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Relationships between the presence of PCBs, OCPs and metals in aquatic ecosystems and the ecological water quality were investigated by combining datasets of long-term monitoring of chemicals in European eel (Anguilla anguilla, N = 1156) in Flanders (Belgium) and the Ecological Quality Ratio (EQR), based on the assessment of fish assemblages at 185 locations. For most pollutants, EQR scores were lower when pollutant levels were higher. Threshold concentrations for a good quality could be formulated for PCB's, most metals and OCPs. Mixed models suggested that the ecological water quality was significantly correlated with the presence of PCBs. However, the low R2 indicates that other environmental pressures may significantly influence the biotic integrity of fish communities. Empirical data and their analyses are essential to enable defining threshold values of bioaccumulated levels to allow better protection of the aquatic environment and its biota through associated food webs as demanded by the Water Framework Directive.

Archived otolith collections represent an invaluable source of information to study demographic and genetic changes in commercially important fish populations. Studies combining both approaches are however rare and reliable extraction of molecular and population demographic data from the same collection of otoliths has never been assessed in the endangered European eel ( Anguilla anguilla L.). Here we evaluate various DNA extraction protocols to compare DNA yield, microsatellite amplification success, genotype integrity and precision of age determination for eel otoliths that have been archived for 4 weeks, and for 28 and 48 years. Our results show a high amplification success and an equal genotype integrity for DNA fragments extracted from both recently sampled otoliths and high quality reference DNA tissue. Although historical samples yielded low amounts of DNA, PCR amplification was successful and genotyping reliable for short fragments, but decreased significantly with PCR fragment size. None of the extraction protocols caused physical damage to the otoliths and precision of age determination was high for both treated and untreated otoliths. Hence, the methodology can be applied as a standard for the further joint analysis of past demographic and genetic changes during the last century in the highly exploited European eel and in other fish requiring urgent conservation measures.

The evolutionary enlightened management of species with complex life cycles often requires the development of mathematical models integrating demographic and genetic data. The genetic structure of the endangered European eel (Anguilla anguilla L.) has been thoroughly analyzed in several studies in the past years. However, the interpretation of the key demographic and biologic processes that determine the observed spatio-temporal genetic structure has been very challenging owing to the complex life cycle of this catadromous species. Here, we present the first integrated demographic-genetic model applied to the European eel that explicitly accounts for different levels of larval and adult mixing during oceanic migrations and allows us to explore alternative hypotheses on genetic differentiation. Our analyses show that (i) very low levels of mixing occurring during larval dispersal or adult migration are sufficient to erase entirely any genetic differences among sub-populations; (ii) small-scale temporal differentiation in recruitment can arise if the spawning stock is subdivided in distinct reproductive groups; and (iii) the geographic differentiation component might be overestimated if a limited number of temporal recruits are analyzed. Our study can inspire the scientific debate on the interpretation of genetic structure in other species characterized by complex life cycle and long-range migrations.

Anguillid freshwater eels show remarkable life histories. In the Atlantic, the European eel (Anguilla anguilla) and American eel (Anguilla rostrata) undertake extensive migrations to spawn in the oceanic Sargasso Sea, and subsequently the offspring drift to foraging areas in Europe and North America, first as leaf-like leptocephali larvae that later metamorphose into glass eels. Since recruitment of European and American glass eels has declined drastically during past decades, there is a strong demand for further understanding of the early, oceanic phase of their life cycle. Consequently, during a field expedition to the eel spawning sites in the Sargasso Sea, we carried out a wide range of dedicated bio-physical studies across areas of eel larval distribution. Our findings suggest a key role of oceanic frontal processes, retaining eel larvae within a zone of enhanced feeding conditions and steering their drift. The majority of the more westerly distributed American eel larvae are likely to follow a westerly/northerly drift route entrained in the Antilles/Florida Currents. European eel larvae are generally believed to initially follow the same route, but their more easterly distribution close to the eastward flowing Subtropical Counter Current indicates that these larvae could follow a shorter, eastward route towards the Azores and Europe. The findings emphasize the significance of oceanic physical–biological linkages in the life-cycle completion of Atlantic eels.

