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Food and feeding activity of glass eel Anguilla anguilla (L.) stocked in earthen ponds. Ir Fish Investig Ser A (Freshw)
... Apparently because of these structural changes to the body, their feeding ceases completely during metamorphosis. This feeding cessation through metamorphosis from leptocephalus to glass eel marks a major shift in diet from consuming marine snow, which consists of a wide range of detrital materials and carbohydrate exudates from plankton and bacteria that form soft aggregates that is floating free in the ocean surface layer (Miller et al., 2020), to eating benthic aquatic animals in estuaries and rivers (Belpaire et al., 1992). ...
... This point needs to be noted with respect to the data and should be improved in future studies. Although the paste diet was developed for feeding glass eels, we cannot completely rule out the possibility that the individual variations in the feeding initiation time were the result of feeding the eels a diet that is different from their natural diets, which are likely to be small aquatic animals such as amphipods (Mordenti et al., 2016), or insect larvae of the Chironomidae, Caenidae and Diplostraca (Belpaire et al., 1992). However, even in a comparison within the same weeks after completion of metamorphosis, eels with more developed digestive organs such as the stomach length or higher expression of digestive enzymes were observed, suggesting that the individual variations in the timing of feeding starting was not solely due to the use of an artificial diet. ...
... The remarkable elongation of the stomach, and differentiation and numerical increase of gastric glands in post-metamorphic glass eels seems to be related to a rapid development of their ability to digest more protein, which is required in association with a diet shift from the marine snow diet of leptocephali (Miller et al., 2013(Miller et al., , 2020 to feeding on aquatic invertebrates as young eels (Belpaire et al., 1992). Pepsinogen secreted by the gastric glands reacts with gastric acid that is likewise secreted by the gastric glands and is converted to active pepsin, which contributes to proteolysis. ...
Temporal changes of feeding incidences, digestive organ tissues, and mRNA expression of digestive enzymes
were investigated in artificially reared Anguilla japonica glass eels. Incidences of daily feeding and cumulative
first feeding exceeded 50% at day 40 and 33, respectively, but tended to increase gradually. Time until first
feeding varied from day 11–61 among individuals. Glass eels without food after metamorphosis showed notable
developmental changes in their digestive system that included increases in esophagus goblet cells and blood
vessel diameters, elongation of the stomach, differentiation and increased number of gastric glands, and increases
in gall bladder sizes. A distinct qualitative change, which was gastric gland differentiation, occurred in all
eels until week 2 after metamorphosis. All quantitative character values showed trends of gradual changes in
their averages and had high coefficients of variation, especially for goblet cell numbers. Few deaths and no
histological features related to severe starvation such as notably more hepatocyte necrosis or desquamation of
intestinal epithelial cells were observed even after 9 weeks. Hepatocyte-vacuole stored glycogens and pancreatic
zymogen granules were also found, suggesting that they were not expressing symptoms of starvation. Relative
expressions of five digestive enzymes were lowest at week 0 and increased gradually in their averages with the
passage of weeks, along with large variations among fish. This study found that half of the post-metamorphic
glass eels began feeding after 4 weeks, but that there were large individual variations in the timing of feeding
onset, the degree of developmental of their digestive organs, and in digestive enzyme expressions.
... Stocking practices were mainly conducted in estuaries, lakes and lagoons (e.g. Belpaire et al., 1992;Edeline & Elie, 2004;Rigaud et al., 2015), which made monitoring a difficult process due to the size, and biotic and abiotic conditions of the area where recaptures of the released specimens needed to take place. So, although contributing to an improvement in local fisheries (Ciccotti, 1997;Pedersen, 1997;Rosell, 1997), the stocking did not seem to mitigate the continuous declining trend of recruitment (Feunteun, 2002;ICES, 2014;Moriarty & Dekker, 1997), raising some doubts about the effectiveness of stocking programmes for recovery of the species (Feunteun, 2002). ...
