Agonistic behaviour and growth in the European eel,Anguilla anguilla L., in relation to warm-water aquaculture*

Agonistic behaviours were studied at high densities and in tournament experiments with pairs of small eels with and without prior isolation. Although innate differences probably exist, eels appear to learn their average positions in diffuse social and size hierarchies from the outcome of repeated agonistic encounters. Dominance or avoidance were most marked when the larger fingerling eels were about 1.5 times the body weight of the smaller. Larger eels tended to attack more readily, be dominant in threat encounters, be less active and spend less time swimming high in the water, consume more food, and show higher growth rates and lower mortality. Implications for warm-water aquaculture are discussed, and practical management techniques to minimize mortality and maximize productivity are suggested.     

Cholinesterase activities and sensitivity to pesticides in different tissues of silver European eel, Anguilla anguilla

Cholinesterase (ChE) activities were characterized in silver European eel, Anguilla anguilla, grown in the brackish lagoon of Comacchio (Italy). All specimens were harvested at the “lavoriero”, a traditional eel trapping weir that captures eels while leaving internal waters at the onset of reproductive migration. To our knowledge, no investigation on ChE was reported in silver eels. Therefore a first characterization of enzyme activity in muscle, brain, liver and plasma of silver eel was carried out, in the presence of different substrates, selective inhibitors, and four pesticides representative of the carbamate and organophosphate classes. Brain and white skeletal muscle showed similar ChE activities, 5- and 10-fold higher than those detected in liver and plasma, respectively. Km values of 0.31 and 0.30 mM, and Vmax values of 40.28 and 35.47 nmol min^–1 mg protein^–1 were obtained in brain and muscle ChE, respectively. Acetycholinesterase was the predominant ChE form in all tissues, as concluded by comparing the effects of BW 284c51, iso-OMPA and eserine. ChE activities in brain and muscle were significantly inhibited by in vitro treatment with pesticides, with the following order of potency: carbofuran > carbaryl > chlorpyrifos ≥ diazinon.     

Scanning electron microscopic analysis of annulus microstructure in otolith of European eel, Anguilla anguilla

To validate the yearly periodicity of annulus formation in the otolith of the eel, the structure of annuli in otoliths of the European eel, Anguilla anguilla , stocked for 7 and 12 years in Lake Ommen on the east coast of southern Sweden, was examined. The population was stocked from elvers imported both from France (Bay of Biscay) in April 1979 and England (River Severn) in March–April 1984. The microstructure of an annulus consisted of single, double and/or composite tings depending on the location in the otolith. The counts of annuli in otoliths of these eels were approximately consistent with the expected age. However, supernumerary false annuli and/or annulus underestimation frequently occurred. The methodology for annulus discrimination with light and scanning electron microscopes is described.     

Different effects of cGMP and cAMP in the intestine of the European eel,Anguilla anguilla

The regulation of salt absorption in the sea water cell intestine was studied by evaluating the effects of theophylline, 8 Br cyclic adenosine monophosphate, 8 Br cyclic guanosine monophosphate, atriopeptin III, porcine vasoactive intestinal peptide and prostaglandin E ₁ on the short-circuit current, the transepithelial voltage difference and conductance and on the dilution potentials. It was shown that theophylline increased the transepithelial conductance and reduced the magnitude of the dilution potentials, indicating that the drug increase the anion conductance of the tight junctions. In addition its inhibitory effect on short-circuit current and transepithelial voltage difference suggests that theophylline also affects the transcellular transport mechanisms. It was shown that 8 Br cyclic guanosine monophosphate and 8 Br cyclic adenosine monophosphate affect transcellular mechanisms underlying Cl^– transport since both compounds reduced short-circuit current and transepithelial voltage difference; however, cyclic adenosine monophosphate is less effective since unlike cyclic guanosine monophosphate, even at maximal concentration, it was not able to completely abolish transepithelial voltage difference and short-circuit current. The effects of cyclic guanosine monophosphate and cyclic adenosine monophosphate were not additive even if cyclic guanosine monophosphate may produce further inhibition of ion transport in 8 Br cyclic adenosine monophosphate-treated tissues. In addition, cyclic guanosine monophosphate but not cyclic adenosine monophosphate reduced the magnitude of the dilution potentials, suggesting that cyclic guanosine monophosphate acts also on the paracellular pathway. Rat atriopeptin III, a peptide known to increase cyclic guanosine monophosphate cellular levels, behaved like 8 Br cyclic guanosine monophosphate since it lowered the dilution potentials and reduced short-circuit current and transepithelial voltage difference to near zero values, suggesting that the hormone modulates both paracellular and transcellular transport mechanisms, probably acting on the Na-K-2Cl cotransport. Agents acting via cyclic adenosine monophosphate, like porcine vasoactive intenstinal peptide and prostaglandin, behaved like 8 Br cyclic adenosine monophosphate. They were less effective in inhibiting ion transport and did not interfere with the paracellular pathway.Abbreviations AP III rat artriopeptin III - 8 Br cAMP 8 Br cyclic adenosine monophosphate - 8 Br cGMP 8 Br cyclic guanosine monophosphate - g _t transepithelial conductance - I _sc short circuit current - IC ₅₀ half-maximal inhibitory concentration - NaK ATPase Na-K-adenosine monophosphate - NPPB 5-nitro-2-(3-phenylpropylamino)-benzoic acid - PGE prostaglandin E ₁ - R _t tissue resistance - SITS 4-acetamide-4-isothiocyano-stilbene-2,2-disulfonic acid - V _t transepithelial voltage difference - VIP porcine vasoactive intestinal peptide     