European eels (Anguilla anguilla) undertake spawning migrations of more than 5000 km from continental Europe and North Africa to frontal zones in the Sargasso Sea. Subsequently, the larval offspring are advected by large-scale eastward ocean currents towards continental waters. However, the Sargasso Sea is oligotrophic, with generally low plankton biomass, and the feeding biology of eel larvae has so far remained a mystery, hampering understanding of this peculiar life history. DNA barcoding of gut contents of 61 genetically identified A. anguilla larvae caught in the Sargasso Sea showed that even the smallest larvae feed on a striking variety of plankton organisms, and that gelatinous zooplankton is of fundamental dietary importance. Hence, the specific plankton composition seems essential for eel larval feeding and growth, suggesting a linkage between eel survival and regional plankton productivity. These novel insights into the prey of Atlantic eels may furthermore facilitate eel larval rearing in aquaculture, which ultimately may replace the unsustainable use of wild-caught glass eels.

European eel (Anguilla anguilla (L.)) stocks are in decline in most of their distribution area and their status is considered below safe biological limits. There are numerous possible causes for this decline. Recently, there is anincreasing awareness that spawner quality might be an essential element in the decline of the species. Pollution by chemical substances may have a large impact on the reproduction success of the eel. This study gives a literature overview of the consequences of these contaminants on the biology and fitness of the European eel in order to document the role of pollution, within the decline and to support the eel management and restoration plans.

World-wide, underwater background noise levels are increasing due to anthropogenic activities. Little is known about the effects of anthropogenic noise on marine fish, and information is needed to predict any negative effects. Behavioural startle response thresholds were determined for eight marine fish species, held in a large tank, to tones of 0.1–64 kHz. Response threshold levels varied per frequencywithin and between species. For sea bass, the 50% reaction threshold occurred for signals of 0.1–0.7 kHz, for thicklip mullet 0.4–0.7 kHz, for pout 0.1–0.25 kHz, for horse mackerel 0.1–2 kHz and for Atlantic herring 4 kHz. For cod, pollack and eel, no 50% reaction thresholds were reached. Reaction threshold levels increased from For cod, pollack and eel, no 50% reaction thresholds were reached. Reaction threshold levels increased from For cod, pollack and eel, no 50% reaction thresholds For cod, pollack and eel, no 50% reaction thresholdswere reached. Reaction threshold levels increased from - 100 dB (re 1 uPa, rms) at 0.1 kHz to - 160 dB at 0.7 kHz. The 50% reaction thresholds did not run parallel to the hearing curves. This shows that fish species react very differently to sound, and that generalisations about the effects of sound on fish should be made with care. As well as on the spectrum and level of anthropogenicsounds, the reactions of fish probably depend on the context (e.g. location, temperature, physiological state, age, body size, and school size). 2008 Elsevier Ltd. All rights reserved.

Belpaire, C., Goemans, G., Geeraerts, C., Quataert, P., and Parmentier, K. 2008. Pollution fingerprints in eels as models for the chemical status of rivers. – ICES Journal of Marine Science, 65: 1483–1491. The 2006 EU Water Framework Directive (WFD) proposed the monitoring of a selection of priority substances in the aquatic phase, including lipophilic substances. However, there are strong arguments for measuring lipophilic substances in biota. Yellow eel is a good candidate because it is widespread, sedentary, and accumulates many lipophilic substances in its muscle tissue. Several authors have described the indicative value of measured concentrations, yet few studies have investigated to what extent the spectrum of contaminants present characterizes the local environmental pollution pressure. To evaluate the value of the pollution profile of an eel as a fingerprint of the chemical status of the local environment, two datasets were selected from the Flemish Eel Pollutant Network database. The pollution profiles in individual eels along a river (even at distances