... However, comparability with other studies is limited, as these have been conducted in experimental ponds (e.g. Belpaire et al., 1992;Edeline & Elie, 2004), in reaches closer to the coastline (e.g. Rigaud et al., 2015) or in lakes, and stocked with farmed larger eels (Lin et al., 2007;Simon et al., 2013). ...
... Nonetheless, growth differences between locations are largely due to different habitat conditions that may be more or less favourable for eel growth. Under low-density conditions where competition for food does not affect growth (Belpaire et al., 1992), the quality of prey may play an important role, despite the high dietary plasticity of eel (Belpaire et al., 1992;Yalçın-Özdilek & Solak, 2007). In the present study and in the studies by Ovidio et al. (2015) and Nzau Matondo et al. (2019), eels were stocked at different, but all low, densities, allowing a comparison of results, and in the present study, eels showed a faster growth at higher densities. ...
One of the actions that has been implemented to support the recovery of the panmictic population of European eel is stocking of waterbodies where natural recruitment is low or null. However, growth conditions of the stocked eels can vary greatly. This circumstance emphasises the importance to determine ideal habitat conditions to contribute to the success of stocking actions and, consequently, to increase the production of silver eels. This study aims to evaluate the early settlement and growth of stocked glass eels in the upper reaches of a fragmented river. Stocking was carried out, in 2014, at three sites of an inland tributary of the Mondego river basin (Central Portugal), and its monitoring was conducted during the following two years, until 2016, along with the collection of environmental and hydromorphological parameters. The results showed a successful dispersion throughout the study area. Growth varied spatially, although environmental parameters have not clearly explained this variation, but overall with high growth rates and a positive allometric growth in this early stage of stocking, suggesting a good condition of the stocked individuals. This study showed that these upper reaches of fragmented watercourses, a shared feature amongst most European rivers that are currently inaccessible for natural recruitment, may be suitable habitats for eel stocking.
... Thus, important questions addressing ecological issues, such as the impact caused on other species, either through predation (over lower trophic groups) or competition (other fish species) remain unanswered or fall short. Although eels show a high dietary plasticity on a spatial and temporal scale (Belpaire et al., 1992;Yalçın-€ Ozdilek and Solak, 2007), in freshwater, insect larvae compose the main diet in early stages (Belpaire et al., 1992;Tesch, 2003;Yalçın-€ Ozdilek and Solak, 2007) and, thus, having the potential to be highly impacted. Inter-specific competition for food, on the other hand, depends on the level of dietary overlap. ...
... Thus, important questions addressing ecological issues, such as the impact caused on other species, either through predation (over lower trophic groups) or competition (other fish species) remain unanswered or fall short. Although eels show a high dietary plasticity on a spatial and temporal scale (Belpaire et al., 1992;Yalçın-€ Ozdilek and Solak, 2007), in freshwater, insect larvae compose the main diet in early stages (Belpaire et al., 1992;Tesch, 2003;Yalçın-€ Ozdilek and Solak, 2007) and, thus, having the potential to be highly impacted. Inter-specific competition for food, on the other hand, depends on the level of dietary overlap. ...
... This indication of an absence of inter-specific competition may be related to a high carrying capacity of the habitat but also to prey biometry. Nonetheless, the well-known dietary plasticity of the European eel (Belpaire et al., 1992;Yalçın-€ Ozdilek and Solak, 2007), may allow a change in feeding preferences, depending on potential competitors, which, in turn, favours a good adaptation to different habitat types observed throughout the eel distribution range. ...
In an attempt to assist the recovery of the panmictic population of the European eel, declining since the late 1980s, the restocking of areas with low or no natural recruitment has been one of the measures adopted to reverse this trend. However, the main focus in several monitoring programmes for these actions, has been in the best interest of its viability and cost/benefit relationships and, for that, the condition of the released stocks has been the main concern. Yet, so far, no studies have assessed the potential ecological impacts that restocking might have on other biological communities. This pioneer pilot study aimed to evaluate the early ecological impact of a restocking event on other biological communities, considering inter-specific competition (other fish species) and feeding impact (macroinvertebrates).
