et des LAC simples et étroites (< 5 um) représentatives de zones non saisonnières. Tous les otolithes des anguilles fluviales présentaient les deux catégories de marques. Parmi les otolithes des anguilles estuariennes, seuls 5 % de l'échantillon de la Vilaine et 8% de celui de la Loire présentaient des LAC simples.La zonation de l'otolithe, interprétée par rapport à la croissance des poissons, décrit deux écotypes dans la population d'anguilles de la Vilaine : un écotype fluvial dont la croissance est perturbée et ralentie, et un écotype estuarien dont la croissance est plus régulière. Cette partition de la population est cohérente avec les caractéristiques environnementales des deux milieux. Elle pourrait par ailleurs constituer un indice de l'impact des aménagements des bassins versants sur la croissance, la durée du cycle biologique et la dynamique des populations d'anguilles. Il apparaît alors nécessaire de considérer séparément les sous-unités estuariennes et fluviales des stocks d'anguilles des bassins versants de la Vilaine et de la Loire, en particulier pour étudier la diminution des abondances.Mots-clés : anguille, Anguilla anguilla, croissance, identification des stocks, otolithe. FLUVIAL AND ESTUARINE EELS : CONTRIBUTIONS OF OTOLITHOMETRY. ABSTRACTFemale eels (Anguilla anguilla) have been sampled in two water basins, the Vilaine river (n = 569) and the Loire river (n = 221), both in estuaries and fluvial streams. Otoliths have been ground on the sagittal plane and pattern of growth zones (LAC, stained by toluidine blue) was described with a special attention to the sampling site, fluvial or estuarine. For fish of the same length class, LAC were up to twice as numerous in fluvial eel otoliths than in estuarine ones. The microstructure of LAC was observed with a scanning electronic microscope and two types of LAC were distinguished : composite and large (> 10 um) LAC that have been validated as seasonal annuli, and simple and narrow (< 5 um) LAC which correspond to non-seasonal zones. All the otoliths of fluvial eels displayed both types of LAG. Among the otoliths of estuarine eels, only 5 % of the Vilaine sample and 8 % of the Loire one di...

A double mark, called a transition ring, or elver mark, was identifiable in light microscopy on otoliths of young yellow eels. In the Vilaine watershed, the radius of this mark decreased from 178 µm in yellow eels corresponding to glass eels arriving in autumn 1997 to 163 µm in yellow eels arriving in spring 1998. The mean transition ring radius of the freshwater eel population in the Vilaine river had an intermediate value between spring and autumn recruits. This implies that it consisted of a mixture of spring and autumn recruits. In the Vilaine estuary and the Frémur populations, the mean radius of the transition rings was close to the autumn one. The springtime recruits formed 68 % of freshwater and 15 % of estuarine population in the Vilaine for the 1998 cohort. This result was in sharp contrast with the available assessments of recruitment, which both in estuary and in the fluvial part of the watershed, were dominated by spring recruits. This contrast is possibly the consequence of density-dependent mortality, which would have been particularly important in springtime for glass eels whose migration was inhibited by the dam.

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