Belpaire, C., and Goemans, G. 2007. Eels: contaminant cocktails pinpointing environmental contamination. – ICES Journal of Marine Science, 64: –. There is growing concern that insufficient somatic and health conditions of silver European eels (Anguilla anguilla) emigrating from European waters to oceanic spawning areas might be a key causative factor in the decline of the stock. One factor that could contribute to deterioration in the status of eels is high contaminant accumulation in their body. Contaminants may affect lipid metabolism and result in lower energy stores. A high body burden of contaminants and low energy stores might be responsible for failure of migration and/or impairment of successful reproduction. During a 12-year study on a relatively small area within the river basins of IJzer, Scheldt, and Meuse (ca. 13 500 km2), 2613 eels were sampled covering a dense monitoring network of 357 stations. Eels were analysed for ca. 100 chemicals. These included PCBs, organochlorine pesticides, heavy metals, brominated flame retardants, volatile organic pollutants (VOCs), endocrine disruptors, dioxins, perfluorooctane sulphonic acids (PFOSs), metallothioneins, and polycyclic aromatic compounds. This series represents only a very small fraction (30 000 chemicals currently marketed and used in Europe. The biomonitoring value of eels as a tool for monitoring environmental contamination is illustrated. Two major conclusions were drawn: (i) the eel is a highly suitable biomonitor for environmental contaminants, for both local and international purposes, e.g. to evaluate the chemical status for the Water Framework Directive, and (ii) dependent on the degree of pollution in their habitat, the levels of certain contaminants reported in yellow eels can be high, and might affect their potential for reproduction.

Summary A standardized method was developed for artificially infecting the European eel with Anguillicola crassus, a parasitical swim bladder nematode. For this purpose, an intermediate host, Paracyclos fimbriatus, and a paratenic host, Cyprinus carpio, were used. Parasite prevalence in the artificially infected eels was very high, although there was a large variation in the number of worms per eel. Compared to other methods of artificial infection described in the literature, this method apears to be more efficient and reliable; there were also more infectious larvae that were recovered and tRe mortality rate of eels under treatment was lower. Resume Une methode amelioree pour l'infection artificielle de l'anguille europeenne Anguilla anguilla par Anguillicola crassus (Nematoda, Dracunculoidea) Une methode standardisee a ete developpee dans le but d'infecter artificiellement l'anguille europeenne par Anguillicola crassus, un nematode parasitique qui se trouve dans le vessie natatoire de l'hote. Dans ce but, un hote intermediaire Paracyclops fimbriatus et un hote paratenique Cyprinus carpio ont ete utilises. La prevalence du parasite dans les anguilles infectees artificiellement etait haute, bien que le nombre de vers retrouves ingviduellement etait tres varie. Compare sa d'autres methodes d'infection artificielle decrites dans la litterature, cette methode demontre ere plus efficace: le nombre de larves contaminantes rassemblees etait plus eleve, la mortalite parmi les anguilles traitees etait moins importante et la methode etait plus fiable. Zusammenfassung Eine verbesserte Methode der kunstlicben Infektion des Europaiscben Aales, Anguilla anguilla, rnit Anguilhcola crassus (Nematoda, Dracunculoidea) Es wurde eine standardisierte Methode zur Infektion des europaischen Aals rnit dem Schwimmblasenwurm Anguillicola crassus entwickelt. Zu diesem Zweck wurden ein Zwischenwirt, Paracyclops fimbriatus und ein Wirt, Cyprinus carpio, eingesetzt. Die Haufigkeit des Parasiten in den kunstlich infizierten Aalen war sehr hocg, jedoch konnte eine hohe Variabiliat individuellen Wurmbefdls festgestellt werden. Im Vergleich zu anderen Infektionsmethoden, die in der Literatur beschrieben sind, erscheint die mir erprobte Methode effizienter: die Wiederauffindungsrate infektiver Larven war hoher und die Mortalitatsrate der behandelten Aale geringer. Die Methode wird auch als zuverlassiger angesehen.

Concentrations of the sum of the seven indicator PCBs (b.Sigma7 iPCBs) measured in non-commercial European eel (Anguilla anguilla L.) in Flanders are high: in 80% of all sampled localities, the Belgian PCB standard for fish was exceeded. The objective of this study was to assess the intake of the b.Sigma7 iPCBs through consumption of eel by recreational fishermen and to compare it to the intake of a background population. The median estimated intake for recreational fishermen varied between 18.4 and 237.6 ng iPCBs kg-1 bw day-1, depending on the consumption scenario, while the estimated intake of the background population (consumers only) was 4.3 ng iPCBs kg-1 bw day-1. Since the levels of intake via eel for two intake scenarios were, respectively, 50 and 25 times higher than the intake of the background population, the body burden (BB) might be proportionally higher and reach levels of toxicological relevance. The intake of the seven iPCBs via consumption of self-caught eel in Flanders is at a level to cause serious concern. The Flemish catch-and-release obligation for eel, established in 2002, should be maintained and supervised (more) carefully.