The reference condition of the biological communities of an inland tributary of the Mondego river, the River Ceira, was determined in three sites inaccessible to the natural recruitment of eels, followed by a post-stocking assessment. The results showed no significant changes in the fish assemblages in restocked areas, contrary to the macroinvertebrate community. However, the ecological status for the macroinvertebrate community showed no deleterious effects, with the results suggesting exactly the opposite. This may be related to the low density of the restocked eels and factors influencing the local trophic web. This study confirms the suitability of the habitat for restocking with glass eels, during its early stages, without disrupting the local ecological status, using densities close to those of natural recruitment.
... Bouchereau et al. (2009a) found that eels tend to consume benthic prey that are most abundant at a particular time. In general, eels appear to be bottom-dwelling predators feeding mainly on epibenthic and periphytic invertebrates and small fishes (Bergersen and Klemetsen, 1988;Belpaire et al., 1992;Do¨rner et al., 2009). Do¨rner et al. (2009 even showed that piscivory by eels appeared negligible when macroinvertebrate (insects) availability was high. ...
... As expected, ingested macro-invertebrate taxa were found to be present in the Schelde-Lippenbroek ecosystem (Beauchard et al., submitted). Since eels adapt their diet according to the available resources offered by the ecosystem (Belpaire et al., 1992;Bouchereau et al., 2006Bouchereau et al., , 2009aDo¨rner et al., 2009), the diversity of food items in their diet is determined by the diversity of food in that particular ecosystem. Indeed, a high relative abundance of prey taxa in the Schelde estuary (Beauchard et al., submitted) was accorded to a higher numeric percentage of the ingested prey by many of the studied eels in the Schelde. ...
... Differences in stomach contents between sites could be caused by the specific digestion rates of prey, resulting in a different probability for a certain prey item to be found in the samples, which in turn could influence the caloric value estimates. Studies on different digestion rates of prey in eel are scarce, but one study estimated a general digestion rate of 3-4 h for elvers (Belpaire et al., 1992). In our study, eel stomachs were sampled within the time frame where the degree of fullness is >50% (Belpaire et al., 1992), assuring that most prey items were still present. ...
Implementation of the Controlled Reduced Tide (CRT) technique could increase the total surface of tidal freshwater marshes in Europe and ease implementation of restoration projects in coastal defense and riverine ecosystems. The goal was to determine whether a regularly flooded area connected to a freshwater tidal river could act as an important foraging area for European eel, and if so, to what extent the diet of eels in this flooding area differed from that of eels foraging in the river itself. The stomach contents of eels from the River Schelde were compared with eels from the Lippenbroek, an adjacent CRT area. Prey diversity (H′) of individual eels was about four times higher in the Lippenbroek than in the River Schelde. Moreover, 12 prey categories in eel stomachs from the Lippenbroek were found whereas only three categories were retrieved from eels in the River Schelde. In the Lippenbroek, eels fed on terrestrial organisms (lumbricids, caterpillars and other insects), but also on fish and fish eggs and to a lesser extent on other aquatic prey (Lumbricullidae, chironomids and Hirudinea). In contrast, eels from the main river fed mainly on tubificids, fish, and some gammarids. Consequently, eels in the Schelde estuary are opportunistic feeders, but with a preference for large benthic prey. The number and weight of aquatic organisms ingested by eels in the Lippenbroek is not significantly different from the River Schelde. However, eels foraging in the Lippenbroek area had consumed significantly more terrestrial prey. Furthermore the total caloric value estimated for the ingested prey of eels from the Lippenbroek (derived from the literature) was about twice as high as that for eels from the River Schelde. While the condition index remained inconclusive, an Ancova revealed that eels captured in the Lippenbroek were significantly heavier for a given length than eels captured in the Schelde. The study showed that with a controlled reduced tide to restore lateral connectivity of large tidal rivers with their adjacent floodplains, high quality habitats for the European eel are created. These measures could significantly contribute to the production of eels in better condition, which have better chances to reproduce successfully. Hence, wetland restoration could enhance the recovery of the European eel stocks.
