The yellow European eel (Anguilla anguilla L.) may adopt a sedentary lifestyle in inland freshwaters

We analysed the movements of the growing yellow phase using a long-term mark–recapture programme on European eels in a small catchment (the Frémur, France). The results showed that of the yellow eels (>200 mm) recaptured, more than 90% were recaptured at the original marking site over a long period before the silvering metamorphosis and downstream migration. We conclude that yellow European eels >200 mm may adopt a sedentary lifestyle in freshwater area, especially in small catchment

Le marais doux endigué de Bourgneuf-Machecoul (Pays de Loire) Premier éléments de connaissance du peuplement piscicole. Relation ichtyofaune-habitat et problèmes majeurs de gestion (Maroc)

Le peuplement ichtyologique d'un marais littoral endigué, géré en eau douce, a été étudié sur une zone de 2 700 ha, située au nord du marais Breton-Vendéen (Loire-Atlantique, France). Le réseau hydraulique, qui représente près de 16 % de cette surface, se compose, d'environ 91 m de linéaire de fossés par ha (en tout 234 km) et de bassins présents uniquement dans la partie d'origine salicole du marais. Les hauteurs d'eau, l'envasement et le recouvrement par la végétation aquatique dépendent de la gestion humaine et sont très variables (moyennes respectives : 42 cm; 43 cm; 70 %). Cela se traduit par un morcellement spatial de l'habitat pour les poissons. La stratégie d'échantillonnage adoptée, qui tient compte de cette hétérogénéité, a permis de décrire un peuplement comportant 21 espèces. Dominé par les poissons-chats et par les anguilles, ce dernier est caractéristique de la zone à brèmes des cours d'eau. Les abondances sont relativement élevées (en moyenne 315 kg/ha et 11 460 poissons/ha), mais elles sont très hétérogènes. L'évolution qualitative et quantitative de la répartition spatio-temporelle est décrite à l'aide d'une analyse factorielle des correspondances portant sur 74 échantillons prélevés par pêche électrique entre 1987 et 1989. Bien que l'approche de ce milieu soit complexe et les références bibliographiques relativement rares, l'analyse des premières données permet d'ores et déjà d'identifier quelques problèmes de gestion ayant des répercussions directes sur le peuplement piscicole de cette zone.The dammed up marshes of the French Atlantic coast cover about 200 000 ha between River Vilaine and the « Bassin d'Arcachon ». Eighty eight % are managed with freshwater. They constitute original environments initially created for agriculture or for salt production, and they are now threatened by land abandonment within the next decade. Concurrently to aquaculture (in created or existing ponds), exploitation of the fish stocks in the ditchweb is likely to encourage a diversification of agricultural activities. Unfortunately, bibliographic analysis reveals the relative scarceness of research about sampling methods and qualitative or quantitative characteristics of these fish communities. This is quite surprising considering the importance of the ditchwed of this kind of environment outlined by several authors. In the Netherlands, BELTMANN (1984) assessed that there is a total of 400 000 km of ditches. In France, the littoral dyked marshes of the Atlantic coast couid comprise 20 000 km of ditches and about 24 000 ha of open water. The present work provides for the first data on the fish community of Bourgneuf marsh.The northern part of the marsh of Bourgneuf, 2 700 ha provided whith f reshwater, contains nearly every kind of landscapes found throughout the whole Breton-Vendéen marsh. The pattern of the ditch network strongly changes from a zone to another (fig.1) : presence of former salt pans in the western part, regular geometric shapes in the recently created polders next to the River Falleron, irregular ditchweb pattern in the eastern part. The average density of the ditch network is 91 m of ditches per ha, totalizing 234 km in the study area. The total surface of open water, composed of ditches and basins (former salt pans), covers 411 ha (over 15 % of the study site). Diversity of ditch types occurs at fine scales (<1 000 m2), they vary according to their widths (0,3 to 7 m), depths (average, 42 cm; SD, 20,4), thickness of silt layer (average, 43 cm; SD, 42) and their hydrophyte vegetation cover (average, 70 %; SD, 60 %). As a consequence of this heterogeneity, available habitats are scattered over the marsh (mosaïc distribution). A nested sampling (FRONTIER, 1983) was carried out to take into account this high heterogeneity : 5 sampling areas were selected randomly. In each one, 3 to 5 ditches were chosen according to their characteristics (see above). Sampling stations were delimited by 2 stop nets (5 mm mesh) settled 30 m apart, in order to avoid fish migration. Field work was conducted using « Heron » electric fishing material (see LAMARQUE et al., 1978). In each ditch-section, we carried out as many successive catches as necessary to apply the maximum likelihood weighted estimation method of CARLE and STRUB (1978). Nine to 19 stations were sampled at 5 periods, between 1987 and 1989. A total of 74 samples were collected.The fish community was composed of 21 species (table 1) and corresponded to the bream zone of Verneaux's classification (1977). The densities and biomass were quite high (on average 315 kg/ha and 11 460 fishes/ha) but very variable (0 to 2 120 kg/ ha and 0 to 39 300 fishes/ha). The catfish, Ictalurus nebulosus (170 kg/ha), the eel, Anguilla anguilla (47 kg/ha) and the tench, Tinca tinca (28 kg/ha), represented on average 77,5 % of the standing crop, but their spatial distribution was very irregular. These estimates are assumed to be reliable considering that the data used for the calculations were provided by a sampling design which permits to respect the basic assumptions of the removal method. (f) The population size could only change because of the fishings (no migration because of the stop nets; no recruitment/death because of the short duration of the fishing sequences). (ii) The standard sampling design permitted to reduce the variations of the catch probabilities between the successive removals. Several studies have shown that this removal method under-estimates by about 20 % the true size of the fish populations (e. g. BOHLIN and SUNDSTROM, 1977; MAHON, 1980). But they were based on Zippin's method, and the estimator of CARLE and STRUB (1978), that we used, was shown to be more robust (COWX, 1983; GERDEAUX, 1987). Nevertheless, we assume that the values presented in this paper provide for an approached information on the sizes of the studied fish populations.To assess the fish-habitat relationship, a correspondence analysis (fig. 2) was performed on the 74 samples X 17 species matrix (excluding the sticklebacks, Gaslerosteus aculeatus and Pungitius pungitius, which population size estimations failed because of their low catchabilities). Four groups of samples were ordinated according to their specific richness and the species they contained. Several habitat parameters were projected on F1-F2 factorial map (fig. 3). Hydrophyte cover, thickness of silt layer, water depth (fig. 3 and 4), which are directly controlled by human maintenance, appeared to be the major structuring habitat parameters for the fish community. In the deepest and less silted stations, the communities were rich (on average 11 species; group 4, fig. 2). Predators such as pike-perch, Stizostedion lucioperca, and perch, Perca fluviatilis, occurred, and cold water species were found, such as minnow, Phoxinus phoxinus, or chub, Leuciscus cephalus. When the silt layer was thicker and the water level was intermediate, the specific richness decreased (average, 6,2 species) and the community was either dominated by the cattish (group 2, fig. 2) or by the rudd, Scardinius erythrophtalmus (group 3, fig. 2), according to the importance of the aquatic vegetation cover. Habitats with thickest silt layers, shallowest waters and maximum aquatic vegetation cover contained the poorest communities (average 3,9 species) dominated by eel (group 1, fig. 2). There is also evidence that the diversity of the community has progressively decreased since 1987 (fig. 5). The most stenothermous species disappeared, and the importance of the catfish increased : it doubled between May 1987 and September 1989 (fig. 6). Although the eel is the species most adapted to this environment, we emphasize the diminution of its biomass (fig. 7). These phenomena could be partly due to the climate (cold winter in 1987, important swelling in January 1988 and 2 droughts in summers 1988 and 1989). But they are mainly caused by the water management policy which is intended to favour agriculture by keeping stable water levels (evacuation of swellings) and by preventing the freshwater part from the marine influence (collective sluice gates). This does mot permit an optimal breeding of the species that have to spawn on flooded meadows, neither a proper colonisation of the marsh by elvers