Marine organisms experience a broad range of intrinsic and extrinsic influences during their lives, which impact their population dynamics and genetic structure. Subtle interpopulation differences reflect the continuity of the marine environment, but also pose challenges to those wishing to define management units. The catadromous European eel (Anguilla anguilla) is no exception. Its spawning habitat in the Sargasso Sea and long migration across the North Atlantic qualify it as marine. However, the synergy between hydrographic variability, changing climate, and the impacts of habitat degradation and overfishing in continental waters has negatively affected stock sizes. Its protracted spawning period, variance in age-at-maturity, parental contribution and reproductive success, and the difficulty in sampling the spawning region together may mask a weak geographical genetic differentiation. Recent molecular data report evidence for spatial as well as temporal differences between populations, with the temporal heterogeneity between intra-annual recruitment and annual cohorts exceeding the spatial differences. Despite its common name of "fresh-water eel", the European eel should really be managed on a North Atlantic scale. The fishery may have to be curtailed, migration routes kept open and water quality restored if it is to survive. Eel aquaculture has to focus on efficient rearing in the short term and controlled breeding in the long term. Future research on eel genetics should focus on (i) sampling and analysing spawning populations and recruitment waves to detect spatio-temporally discrete groups, and establishing a biological baseline from pre-decline historical collections for critical long-term monitoring and modelling of its genetic composition; (ii) the analysis of adaptive genetic polymorphism (genes under selection) to detect adaptive divergence between populations, perhaps requiring separate management strategies; and (iii) improving artificial reproduction to protect natural stocks from heavy exploitation, especially now the species has been categorized as endangered.

The European eel (Anguilla anguillaL.) has been a prime example of the panmixia paradigm because of its extraordinary adaptation to the North Atlantic gyral system, semelparous spawning in the Sargasso Sea and long trans-oceanic migration. Recently, this view was challenged by the suggestion of a genetic structure characterized by an isolation-by-distance (IBD) pattern. This is only likely if spawning subpopulations are spatially and/or temporally separated, followed by non-random larval dispersal. A limitation of previous genetic work on eels is the lack of replication over time to test for temporal stability of genetic structure. Here, we hypothesize that temporal genetic variation plays a significant role in explaining the spatial structure reported earlier for this species. We tested this by increasing the texture of geographical sampling and by including temporal replicates. Overall genetic differentiation among samples was low, highly significant and comparable with earlier studies (FST=0.0014;p

