This is the first report on the presence of acanthocephalan parasite Echinorhynchus truttae in brown trout (Salmo trutta) from Spain. A total of 343 fish were captured by local anglers from 19 rivers in the adjacent Tambre and Ulla watersheds in Galicia (north-western Spain). Macroscopic and microscopic analyses of the intestinal contents revealed the presence of adults and/or eggs of E. truttae in 53 of the 123 trout from Tambre river basin (43.1%). By contrast, parasitic forms of this acanthocephalan were only observed in eight of the 220 fish from Ulla basin (3.6%), showing significant differences between the prevalences obtained in two adjacent watersheds (P < 0.001, odds ratio 19.0). Prevalence was significantly higher in specimens >3 years (length >26.0 cm) than in younger specimens (P < 0.05). The absence of Gammarus pulex in the region suggests that native gammarid species in Galicia (Echinogammarus lusitanicus and Echinogammarus beriyoni) may act as intermediate host in the life cycle of E. truttae. Different prevalences of E. truttae indicate that the presence/abundance of the intermediate crustacean host may be different in the two river basins, probably as a consequence of various abiotic factors and anthropogenic activities.

Population dynamics, site selection, growth and maturation of the cestode Eubothrium crassum in a natural population of Salmo trutta in a small lake were studied over a period of 1 year, the life-span of a cohort in fish. Infection of fish commenced in spring but peaked in July. Small, plerocerciform parasites initially located in the intestine, but then some moved into the pyloric caecae whilst others, the majority in heavy infections, were lost from the fish causing a fall in abundance from 460 to 10 over 2 months. This mortality was density dependent. Initially, parasites were distributed more evenly throughout the caecae but as time increased larger parasites were found preferentially in the anterior caecae before moving back into the intestine when gravid, preparatory to being lost in the following summer. Only a small proportion of the infrapopulation became gravid. Although the proportion of caecae occupied was initially density dependent, by the time of maturation several preferred anterior caecae remained unoccupied and mean intensity always exceeded unity. Neither growth nor maturation was affected by intraspecific competition. It was concluded that caecal availability did not set a limit or threshold of infrapopulation density, and in this respect E. crassum–S. trutta differed from some acanthocephalan-fish systems but was similar to others. Heavy infection followed by heavy mortality appeared to be typical of this parasite-host system in other localities, and of several other cestode-fish systems. The implications of this for population regulation are discussed.

Brown trout (Salmo trutta) from four lakes within an area of approximately 40 km2 were found to harbour nine helminth species. Crepidostomum metoecus could be classified as core species, while Crepidostomum farionis had an intermediate position. All the others could be classified as satellite species. Salmon (Salmo salar) fingerlings were grown to smolts in an earthpond over the summer. The fingerlings became infected with Crepidostomum spp. which may indicate a superior colonization ability in the core species. The results support the ‘rescue effect hypothesis’ rather than the ‘carrying capacity hypothesis’.

Five myxozoan species, Tetracapsuloides bryosalmonae, Sphaerospora truttae, Chloromyxum schurovi, Chloromyxum truttae and a Myxobolus species were detected in farmed brown trout, Salmo trutta L. from Central Scotland. Using PCR and in situ hybridization, this study investigated the seasonal occurrence and tissue location of these species in young of the year brown trout. C. schurovi, C. truttae and Myxobolus sp. were first detected in brown trout in April, 2 months before T. bryosalmonae and S. truttae. T. bryosalmonae and S. truttae showed proliferation in the blood with intravascular stages of T. bryosalmonae accumulating in the heart. In contrast, only small amounts of PCR products of C. schurovi and C. truttae were obtained from the blood, suggesting that these species use the vascular system for transport but proliferate only in their target tissues from which large amounts of PCR product were obtained and where parasites were visible in histological sections. Large amounts of PCR product were obtained for T. bryosalmonae, S. truttae and both Chloromyxum species from the gills of brown trout, suggesting the gills as entry locus for these species. The neurotropic Myxobolus species formed plasmodia predominantly in the peripheral nerves, possibly indicating an entry route through the skin. Presporogonic stages of all other species had disappeared by September and mature spores were present from August onwards.