Infection status of the swimbladder worm, Anguillicola crassus in silver stage European eel, Anguilla anguilla, from three different habitats in Danish waters

The infection status of silver stage eels, Anguilla anguilla , infected with Anguillicola crassus from three locations, ranging from a lake to seawater, were investigated. Data show a significant difference in mean intensity of Anguillicola crassus in silver stage eels according to the salinity of the habitats.     

Olfactory sensitivity to amino acids in the juvenile stages of the European eel Anguilla anguilla (L.)

Scanning electron micrograph observations of the olfactory mucosa from both unpigmented glass eel(GE)andpigmentedelvers(EL)of the European eel, Anguilla anguilla (L.), revealed the presence of various cell types; amongst these, the ciliated and microvillous ones are likely to possess a chcmosensory function. Recording of underwater electro-olfactograms (EOGs) showed that various amino acids (glycine, L-alanine, L-valine, L-leucine, L-asparagine, L-glutamine and L-methionine) are effective stimulants for the olfactory mucosa. Dose response curves of stimulus concentrations v. EOG amplitudesfit regression linesat both GE and EL stages. Leucine was more stimulatory at the GE than at the EL stage. The stimulatory effect of the other six amino acids tested was similar at both developmental stages. The possible role of olfactory sensitivity in animal behaviour at different developmental stages is discussed.     

Populations of Bothriocephalus claviceps (Goeze) (Cestoda) in the European eel, Anguilla anguilla (L.), in three localities in southwest England

The population biology of Bothriocephalus claviceps (Goeze) has been investigated in the European eel, Anguilla anguilla (L.), in three localities in southwest England. Monthly changes in prevalence and abundance show no statistically significant seasonality, in contrast to growth and reproduction of the cestode. The cestode reaches maximum size and becomes gravid mainly in summer. Gravid cestodes die and are lost soon after reproduction. This overlaps with recruitment of the new generation into eels, and may result in a rapid increase in infection levels and in a more over-dispersed distribution of the parasite in some localities, or in a more gradual change in infection levels and over-dispersion in others. Development and growth of the new generation may cease until the following spring, or continue at a slow rate over winter. Parasite numbers in each host decrease well before they actually reach maturity, since the majority of gravid individuals are present in those eels which harbour only one parasite. This suggests that few cestodes develop to maturity and most are lost from hosts as development proceeds.     

Antigenicity of Pseudodactylogyrus anguillae and P. bini (Monogenea) in the European eel (Anguilla anguilla,L.) under different oxygenation conditions

The antibody response of European eels (Anguilla anguilla, L.) to the branchial parasites Pseudodactylogyrus anguillae and P. bini under hyperoxygenation conditions was studied. The antigenic fractions of parasites were detected by means of electrophoretic techniques (SDS-PAGE) and by Western blot analysis. The results obtained demonstrate that under hyperoxygenation conditions, the eels responded to a greater number of proteins, and this was correlated with a decrease in the level of infestation.     