... Ces deux hypothèses de travail sont en tous cas compatibles avec les résultats de FONTAINE et CALLAMAND (1941) Si quelques études ont révélé l'influence de la densité d'individus sur la croissance de l'anguille jaune (VOLLESTAD et JONSSON, 1988 ;DE LEO et GATTO, 1996), cette influence a été peu étudiée sur le stade civelle. Des suivis d'alevinages de civelles en petits plans d'eau clos (BELPAIRE et al., 1989(BELPAIRE et al., , 1992KLEIN-BRETELER, 1992 ;RIGAUD et MASSÉ, 2001) ont mis en évidence l'influence de la densité initiale sur la croissance moyenne individuelle au cours du premier été surtout à partir d'une densité de 1 ind./m 2 . Elle pourrait traduire une compétition significative pour l'utilisation des sites (proies ou caches). ...
... Dans les plans d'eau, les civelles avaient l'opportunité d'une reprise alimentaire naturelle et/ou d'une émigration. Divers essais sur 5 mois en plans d'eau clos avaient révélé une croissance nettement densité dépendante pour des niveaux de densité bien inférieurs à ceux utilisés dans cette expérimentation (BELPAIRE et al., 1989(BELPAIRE et al., , 1992KLEIN BRETELER, 1992 ;RIGAUD et MASSE, 2001). On pouvait donc supposer qu'ayant la possibilité de sortir des plans d'eau, elles le feraient massivement notamment dans les contextes de forte densité initiale (9 à 12 ind./m 2 ). ...
Glass eel movements and shelter use were studied under experimental conditions in small indoor channels and in ponds. Shelters densities (2 stems of artificial plant, 9 stems) were controlled in indoor channels stocked with 55 ind./m2. Glass eel densities were controlled in ponds (1,6 - 6 - 9 and 12 ind./m2). Experiments lasted for two months and half in channels, and one month and a half in ponds. In indoor channels, glass eels highly looked for shelters after 5-6 days. During this phase, which lasted for 1 month and half, individuals were very gregarious. Thereafter, fish became aggressive and stopped using shelters. In ponds, emigration at the outlet was almost null. At the inlet, 3 phases were observed : the first one (15 days) with many captures, the second one (1 month) without any emigration and the third one, just before the end of the experiment, with few catches. The emigration rate was independent from initial densities, but increased in ponds with no hydrophyte development. The emigration at the inlet occurred for only a short period in the high-density ponds (first 5-6 days), whereas it lasted longer in the low-density ponds (15-20 days). After four weeks without captures, growth appeared high, heterogeneous and associated to low survival rates in the ponds with initial densities less than 10 ind./ m2. At higher densities, growth was lower, but homogeneous and associated with good survival rates. That suggests the development of agonistic behaviour or cannibalism in the lowest density ponds linked with a faster growth of some individuals. In high-density ponds, glass eels would exhibit a gregarious behaviour, leading to high survival and homogeneous but low growth. In these ponds the development of agonistic behaviour would be delayed possibly until fish would have reached a certain size and/or large inter-individual differences in size have appeared.
... This might mark a shift towards the pelagic as primordial feeding ground where the glass eels principally prey on the most available prey. Such planktonic feeding behaviour by juvenile eels is certainly not exceptional because the authors' observation of planktonic crustaceans as favourite prey is shared with cultivation experiments in aquaria (Tesch, 2003) or outdoor earthen ponds (Belpaire et al., 1992;Lecomte-Finiger, 1983). Kara and Quignard (2018) as well claimed that pigmented glass eels start feeding on planktonic prey first before progressively adopting a more generalist non-selective benthic feeding behaviour as also observed for other eel species (Pitman & Schmidt, 2012;Wasserman et al., 2012). ...