The level of genetic differentiation between anadromous and non-andromous trout (Salmo trutta L.) coexisting in a Norman river (Oir river) was examined at 15 microsatellites markers. Despite a large amount of variation, no genetic differentiation was found between forms. Four domesticated stocks were also genotyped with the same loci. Highly significant differences were found with natural samples allowing to exclude a recent stocking effect on the Oir population. We conclude that the resident and anadromous trout form a single naturally reproducing unit in this river system. However, this conclusion could reflect particular ecological or demographic conditions in the Oir basin and needs to be validated in other hydrographical and ecological contexts.

The abundance of prey ingested by brown trout (Salmo trutta) were compared with the abundance of drifting invertebrates in a mountain stream managed by hydropeaking, upstream (site A) and downstream (site B) of a hydroelectric power plant. During power generation, flow and temperature were the two main environmental factors modified. The natural flow in the river below the outlet may be enhanced several times a day from 1 to 11 m3.s–1 in summer and winter, and from 5 to 15 m3.s–1 during spring spates. During hydropeaking, the water was cooled by an average of 6 °C in summer and warmed by an average of 2 °C in winter. Overall drift density was higher at site B than at site A. There was a clear diel pattern of drift at site A, with low drift density during the day and high drift density at night, whereas no clear diel pattern was observed at site B. Below the plant, at site B, drift pattern appeared to be influenced most by hydropeaking. The flushing action of peaking flows caused a catastrophic drift, which was highest in autumn when the difference between natural and peak flows was greatest. Juvenile trout were adversely affected by hydropeaking conditions and subsequently their density and biomass were reduced by 30% from site A to site B, whereas no significant difference was noticed for adults. Gut contents analysis showed that brown trout chiefly fed on the most available prey items at both sites. Fish did not seem to feed in response to diel drift patterns above the plant, whereas they chiefly used drift pulses generated by peaking flows below the outlet. Under natural conditions, fullness indices increased from autumn to summer, suggesting they may be related to prey availability and changes in water temperature. In the regulated section, fullness indices were the lowest in spring, i.e. the season when peak flows added to snowmelt floods, suggesting a prominent role of high current velocities through habitat suitability, position maintenance, and ability to capture preys. Although hydropeaking is known to disturb trout population dynamics in this and other rivers, this kind of river regulation (natural discharge except during periods of power generation, and intermittent hydropeaking from a separate reservoir) allowed the maintain of brown trout below the outlet, probably because the river returned to natural conditions when the plant was inoperative, and because daily artificial fluctuations in flow and temperature remained within the limits of natural seasonal variations.

Radio tracking has been extensively used to enhance our knowledge on the movement and home range of fish in general and salmonids in particular. However, the use of various temporal sampling protocols is likely to overlook fish movements, and produce experimental artefacts, the amplitude of which is unknown and may compromise comparison of fish behaviours revealed by different protocols. Starting from a day-by-day tracking study of brown trout in Belgian waters (Aisne stream, nine fish, minimum 39 daily locations per fish), we re-examined their home range and mobility, through a subsampling process, as if the fish had been located at longer time intervals (2–14 days). The estimates from subsamples were compared to the original data set in order to quantify the corresponding reduction of accuracy from observed data, and how far this could be predicted on the basis of locating frequency. The results clearly indicate that all intervals longer than one day generally generate substantial biases (reduction of accuracy from 0 to 82% for home range and from 5 to 92% for mobility) but these can be partly corrected through the use of predictive models. This analysis demonstrates that any comparison between studies relying on different locating frequencies can generate some ambiguity when interpreting biological phenomena or geographical differences.

Brown trout and rainbow trout (average weight 100 g) were reared in fresh water at 12 °C under the same conditions before transferring brown trout to sea water, in order to compare phosphorus utilisation in both species. Apparent phosphorus availability, orthophosphate excretion and phosphorus accretion in the fish were directly determined. Thus, actual phosphorus mass balance was built. Rainbow trout raised in fresh water had a higher phosphorus retention coefficient (maximum 50 %) than brown trout reared in fresh water (maximum 45 %). Transferring brown trout to sea water induced a reduction in phosphorus retention (maximum 39 %). Orthophosphate excretion, ranging 7-20 mg phosphorus per kg wet weight per day, represented 10-20 % of ingested phosphorus. Phosphorus availability was lower in brown trout raised in sea water (65 %) than brown trout raised in fresh water (76 %). Phosphorus balance measurements showed that 90 to 98 % of phosphorus flow could be accounted for.