Impact of sheep grazing on juvenile sea bass, Dicentrarchus labrax L., in tidal salt marshes

The diet of young of the year sea bass, Dicentrarchus labrax L., from sheep grazed and ungrazed tidal salt marshes were com-pared qualitatively and quantitatively in Mont Saint-Michel Bay. In areas without grazing pressure, the vegetation gradient changes from a pioneer Puccinellia maritima dominated community at the tidal ¯at boundaries through a Atriplex portulacoides dominated community in the middle of the marsh to a mature Elymus pungens dominated community at the landward edge. The A. portula-coides community is highly productive and provides important quantities of litter which provides a habitat and good supply to substain high densities of the detrivorous amphipod Orchestia gammarellus. In the grazed areas, the vegetation is replaced by P. maritima communities, a low productive grass plant, and food availability and habitat suitability are reduced for O. gammarellus. Juvenile sea bass colonise the salt marsh at ¯ood during 43% of the spring tides which inundate the salt marsh creeks. They forage inside the marsh and feed mainly on O. gammarellus in the ungrazed marshes. In grazed areas, this amphipod is replaced by other species and juvenile sea bass consume less food from the marsh. This illustrates a direct effect of a terrestrial herbivore on a coastal food web, and suggests that management of salt marsh is complex and promotion of one component of their biota could involve reductions in other species