Marine organisms usually exhibit a high genetic diversity, a subtle population structure and a low level of genetic differentiation, compared to freshwater organisms. The subtle genetic differences in time and space reflect the continuity of the marine environment. Marine organisms experience a wide range of intrinsic and extrinsic influences during their life cycle, which considerably impact their biological population size and genetic population structure. Furthermore, genetic variability is crucial for the survival of organisms as it enables evolution while maintaining fitness. Marine species however have a high genetic load, affecting the population even more during a population decline or bottleneck. The European eel Anguilla anguilla (Anguillidae; Teleostei), although inhabiting fresh- and saltwater, represents no exception. Its spawning habitat in the Sargasso Sea and extensive migrations across the North Atlantic Ocean qualify it fully as a marine species. This thesis describes the multiple evolutionary consequences of the catadromous life-strategy on the genetic structure of the European eel.Recent data based on microsatellite markers show a subtle genetic structure in the European eel following an Isolation-by-Distance (IBD) pattern. But since genetic introgression from the American eel into the European eel has been suggested in the North Atlantic Ocean, reliable tests were developed to define the species status of the European eel. In this first part the interspecific conservation of a set of microsatellites was tested on other Anguilla taxa and the power of species discrimination was assessed. We then applied this knowledge by screening Icelandic and European samples for introgression of American eel. Indications of unequal but restricted hybridisation were detected, likely maintained through selection against hybrids and the preservation of migrational cues.In the second part, the genetic variability and differentiation between various glass and silver eel populations was compared over a broad geographical range (Iceland to Morocco; Spain to Turkey) with temporal replications. In the first instance a pattern of Isolation-by- Distance was detected in adult populations using allozyme markers. Following a more extended geographical sampling, the temporal stability of this pattern could not be confirmed; the temporal differentiation between populations clearly exceeded the geographical component. By sampling recruiting glass eels over a three-year period, a stronger genetic differentiation was found between temporally separated cohorts. Inter-annual differentiation was much higher than the geographical differentiation. The population genetic structure of eel is likely determined by a double process: (I) a large scale pattern of Isolation-by-Time (IBT) among spawning cohorts, and (2) a smaller scale variance in adult reproductive success (genetic patchiness) among seasonally separated cohorts, most likely originating from oceanic and climatic influences.In the third part, the relation between multi-locus heterozygosity (MLH) and fitness components was studied. If an association exists between genetic variability and fitness traits, it is even more important to maintain the population size of European eel. The catastrophic decline of the European eel might be the consequence of an accelerated loss of genetic diversity, with extinction as possible outcome. This hypothesis was tested in a polluted natural environment and in an eel farm. Eel from three Belgian drainage basins were screened for fitness, heavy metal bioaccumulation and genetic variation. There was a strong negative correlation between MLH and bioaccumulation in highly polluted eels. In a second study, aquacultured eel were screened for fitness and genetic variation; MLH was correlated to growth rate. In both studies, this effect was mainly attributed to metabolical enzymes important in the energy cycle, which points to the importance of

Eel (Anguilla anguilla L.), a commonly consumed fish, is known to bioaccumulate lipophilic contaminants such as polychlorinated biphenyls (PCBs) and organochlorine pesticides. Since 1994, the Flemish eel pollutant monitoring network monitors about 300 different sites in Flanders (the northern part of Belgium). This includes rivers, canals, polder waters and closed water bodies. The concentrations of the sum of the 7 indicator PCBs (PCB congeners 28, 52, 101, 118, 138, 153, 180) measured by this monitoring network, are very high: in 80% of all sampled localities, the Belgian PCB standard for fish (75 ng/g fresh weight) is exceeded. Therefore, in 2002, the Flemish authorities have issued a catch-and-release obligation for all fish in the 5 most polluted waters in Flanders and an overall catch-and-release obligation for eel in Flanders. In spite of this restriction, some sport fishers still take their eel home for consumption. The objective of this study was to assess the intake of the 7 indicator PCBs (iPCBs) in this subgroup and to compare their intake with the exposure of a Flemish background population.

The genetic variability and structure of the European eel (Anguilla anguilla L.) in populations throughout Europe was reassessed using 15 allozymic loci, seven of which were polymorphic. Seven sites were sampled on a latitudinal gradient across the natural continental range, extending from southern France to southern Norway. Heterozygosity (He = 0.05) and level of polymorphism (P = 0.43) were comparable to other marine fish. Populations were poorly differentiated (GST = 0.014, FST = 0.002), which is not surprising considering the high dispersal capability of the European eel. However, a significant geographical cline was detected at two alleles (IDh-1*100 and GPI-1*110), andgenetie distances (DCE) were concordant with geographical coastal distances. Mantel tests, pairwise FST's and multidimensional scaling analyses identify three distinct groups: Northern Europe, Western Europe and the Mediterranean Sea. We propose that the clinal genetic structure in the European eel may be due to (1) isolation by distance (as recently detected with microsatellites), (2) temporal reproductive separation, (3) post-larval selective forces, (4) contact between formerly separated groups or (5) some combination thereof.

Clones (Anguilla anguilla) were obtained from a size-selected library (300-800 bp HaeIII, PvuII, EcoRV-digested inserts), by hybridization with gamma super(32)P-labelled (dG-dT) sub(15). The sequences of the loci Aan01, Aan02, Aan03 and Aan04 have been submitted to GenBank/EMBL (acc. nos U67163, U67164, U67165 and U67166).