Neural networks provide a "black box" model for explaining and predicting trout abundance with 8 environmental variables. This work investigates the specific effect of each variable, by inputting fictitious configurations of explanatory variables and by checking the responses of the model. The comparison between this response of the model to environmental variables on one hand, and results from field observations on the other hand, shows similarities and indicates neural network modelling can be trusted. The elevation appears to be the major explanatory factor. The influence of shelters, bottom velocity and Froude number also play an important role. When considered separately, depth does not have a notable influence on the density of trout. Such confirmations of field observations suggest that these models can be used to obtain a clear identification and hierarchization of the factors influencing the abundance of trout and the mode of action of the factors. This approach can be extended to other applications in quantitative ecology in which non-linear relationships are usually observed.

Samples of trout (Salmo trutta L.) were taken from four rivers in the Conwy system, North Wales: the Roe and the Nant-y-goron, which support sea trout stocks, and the Iwrch and Machno which are situated above impassable waterfalls and contain resident trout. Tissue samples from the fish were used in electrophoretic analysis of enzymes. Fourteen loci were scored of which six were polymorphic (Pgi-2, Gpd-2, Ldh-5, Mdh-2, Mdh-4 and Ck-2). Allele frequency data from these loci were used to test interpopulation heterogeneity, which indicated that the two upstream samples were probably reproductively isolated from each other and also from the downstream samples. Mean heterozygosity was generally lower in the upstream stocks compared with the sea trout populations. UPGMA cluster analysis based on Nei's genetic distance (D) revealed a clear dichotomy between those stocks above, and those below waterfalls. This was best explained by the presence of two strains of trout in the system. One strain, the resident trout occurring above impassable waterfalls, was characterized by high frequencies of the Ldh-5(100) allele, the other strain, principally consisting of sea trout, exhibited high frequencies of the Lhd-5(90) allele.

Introgression between introduced domesticated stocks and natural populations of brown trout was investigated by protein electrophoresis in three Mediterranean rivers. From each river, one undisturbed site and one, closely located, yearly stocked site were sampled. Comparison of the electrophoretic variation observed in samples from the undisturbed sites with that in the introduced hatchery strains revealed several specifically "domestic" variants. The genetic control of these variants was demonstrated by breeding experiments. The samples collected from the stocked sites showed introgression rates ranging from approximately 0 to 40%. The genotype frequencies observed in these samples suggested that no reproductive barrier exists between introduced and native stocks and that they form random mating populations.

Analyse des écailles de truites acclimatées aux îles Kerguelen, archipel du domaine subantarctique, a permis de démontrer la périodicité annuelle des anneaux par différentes méthodes donnant des résultats convergents (analyse des fréquences de longueur, marquage-recapture, chronologie de l'apparition des anneaux sur la marge des écailles). Après un long arrêt de croissance hivernal, l'anneau apparaît sur la marge de l'écaille en octobre-novembre, lors de la reprise de croissance. Les critères objectifs d'identification de l'anneau sont identiques pour les truites de mer et pour les truites non migratrices; ils mettent en évidence un arrêt complet de croissance plutôt qu'un changement de rythme correspondant à un ralentissement de cette dernière. Les basses températures hivernales (5 mois à moins de 3 °C) alliées à une faible disponibilité de faune endogène expliquent l'arrêt de croissance. Malgré les faibles variations saisonnières des facteurs environnementaux (températures moyennes estivales ne dépassant pas 10 °C en eau douce et 7 °C en mer), on constate une grande variabilité de la croissance scalaire entre individus. En conclusion, les écailles peuvent être utilisées pour les étudesdémographiques, sous réserve de validation des déterminations à tous les âges.