Composition of fish communities in macrotidal salt marshes of the Mont Saint-Michel bay (France)

At least 100 fish species are known to be present in the intertidal areas (estuaries, mudflats and salt marshes) of Mont Saint-Michel Bay. These and other comparable shallow marine coastal waters, such as estuaries and lagoons, play a nursery role for many fish species. However, in Europe little attention has been paid to the value of tidal salt marshes for fishes. Between March 1996 and April 1999, 120 tides were sampled in a tidal creek. A total of 31 species were caught. This community was largely dominated by mullets (Liza ramada represent 87% of the total biomass) and sand gobies(Pomatoschistus minutus and P. lozanoi represent 82% of the total numbers). These species and also Gasterosteus aculeatus, Syngnathus rostellatus, Dicentrarchus labrax, Mugil spp., Liza aurata and Sprattus sprattus were the most frequent species (>50% of monthly frequency of occurrence). In Europe, salt marshes and their creeks are flooded only during high spring tides. So, fishes only invade this environment during short immersion periods, and no species can be considered as marsh resident. But, the salt marsh was colonized by fish every time the tide reached the creek, and during the short time of flood, dominant fishes fed actively and exploited the high productivity. Nevertheless, this study shows that there is little interannual variation in the fish community and there are three ‘ seasons ’ in the fish fauna of the marsh. Marine straggler and marine estuarine dependent species colonize marshes between spring (recruitment period in the bay) and autumn before returning into deeper adjacent waters. Estuarine fishes are present all year round with maximum abundances in the end of summer. The presence of fishes confirms that this kind of wetland plays an important trophic and nursery role for these species. Differences in densities and stages distribution of these species into Mont Saint-Michel systems (tidal mudflats, estuaries and tidal salt marshes) can reduce the trophic competition

Comparative ecology of the European eel, Anguilla anguilla (L.1758), in a large Iberian river

A total of 1,816 eels were sampled in 1988, from seven sampling areas. Four areas were located in brackish water and the remaining three were located in freshwater reaches of the Tagus river basin. Eels were more abundant in the middle estuary and decreased both in the upstream and in the downstream directions, with a predominance of males in higher density areas. Smaller individuals preferred more peripheral areas, such as margins and upper reaches in the brackish water zone, and the tributaries of the freshwater habitats. It was assumed that this distribution pattern resulted from three main factors: (i) the dominance of larger specimens; (ii) the need to avoid predators and; (iii) the search for better trophic conditions. The condition of the individuals generally decreased toward the upper reaches, apparently due to a corresponding decrease in feeding intensity. The presence of the Belver dam in the main river, 158 km upstream from the sea, seemed to impose major alterations to the described patterns. The concentration of specimens below this impassable obstacle yielded a reduction in the proportion of females and a decrease in the condition and survival of the eels, contributing to a reduction in the spawning success of this population. Suggestions to diminish the effects of the dam, and to preserve the fishery are also presente

Corner and sloped culvert baffles improve the upstream passage of adult European eels (Anguilla anguilla)

Installation of baffles intended to improve fish passage through culverts can reduce discharge capacity and trap debris, increasing flood risk. A sloping upstream face may reduce this risk, but new designs must be tested for fish passage efficiency. The European eel (Anguilla anguilla) is a critically endangered species, yet the suitability of even common baffle types to aid upstream movement has not been tested. This study compared the water depth, velocity, turbulent kinetic energy (TKE), and upstream passage performance of adult yellow-phase eels, between three 6 m long culvert models: smooth and unmodified (control); containing corner baffles (treatment 1); and with prototype sloped baffles installed (treatment 2). Passage of individual fish was assessed during 25 one-hour trials per model. Performance was quantified as entrance efficiency, number of entries per fish, passage efficiency, and overall efficiency. Total and passage delay, and successful passage time were also evaluated. Despite some individuals being able to swim against unexpectedly high water velocities (&gt;1.5 m s?1 for 4 m), passage performance in the control was poor, with an overall efficiency of 28%. Compared to the control, both treatments increased the mean centreline water depth by approximately 0.11 m, created heterogeneous flow conditions with low velocity resting areas, and reduced maximum velocities. As a result, entrance rate and all efficiency parameters were higher for the treatments than for the control (overall efficiency = 84%), despite longer passage delay. The TKE was slightly higher in treatment 2 than 1, but there was no difference in water depth or overall efficiency. The findings show that both corner and sloped baffles can mitigate for impeded upstream adult eel movement. The extent to which the sloping upstream face will improve debris transport should be explored further