Cannibalism in non-native brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss stream-dwelling populations.

Introduced and allopatric populations of brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss were sampled in Slovenia for stable isotope analysis to assess dietary niche shifts through ontogeny and estimate the propensity for cannibalism. Both S. trutta and O. mykiss are cannibals, with higher average relative contribution of conspecific assimilated energy for S. trutta (27·9%) compared with O. mykiss (7·7%). The smallest cannibal was 166 mm in the S. trutta population and 247 mm in the O. mykiss population.

Temporal changes in allele frequencies in a small marble trout Salmo marmoratus population threatened by extreme flood events.

The effect of extreme floods on the genetic composition of marble trout Salmo marmoratus living in Lipovscek, a tributary of the Soca River in Slovenia, which has been affected by multiple destructive flood events for centuries was investigated. By monitoring genetic variability during the period 2004-2011, apparent signatures of genetic erosion including a decline in observed and expected heterozygosities and allelic richness were observed. Contemporary effective population size was estimated between 11 and 55 individuals, which is congruent with census data. The data suggest asymmetric gene flow between the two sections of the river. The existence of substantial downstream migration (15-19%) was confirmed by paternity analysis. A small (1-3%) upstream migration was also suggested, which was confirmed by tagging data. Overall, low genetic diversity has not prevented the survival of the Lipovscek population, which might be a common feature of salmonid freshwater populations.

Eco-evolutionary dynamics induced by massive mortality events.

An eco-genetic model tuned on a population of marble trout Salmo marmoratus subject to periodic flood events was used to explore how the evolution of growth rates interacting with density-dependent processes can modify size at age and population structure and in turn influence the resilience of populations. Fish with greater growth potential were assumed to have higher mortality rates. The results of simulations were compared between two scenarios, one in which populations may evolve growth rates and the other one in which the distribution of growth rates within a population is kept fixed. Evolving populations had a greater proportion of age 1 year individuals in the population, greater median length at age 3 years (the typical age at sexual maturity for S. marmoratus) and lower population sizes. The slightly smaller population sizes did not affect realized extinction risk. Resilience, defined as the number of years necessary to rebound from flood-induced population collapse, was on average from 2 to 3 years in both scenarios, with no significant difference between them. Moderate heritability of growth, relaxation of density-dependent processes at low densities and rapid recovery to a safe population size combine to limit the capacity to evolve faster recovery after flood-induced population collapses via changing growth rates.

A simplified method to estimate body growth parameters of the European eel Anguilla anguilla.

A simple approach is proposed to fit a body growth model for the European eel Anguilla anguilla to data-poor case studies. The model is a modified von Bertalanffy curve allowing for delayed sex determination and sexual dimorphism. The proposed procedure provides preliminary estimates of model parameters on the basis of average age and body length of silver eels.

An evaluation of indices of gross pathology associated with the nematode Anguillicoloides crassus in eels.

This study compares two alternative indices for quantifying the gross pathology of the swimbladder of eels, Anguilla anguilla (L.), infected with the nematode Anguillicoloides crassus. Two observers recorded twice the scores obtained by the two indices on the same set of 71 wild caught eels (from elver to silver eels, French Mediterranean lagoons). The Length Ratio Index (LRI), performed better than the Swimbladder Degenerative Index (SDI), in three of four predefined criteria of decision. First, the LRI better correlated with an estimate of the swimbladder volume reduction, a functional consequence of the infection (representativeness). Also, the LRI was less prone to subjectivity (inter-observer variability) and more precise (intra-observer variability), although less easy to generate (time needed for measurement/assessment). Using a sub-sample of 32 unaffected eels (showing minor if any swimbladder damage and no living worms at autopsy), we ascertained a linear relationship between the swimbladder length and the total body length, a prerequisite of isometric growth, to definitively accept the new ratio index as a valid alternative to the SDI. Also, because the LRI can be recorded on live specimens with radio-imagery (non-invasive method), we recommend its use, and provide a graph of correspondence between the SDI scores, the LRI scores and the estimated proportion of gas loss in the swimbladder.

Morphological and genetic seasonal dynamics of European eel Anguilla anguilla recruitment in southern France.

The fine scale morphological and genetic dynamics of successive waves of Anguilla anguilla glass eel recruitment was studied over a 2 year period at a southern European Mediterranean location (Camargue, France) with continuous recruitment. Using morphometric [total length (L(T)), mass (M), condition (K) and pigmentation stage] as well as genetic (allozyme) markers, the aim was to test for the existence of temporally separated spawning groups and explore the relation between genetic variability and morphological heterogeneity of recruits. The results showed that L(T), M and K varied over time, being highest from the end of summer to winter (peaking in December) and lowest in spring (lowest in April). The pigmentation stages within monthly samples were highly diverse with a heterogeneous seasonal pattern. Allozyme data showed high genetic variability values within samples, but low genetic differentiation among samples (F(ST) = 0.003, P < 0.05). Pairwise comparisons between samples indicated a positive correlation between genetic differentiation and difference in recruitment time (days), with a marked increase in genetic differentiation around 250 days between monthly recruitment samples. Furthermore, genetic diversity increased with the number of pigmentation stages per sample and was negatively correlated with the North Atlantic Oscillation (NAO) index during the putative year of trans-oceanic migration. No correlation, however, was found between the level of multilocus heterozygosity (MLH) and growth variables. A situation of genetic patchiness with fluctuating parental contribution can thus best explain the patterns observed, although the existence of two separate spawning periods cannot be excluded. More discriminatory and sensitive genetic markers, such as (neutral and adaptive) microsatellites, could probably provide additional insights into the most probable hypothesis explaining the population structure and recruitment heterogeneity of A. anguilla.