Genetic variability is unrelated to growth and parasite infestation in natural populations of the European eel (Anguilla anguilla)

Positive correlations between individual genetic heterozygosity and fitness-related traits (HFCs) have been observed in organisms as diverse as plants, marine bivalves, fish or mammals. HFCs are not universal and the strength and stability of HFCs seem to be variable across species, populations and ages. We analysed the relationship between individual genetic variability and two different estimators of fitness in natural samples of European eel, growth rate (using back-calculated length-at-age 1, 2 and 3) and parasite infestation by the swimbladder nematode Anguillicola crassus . Despite using a large data set of 22 expressed sequence tags-derived microsatellite loci and a large sample size of 346 individuals, no heterozygote advantage was observed in terms of growth rate or parasite load. The lack of association was evidenced by (i) nonsignificant global HFCs, (ii) a Multivariate General Linear Model showing no effect of heterozygosity on fitness components, (iii) single-locus analysis showing a lower number of significant tests than the expected false discovery rate, (iv) sign tests showing only a significant departure from expectations at one component, and, (v) a random distribution of significant single-locus HFCs that was not consistent across fitness components or sampling sites. This contrasts with the positive association observed in farmed eels in a previous study using allozymes, which can be explained by the nature of the markers used, with the allozyme study including many loci involved in metabolic energy pathways, while the expressed sequence tags-linked microsatellites might be located in genes or in the proximity of genes uncoupled with metabolism/growth.     

Factors determining length distribution and abundance of the European eel, Anguilla anguilla, in the River Mondego (Portugal)

1. The distribution and abundance of the European eel, Anguilla anguilla, were investigated in the Mondego River, a strongly modified river in central Portugal. Nine freshwater sites, located below the first impassable obstacle to eel migration, were surveyed for 2 years. The study was conducted monthly during the first year and seasonally during the second year. Fish were caught by electric fishing and, physical and biotic variables potentially influencing eel length distribution and abundance were determined at each sampling session. 2. Canonical correspondence analysis (CCA) was used to explain the relationships between abiotic and biotic habitat variables, and eel length distribution. Spatial variation was strongly associated with distance from the sea, number of obstacles, river width and percentage of instream cover. Depth, river flow, water temperature and cannibalism had weaker effects on the longitudinal distribution, but contributed as explanatory variables for the model. The influence of spatial variables outweighed the importance of temporal variables as predictors in this model. 3. Length distribution changed in space from a dominance of small eels (<100 mm) closer to the sea to larger eels (≥250 mm) in the upper reaches of the river. The smallest length class (eels <100 mm) was strongly associated with wider stretches where instream cover, mainly composed of aquatic submerged macrophytes, was abundant (>75% area coverage). On the contrary, larger eels (≥250 mm) preferred deeper stretches with stronger river flow and less instream cover. 4. The effect of biotic interactions with conspecifics as well as with the non‐native red swamp crayfish Procambarus clarkii, in sites with poor instream cover, modified this general pattern of distribution leading to fewer eels <100 mm at some sites closer to the sea. Therefore, despite the ubiquity and plasticity shown by this species, there were clear ontogenetic variations in habitat use such that the first two canonical axes of a CCA accounted for 58.4% of the spatial variability in size structure.     

The distribution, density and growth of the European eel, Anguilla anguilla, in the freshwater catchment of the River Thames

Improvements in water quality in the Thames Estuary since the 1960s have removed a barrier to fish migration into the Thames catchment. The extent of recolonization by the eel is examined, using data from 235 electrofished and fyke-netted sites, compared with information on pre-pollution distribution. Density is highest in the lower parts of the catchment and decreases significantly with increasing distance from the tidal limit. Only sparse populations are found beyond 50 km from the tidal limit and are characterized by an increasing bias towards larger, slower-growing females. Artificial stocking is believed to be a significant factor in certain areas. The rate of recolonization has been slow and neither the annual natural recruitment nor distribution of the pre-pollution stock of the 1800s appears to have been achieved. The future management of the stock in relation to its commercial scientific and conservation value is considered.     

Population biology of Proteocephalus macrocephalus (Creplin) in the European eel, Anguilla anguilla (Linnaeus), in two small rivers

Seasonal changes in populations of Proteocephalus macrocephalus (Creplin) were investigated for 17 months in the European eel, Anguilla anguilla (Linnaeus), in two rivers in Devon, SW England, but no clear seasonal patterns in prevalence and abundance were apparent. Population levels of the cestode are low in both localities, and it is suggested that natural population levels of P. macrocephalus may generally be low. However, growth and maturation of the cestode show marked seasonality with both occurring mainly in early summer.     

Seasonal population dynamics of Paraquimpevia tenevvima (Linstow) (Nematoda) in the European eel Anguilla anguilla (Linnaeus) in three localities of southwest England

Seasonal population dynamics of Puraquzmperia tenerrima (Linstow) in the European eel Anguillu anguillu (Linnaeus) have been investigated in three localities in Devon, southwest England. The nematode is common and exhibits very similar seasonal patterns in changes of prevalence and abundance in the three localities. The population increases over winter through to late spring or early summer as the recruitment of the new generation occurs throughout this period. Reproduction of the nematode starts in spring. A sharp decrease of prevalence and abundance accompanies or follows reproduction, and infection levels are very low in autumn and early winter. The dispersion pattern of the nematode varies with the infection levels from over-dispersion in spring and early summer to almost random dispersion in autumn and winter. It is suggested that P. tenerrimu is a common specific parasite of eels, especially in small streams or rivers and that insect larvae or species of crustaceans might serve as intermediate hosts for the nematode.     