Six anthelmintic drugs, Ivermectin, Closantel, Safewormer, Masoten, Oxfendazole and L-Levamisol·HCL were screened for their efficacy against Anguillicola crassus, a swimbladder nematode of European eel Anguilla anguilla. All experiments were carried out under saltwater conditions. Of the 6 drugs, L-Levamisole·HCL used in a bath treatment gave the most promising results. Evaluation of the curative dose of L-Levamisole·HCL showed that a long-term treatment (24h). Dosages lower than 20 mg 1-1 showed a lower efficacy against Anguillicola crassus. However, even with a long-term, high-dosage treatment, some recovery of adult nematodes with sublethal damage and/or migration of viable juveniles into the swimbladder lumen could not be excluded. When examined 35d after treatment, the percentage of mobile adults had increased, while a decrease in percentage of dead worms was noticed compared to values obtained immediately after treatment.

Eels were sampled twice a month (March 1990 though March 1991) at the Kolenhaven (Albertcanal, Genk, Belgium) and examined for Anguillicola crassus nematodes in the swimbladder. Averaged over all samples, 90.2% of the 345 eels were infected with a mean intensity (averaged for all stages) of 17 nematodes. Thirty-one percent of all nematodes, which were adult worms or their remnants, were present in the swimbladder lumen, while the other 69% which consisted of the third or fourth larval stage and preadults were found in the swimbladder wall. No clear seasonal fluctuations in parasite prevalence or mean intensity were observed. Both L3-larvae and mature adult worms were present throughout the year. Eggs or L2-larvae were observed in 41% of the infected eels. This presence showed no seasonal pattern, though it was positively correlated with eel length: massive concentrations of L2-larvae were only found in eels bigger than 40 cm. A positive but weak correlation between eel length and number of adult nematodes in the swimbladder was significant, while no correlation was found between eel length and total parasite burden.

Summary Data on the distribution in 1986 of the parasitic nematode in the swimbladder of the European eel (Anguilla anguilla L.) in Flanders (Belgium) are reported. In stations where imported eels have been used for restocking, samplings generally show high infection rates thus enhancing the spread of Anguillicola crassus in Flanders.

Growth in eel populations is not centered around the mean. The model described here is designed to take the growth rate variability into account by classifying the population, step by step in time, into size boxes on which grading and grouping occurs. By selecting options, this model allows the fit to other growth conditions and therefore can be adapted to other species. This growth model is the backbone of a predictive model for aquaculture, built up by adding allometric functions related to basic physiological properties.

Migration patterns of third-stage Anguillicola crassus larvae, and pathogenesis of the lesions induced by third-stage larvae, was investigated in European eel Anguilla anguilla L. Young elvers (1g) were fed infected Paracyclops fimbriatus (Copepoda). Eel samples were collected and examined histologically at varying intervals during 6 mo post-infection period. Third-stage larvae (L-III) migrated directly through the intestinal wall and body cavity to the swimbladder within 17h post-infection. L-IV larvae were detected 3 mo post-infection, and immature adults were detected within 4 mo post-infection. The parasites occasionally showed aberrant migration paths. Pathological effects caused by the parasite were less severe after experimentally induced infections than those detected in some natural infections.

In the regions where eels are common, they became a priced food species. In Europe, this is particularry the case around the North Sea and the Baltic, in most estuaries and large rivers, and in the coastal lagoons of the Atlantic and Mediterranean Sea. In the countries with high eel consumption, the demand exceeds the catches due to a decrease in the fisheries. Importations started from surrounding countries where eels are more or less consumed and where fishing could sustain the trade. This trend led to look for further away producing countries as well as overseas (America, New Zealand, etc.) in order to compensate the decline in the availability of the european eel, even with exotic species. This led also some eel consuming countries, triggered by the example of Japan, to improve traditional and extensive aquaculture practices (such as in lagoons) and to develop new farming technology. In some Northern European countries mainly, the acute need for eels also resulted in the launching of experimental or commercial plants for intensive eel farming in heated water. The aim of this paper is to describe the availability of European eels, the market demands and the importations necessary to cover the consumption. The trends in these matters does not allow optimism with regard to future availability. In this respect, aquaculture and a good management of the wild eel stocks are the solutions to the demand for this species.