Trace elements and chlorinated hydrocarbons in eggs of Pelecanus crispus, a world endangered bird species nesting at Lake Mikri Prespa, North-western Greece.

Eggs of the Dalmatian pelican, Pelecanus crispus, collected from 1984 to 1986 at Lake Mikri Prespa, north-western Greece, contained residues of trace elements and PCBs at low concentrations, and of DDE at rather high concentrations. DDE is negatively related to eggshell thickness. Eggshell thickness was reduced by 12-20% in comparison with the pre-1947 (before DDT use) eggshell thickness. However, this decrease did not affect the reproductive success of this species. The main fish species eaten by the Dalmatian pelican at Lake Mikri Prespa were analyzed for pollutants. All the fish contained low concentrations of residues. The contribution of the diet while the birds are on the wintering grounds is unknown. Recommendations for a monitoring programme of pollutants in eggs of the Dalmatian pelican, a world-endangered species, are proposed.

Anguillicolosis: dynamics of the infection over two decades.

The dynamics of the infection of the European eel Anguilla anguilla L. by the Asian nematode Anguillicola crassus Kuwahara, Niimi and Itagaki, 1974 (i.e. anguillicolosis) was monitored over 2 decades in an oligohaline canal in southern France (Camargue, Mediterranean coast). Since the first mention of the parasite in this canal in 1985, which was also the first record in France, prevalence of pre-adult and adult forms has risen from 32 to 73%. However, during the last 7 yr (1997 to 2003), prevalence seems to have stabilized around values of 60 to 70% and parasite load, though inter-annual variation is substantial, shows no sign of increase (intensity for the last 5 yr: min. = 3.70, max. = 9.66, mean = 6.01). Our results thus confirm the dynamic pattern observed elsewhere in Europe, i.e. a rapid spread following the introduction of the parasite in a water system and then stabilization around ceiling levels. We review possible mechanisms that may explain such a leveling off in the infection spread. We particularly document the possibility that repetitive infections may render the infected organ, i.e. the swimbladder, unsuitable for further A. crassus establishment. In support of this hypothesis, we showed that the infection rate is lower among eels with severely damaged swimbladders.

Spatial and temporal variation in Anguillicola crassus counts: results of a 4 year survey of eels in Mediterranean lagoons.

We present the results of a survey of anguillicolosis in the Rh6ne 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 pre-adults). 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 Rhjne 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.

Long-term monitoring on the occurrence of a myxosporean parasite Kudoa camarguensis (Myxosporean) on the common goby (Teleostei, pisces) Pomatoschistus microps.

The evolution of a host-parasite system composed of Pomatoschistus microps-Kudoa camarguensis was investigated in the Vaccarès lagoon (Rhjne river Delta, France) from 1993 to 1997. During this long-term monitoring, centennial flooding of the Rhjne river occurred, leading to an inrush of about 110 million m3 of freshwater in the Vaccarès lagoon. The salinity drastically dropped from 14 to 5 g l(-1) in 1 wk. We observed that the annual prevalence and abundance of the myxosporean parasite decreased from 12.18 in 1993 to 3.7% in 1997 and from 1.10 in 1993 to 0.27 in 1997, respectively. Here, we discuss the possible reasons for the rapid decline of this host-parasite system following the flood.

Host death: a necessary condition for the transmission of Aphalloides coelomicola Dollfus, Chabaud, and Golvan, 1957 (Digenea, Cryptogonimidae)?

Eggs of the trematode Aphalloides coelomicola were observed free in the abdominal cavity of Pomatoschistus microps from March 1993 to October 1995 (Pisces, Gobiidae) in the Vaccares lagoon (delta du Rhone, France). The eggs were contained in a fine membrane of parasitic origin as no immunological reaction of the fish was noted. The alternatively high prevalence of adults and low prevalence of free eggs, and vice versa, suggest that P. microps is both the second intermediate and final hosts of the trematode Aphalloides coelomicola and that the mechanism of transfer to the first intermediate host is through death of the definitive host.

[Pseudorascora parva (Teleostei, Cyprinidae), an invasive species, a new vector for the maintenance and dissemination of anguillicolosis in France?]]. / Pseudorasbora parva (Téléostéen, Cyprinidae) espèce invasive, nouvel agent du maintien et de la dissémination de l'anguillicolose en France?