The transition from marine to fresh water is a challenging task for juvenile eels. This critical step in the early eels' life is preceded by a metamorphosis from the oceanic larval to the continental glass eel stage, requiring major energy‐demanding morphological, physiological and behavioural modifications during which time these animals do not feed. The success of the glass eels’ inland migration after metamorphosis will largely depend on remaining energy levels, which can be supplemented only by resuming food uptake. Although it is crucial for their survival and the maintenance of the population, the feeding behaviour of glass eels is still an understudied aspect of the eels’ complex life cycle. Many uncertainties about the phenology, diet, potential prey preferences and their relation with migration modus (migratory vs. sedentary) still remain. In this study, the authors analysed the stomach and gut contents of 458 European glass eels (Anguilla anguilla L. 1758) captured in a drainage canal connecting a small mesotidal estuary with an adjacent polder area during the spring migration seasons of 2016 and 2017. They demonstrated that although glass eels started feeding briefly upon arrival in the estuary, food uptake for early arrivals was restricted to a minority that sparsely feed on detritus and some worm‐like benthic invertebrates. Along the season, food uptake intensified eventually engaging all glass eels and their dietary palette diversified including a wide array of planktonic and benthic organisms. Crustacean plankton (mainly cyclopoid copepods) was an important part of the glass eel diet, whereas benthic oligochaetes were less abundant as food source in spite of their high presence in the sediments. No clear differences in feeding behaviour could be observed between migratory and sedentary glass eels. This study showed that glass eels can use highly artificial and dynamic drainage canals as feeding ground during their critical marine/freshwater transition. This outcome is also a plea to improve the accessibility of alternative (unnatural) migration routes between the ocean and suitable freshwater growth habitats for the European eel.
... Feeding does not occur during metamorphosis from leptocephalus to glass eel and during the early glass eel stage, when individuals rely on energy accumulated as leptocephali (Miller, 2009;Okamura et al., 2012;Tesch, 2003). Once feeding commences, A. anguilla glass eels continue to consume marine food items (Bardonnet & Riera, 2005), then switch to benthic feeding once they reach coastal waters (Tesch, 2003) where food items are diverse and seasonally dependent (Belpaire et al., 1992;Tesch, 2003 (Dörner & Berg, 2016). In saline/brackish waters, crustaceans, fish and amphipods constitute the main food items (Dörner & Berg, 2016;Tesch, 2003), which is also the case for A. japonica . ...
Anguillid eels are found globally in fresh, transitional and saline waters and have
played an important role in human life for centuries. The population status of several
species is now of significant concern. The threats to populations include direct
exploitation at different life stages, blockages to migratory routes by dams and other
structures, changes in river basin management that impact habitat carrying capacity
and suitability, pollution, climate change, diseases and parasites. While much has
been done to understand eel biology and ecology, a major challenge is to identify
the key research and management questions so that effective and targeted studies
can be designed to inform conservation, management and policy. We gathered
30 experts in the field of eel biology and management to review the current state
of knowledge for anguillid eel species and to identify the main topics for research.
The identified research topics fell into three themes: (a) Lifecycle and Biology; (b)
Impacts and (c) Management. Although tropical anguillid eels are by far the least well
understood, significant knowledge gaps exist for all species. Considerable progress
has been made in the last 20 years, but the status of many species remains of great
concern, particularly for northern temperate species. Without improved engagement
and coordination at the regional, national and international level, the situation is unlikely
to improve. Further, adaptive management mechanisms to respond to developments
in science, policy and our knowledge of potential threats are required to
ensure the future of these important and enigmatic species.
... Despite that both are predatory species, feeding primarily on invertebrates and small fish species [63,76], there are differences in the feeding ecology between both species in the studied populations. The broader isotopic niche (indicated by SEA c ) indicates that, despite the overlap in isotopic niche area between both species, eels have a more diverse and flexible diet, which might consist of different invertebrate or fish species, compared to perch [9]. For example, De Meyer et al. [22] showed that head morphology of eel Teunen et al. ...