Catecholamine fluorescence in the pituitary of the eel,Anguilla anguilla,with special reference to its variation during background adaptation

Summary In the neuro-intermediate lobe (NIL) of the eel, Anguilla anguilla, a specific formaldehyde-induced fluorescence, indicating a catecholamine (CA) innervation, has been demonstrated in the neural lobe processes. Microspectrofluorimetric analyses and pharmacological treatments indicate noradrenaline or dopamine or both to be responsible for the fluorescence.The fluorescence in the NIL has displayed a definite tendency toward variation during the adaptation to a white and to a black background. The highest amounts of fluorescence were generally found in animals adapted to a black background, especially when adapted for a rather long period, and in animals recently transferred to a white background. The lowest amounts of fluorescence were generally found in animals adapted to a white background.This and the result of injections of CA-depleting drugs suggest that the monoaminergic nerves are active when the animal is on a white background, inhibiting the MSH release directly or indirectly or both, or in co-operation with other factors.Specific green fluorescent structures were also found in other parts of the neural lobe supplying the pars distalis.In some pharmacologically untreated specimens and in animals treated with CA-depleting drugs, the intermedia cells fluoresced. Microspectrofluorimetric analyses indicated that this fluorophore was not a CA.We wish to express our sincere thanks to Miss Ingrid Carlsen for excellent technical assistance, Mr. Lajos Erdös for the photography and the technical staff of the Department of Histology in Lund. We are also indepted to Dr. Anders Björklund for valuable discussion and advice.Supported by grants from the Swedish Natural Science Research Council, the University of Lund, and the Royal Physiographic Society of Lund.     

Isolation and characterization of 12 dinucleotide microsatellites in the European eel, Anguilla anguilla L., and tests of amplification in other species of eels

Twelve polymorphic dinucelotide microsatellites in the freshwater eel Anguilla anguilla L. were isolated and characterized. Genetic diversity was assessed in eels from Lake Constance, Germany. Allele numbers ranged from five to 26 per locus with observed heterozygosities between 0.125 and 0.875. A portion of locus AangCT77 aligns with a transcribed region of the zebrafish gene crystallin beta B2. Cross-species amplification of most markers was possible for nine other Anguilla eel species. The newly developed primer pairs will facilitate population and conservation genetic studies in order to refine the understanding of the subtle population genetic structure typical of eels, and to identify interspecies admixture due to global trade.     

Genetic variation of density responses in relation to wing polymorphism in the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae)

Responses to nymphal density in the determination of wing form were compared between the offspring from brachypter x brachypter crosses and those from macropter x macropter crosses of the oriental chinch bug, Cavelerius saccharivorus. In the offspring from crosses between macropters, there was a strong tendency for macropters to increase to a rather high level with increasing nymphal density in both sexes. In contrast to this, in the offspring from crosses between brachypters, the appearance rate of macropters attained a maximum value in moderately crowded conditions and conversely decreased to a lower level in more crowded conditions in both sexes. Thus density response patterns concerning the determination of wing form were quite different between the offspring from different crosses in the wing form, indicating that there is a genetic basis underlying wing polymorphism in this species. As for the body size of emerged adults, macropters tended to be larger than brachypters in the same crowded condition. Moreover, the rate of decrease of body size with nymphal density was lower in the offspring from crosses between macropters than in the offspring from crosses between brachypters. This indicated that the former offspring are more tolerant of nymphal crowding than the latter. The difference in such a tolerance against nymphal crowding between the offspring from different crosses was considered to be related to the difference in the appearance of macropters in the crowded conditions between them.     

Growth,condition and gonad development of bream (Abramis brama L.) in relation to its feeding conditions in Tjeukemeer

The bream (Abramis brama L.) population of Tjeukemeer was studied for three successive years. The growth, condition, gonad development, diet and feeding conditions of the fish are described. Food competition for two years with the then abundant young planktivorous smelt (Osmerus eperlanus) and perch (Perca fluviatilis) resulted in a decreased growth rate, condition and gonad development of the larger bream. The biomass of chironomids was too small in relation to that of the zooplankton to be important for the bream population, although they were eaten more efficiently.