Pseudorasbora parva (Pisces, Cyprinidae) is an invasive species from eastern Asia. Known in France since 1983, it has only been observed since 1993 in the Rhône delta where large populations occur. 203 specimens of that fish species have been examined for L3-larvae of Anguillicola crassus. 35% of the fish were found to be infected.

Is the brown shrimp Crangon crangon (L.) population of the Vaccarès lagoon (Camargue, France, Rhône delta) an annual population?

To study the population dynamics of Crangon crangon in the Vaccarès lagoon in the Camargue (Rhône delta), the age of brown shrimp was determined using the method of Tiews (1954). This method consists in counting the number of segments of the outer branch of the first antenna. This number is correlated with the number of moults already accomplished by the shrimp. The duration of the intermoult depends on water temperature. Adapting this model to our environmental data, we determined the age of the brown shrimp. Hatching of larvae occurs in the sea in May, and juveniles enter the lagoon from June to September. Our results showed that Crangon crangon in the Vaccarès lagoon reached sexual maturity between 6 and 11 months, and individuals are 1 year old when they leave the lagoon. A few adults from the previous year (1+) migrate back to the lagoon the following year, but these older individuals represent only a very small part of the population, and have only been observed occasionally. The Vaccarès brown shrimp population can therefore be considered as annual, in contrast to northern European populations.

The health state of the eel swimbladder as a measure of parasite pressure by Anguillicola crassus.

We have developed an index based on macroscopic criteria to easily assess the health state of the eel swimbladder as a result of infection by the nematode Anguillicola crassus. In the sampling area (brackish lagoons of the French Mediterranean coast), 92% of the host sample (1,251 eels covering all size classes) showed pathological signs of infection. The general trend was for increasing damage as eel size increased, thus suggesting accumulation of pathological effects. We also revealed a non-linear relationship between the swimbladder index and the abundance of living worms. In particular, we showed that severely damaged swimbladders harboured very few living nematodes. We argue that the swimbladder degenerative index more closely reflects the parasite pressure than does classic parasite count. We found seasonal variation in the swimbladder index, with maximum damage occurring in July and thereafter a trend for healthy individuals. We discuss the possibility that the seasonal decrease in the swimbladder index could reflect the death of the more severely affected individuals during the warmest months.

The genetic diversity of native, stocked and hybrid populations of marble trout in the Soca river, Slovenia.

The marble trout (Salmo marmoratus) is an endangered species in Slovenia (and in Italy, Croatia and Albania) because of hybridization resulting from intensive stocking. Eleven populations of trout from the Soca river basin (Slovenia) were analysed using 31 presumptive enzyme loci. Three European control samples representing the main European genotypes of brown trout were also analysed. This analysis confirmed the existence of extensive hybridization in the main river, resulting in an equal mixture of alleles belonging to marble trout, and Danubian and Atlantic brown trout (Salmo trutta). Despite the high level of introgression observed in the main Soca River, nearly pure populations of marble trout were found in the headwaters of five tributaries. The five stations with pure marble trout will be the basis of a rehabilitation programme. Hardy-Weinberg and linkage disequilibria were observed and hypotheses are discussed to explain this. A scenario for the colonization of the Adriatic region is proposed.

Influence of the trematode Aphalloïdes caelomicola Dollfus, Chabaud & Golvan, 1957 on the fecundity and survival of Pomatoschistus microps (Krøyer, 1838) (Teleostei: Gobiidae).

Monthly data collected during 1993 in the Vaccarès lagoon (Rhône Delta, France) were used to investigate the influence of the trematode Aphalloïdes coelomicola on the survival of the common goby Pomatoschistus microps, which acts both as second intermediate and definitive host. Prevalence of the parasite did not increase with fish size in either sex. Mean abundance and variance to mean abundance ratio gave evidence that the trematode could have an impact on host mortality. Gonad weight of parasitized females was reduced and the parasite seemed to provoke a gonadal regression above a certain parasitic biomass. Fecundity and egg diameters did not differ between parasitized and unparasitized females, we thus hypothesize that the energy content of eggs (composition in lipids and proteins) was probably modified as an adaptation to energy losses. Prevalence of infection in males compared to females and influence of the parasite on host survival and reproductive effort are discussed.

Extreme genetic differentiation among the remnant populations of marble trout (Salmo marmoratus) in Slovenia.

Populations of the marble trout (Salmo marmoratus) have declined critically due to introgression by brown trout (Salmo trutta) strains. In order to define strategies for long-term conservation, we examined the genetic structure of the 8 known pure populations using 15 microsatellite loci. The analyses reveal extraordinarily strong genetic differentiation among populations separated by < 15 km, and extremely low levels of intrapopulation genetic variability. As natural recolonization seems highly unlikely, appropriate management and conservation strategies should comprise the reintroduction of pure populations from mixed stocks (translocation) to avoid further loss of genetic diversity.