Background
Despite specific restrictions on their production and use, per- and polyfluoralkyl substances (PFAS) are still omnipresent in the environment, including aquatic ecosystems. Most biomonitoring studies have investigated the PFAS concentrations in indigenous organisms, whereas active biomonitoring has only been used sporadically. In the present study, accumulated PFAS concentrations were measured in indigenous fish, European perch ( Perca fluviatilis ) and European eel ( Anguilla anguilla ), and in translocated freshwater mussels ( Dreissena bugensis and Corbicula fluminea) at 44 sampling locations within the main water basins of Flanders, the northern part of Belgium. Finally, both human health risk and ecological risk were assessed based on accumulated concentrations in fish muscle.
Results
Among locations, ΣPFAS concentrations ranged from 8.56–157 ng/g ww (median: 22.4 ng/g ww) in mussels, 5.22–67.8 ng/g ww (median: 20.8 ng/g ww) in perch, and 5.73–68.8 ng/g ww (median: 22.1 ng/g ww) in eel. Concentrations of PFOA and PFTeDA were higher in mussels compared to fish, whereas for PFDA and PFUnDA the opposite was true. A comparison of concentrations on a wet weight basis between both fish species showed significantly higher PFDoDA, PFTrDA, PFTeDA and PFOA concentrations in eel compared to perch and significantly higher concentrations of PFDA and PFOS in perch. In mussels, PFAS profiles were dominated by PFOA and showed a higher relative contribution of short-chained PFAS, while PFAS profiles in fish were dominated by PFOS. Furthermore, all mussel species clearly occupied a lower trophic level than both fish species, based on a stable isotope analysis.
Conclusions
Biomagnification of PFDA, PFUnDA and PFOS and biodilution of PFOA and PFTeDA were observed. Translocated mussels have been proven suitable to determine which PFAS are present in indigenous fish, since similar PFAS profiles were measured in all biota. Finally, mean PFAS concentrations in fish did pose a human health risk for eel, although tolerable daily intake values for perch were close to the reported daily consumption rates in Belgium and exceeded them in highly contaminated locations. Based on the ecological risk of PFOS, the standard was exceeded at about half of the sampling locations (44% for perch and 58% for eel).
... Glass eels may feed on various food sources in fresh water habitats. A study carried out by (Belpaire et al., 1992) indicated that in freshwater ponds the most important prey for glass eels were Cladocera and Ceratopogonidae. Both these taxa are also very common in the littoral zone of Võrtsjärv (Kangur et al., 2004;Nõges et al., 2004), making them theoretically a preferred food source for restocked glass eels. ...
Restocking of European eel (Anguilla anguilla) is a widespread practice throughout Europe. Conditions during restocking activities and mortality related to restocking practices have been discussed, however, factors affecting these restocked populations afterwards are mostly not considered. In this study we used a machine learning method followed by generalized linear model to analyze long time eel restocking, commercial fishery and environmental data from Lake Võrtsjärv, Estonia, to detect whether significant relationships exist within these data. It was found that environmental parameters can have an effect on the commercial eel yield both retrospectively and during the particular fishing year. Considering that 7-year old eel was the most common age group in commercial catch, we introduced a 7-year gap between eel restocking and yield to study the most important abiotic and biotic factors during the first year of eel restocking that have an effect on the yield. According to our results, cyanobacterial biomass and summer water temperature during the year of restocking had the strongest negative impact on the yield 7 years after, while the number of restocked individuals and copepod biomass had a positive effect. During particular fishing year, however, the yield was most notably positively affected by total phosphorous concentration, number of individuals restocked 7 years before and metazooplankton biomass in the lake.
The essentials of knowledge of the biology of the yellow eel in temperate waters had been well established by the 1970s and have been ably summarized by Tesch (1977). His book clearly indicated both the very high degree of variability within each species and considerable uniformity between those species of the genus Anguilla for which information was available. Many studies of yellow eel populations have taken place since 1977. While greatly increasing the detail of information available, they have in general supported the conclusions drawn at that time. In this chapter, general statements on the ecology of the yellow eel will be found to be supported in the work of Tesch, which contains an exhaustive bibliography. The references used in preparing this chapter are, for the most part, work that has been published since 1977. Strong selection was necessary because a list of all relevant publications would have occupied the entire chapter.
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