Sex differentiation of the European eel in brackish and freshwater environments: a comparative analysis

Body growth parameters, age and total length at sex differentiation were compared in three European eel Anguilla anguilla populations from Mediterranean sites with different salinity. Whilst body growth was faster in brackish than in freshwater environments, the present analysis shows that body size at sex differentiation might be a physiological invariant.     

Timing and rate of sexual maturation of European eel in brackish and freshwater environments

Maturation rates in three European eel Anguilla anguilla populations increased from September to October and were slightly lower in fresh water than in brackish waters. Average and variance of total length at maturation were larger in females than males.     

Magnetic particles associated with the lateral line of the European eel Anguilla anguilla

Magnetization measurements of the European eel Anguilla anguilla demonstrated the presence of magnetic material concentrated in the region of the mandibular canals of the lateral line system. The data suggest that the material is magnetite, has a size suitable for magnetoreception and is of biogenic origin. The presence of magnetic particles in the lateral line system is discussed in relation to their possible role in allowing the fish to orientate with respect to the geomagnetic field during their extensive oceanic spawning migrations.     

Induction of maturation and spermiation in the male European eel: assessment of sperm quality throughout treatment

Weekly injections of human chorionic gonadotropin (hCG; 1·5 IU g⁻¹ W ) induced spermiation in 87–100% farmed male European eels Anguilla anguilla over a 10-week period, producing 3 ml (100 g)⁻¹ milt volume, at 1˙4×10⁹ spermatozoa ml⁻¹ sperm mobility peaked in ninth week of treatment with 97% of males with motile sperm. Gonadotropin-treated males showed earlier but similar morphological changes to control males. Whereas higher sperm density was obtained 6 h after hCG administration, higher percentage of motile cells and longer sperm mobility were observed 24 h after the treatment indicating that this is the optimum time to obtain good sperm after the hormonal induction. Several monounsaturated fatty acids in sperm increased significantly from week 5 to 9 of treatment (when highest volume, density and mobility of milt were recorded), whereas eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and total n-3 polyunsaturated fatty acids (PUFAs) were significantly reduced. Fatty acid levels returned to initial values after 13 weeks of treatment. These results are in agreement with the reduction of n-3 PUFAs recently observed in the ovary of female Japanese eel during artificial maturation, and an attempt is made to suggest a physiological explanation of its possible relation with sperm quality.     

Migratory patterns and contribution of stocking to the population of European eel in Lithuanian waters as indicated by otolith Sr:Ca ratios

Otolith Sr:Ca ratios were examined to evaluate the contribution of the stocked eel Anguilla anguilla elvers, which have been stocked in Lithuanian waters and mixed with naturally recruited eels for several decades, to the native eel population. Stocked eels were identified by the freshwater signature (Sr:Ca ratios <2·24 × 10⁻³) on the otolith after the glass eel stage. Naturally recruited eels, that had migrated through the North and Baltic Seas, were characterized by an extended seawater and brackish-water signature (Sr:Ca ratios >3·23 × 10⁻³) after the glass eel stage. Of 108 eels analysed, 21 eels had otolith Sr:Ca ratio profiles consistent with stocking while 87 showed patterns of natural recruitment. The ages of naturally recruited eels arriving in Lithuanian fresh waters varied from 1 to 10 years, with a mean ±  s.d . age of 5·2 ± 2·1 years. Eels from the inland Lake Baluošai were all freshwater residents of stocked origin. Stocked eels, however, accounted for only 20% of the eels from the Curonian Lagoon and 2% of eels sampled in Baltic coastal waters. This finding does not support the hypothesis that the eel fishery in the Curonian Lagoon depends mostly on stocking.     

A stage‐structured model to predict the effect of temperature and salinity on glass eel Anguilla anguilla pigmentation development

The pigmentation development process of glass eels Anguilla anguilla from stage V_B to VI_A3 was modelled by gamma cumulative functions. These functions varied with respect to the factors temperature and salinity whose effects were adjusted by beta functions. Temperature was shown to accelerate pigmentation, while salinity acted as a secondary factor slowing down the pigmentation. The model fits the development of 15 samples kept at various temperatures and salinities in the Vilaine River, as well as samples monitored at other dates and places in Europe. It allows the prediction of the duration of estuarine residency for glass eels, in winter and spring, in the Atlantic estuaries.     

Comparison of oxygen consumption in freshwater mussels (Unionidae) from different habitats during declining dissolved oxygen concentration

The rate of oxygen consumption (OC) of 9 species of freshwater mussels was measured under declining dissolved oxygen (DO) concentrations. The effects of temperature for some species also was investigated. The pattern of the OC vs. DO curve for each species was used in a hyperbolic model to compare abilities to regulate OC under low oxygen conditions. At 24.5 °C, Pyganodon grandis (from lakes), Amblema plicata and Quadrula pustulosa (from mud or sand in large rivers), Elliptio complanata (from pool areas in rivers), and Elliptio fisheriana and Elliptio lanceolata (from bank margins of rivers) were better able to maintain OC under low DO than were Villosa iris and Villosa constricta (which inhabit riffles) and Pleurobema cordatum (found in rivers with moderate flow). Villosa iris was especially sensitive to low oxygen conditions. The ability to maintain normal OC at low DO was improved considerably at 16.5 °C for V. iris, P. grandis and E. complanata. It is concluded that oxygen regulation ability appears to be related to the degree of hypoxia a species normally experiences in its habitat type, and it is enhanced at low temperature. The measurement of OC vs. DO may be a useful technique for estimating DO water quality criteria for endangered species because it is noninvasive.     

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.     

Comparative effects of long-term hypoxia on growth, feeding and oxygen consumption in juvenile turbot and European sea bass

When juvenile turbot Scophthalmus maximus and sea bass Dicentrarchus labrax were fed to satiation, growth and food intake were depressed under hypoxia (3·2±0·3 and 4·5±0·2 mg O₂ l^-1). However, no significant difference in growth was observed between fishes maintained in hypoxia and fed to satiation and fishes reared in normoxia (7·4±0·3 mg O₂ l^-1) and fed restricted rations (same food intake of fishes at 3·2 mg O₂ l^-1). Routine oxygen consumption of fishes fed to satiation was higher in normoxia than in hypoxia due to the decrease in food intake in the latter. Of the physiological parameters measured, no significant changes were observed in the two species maintained in hypoxia. This study confirms the significant interaction between environmental oxygen concentrations, feeding and growth in fishes. Decrease in food intake could be an indirect mechanism by which prolonged hypoxia reduces growth in turbot and sea bass, and may be a way to reduce energy and thus oxygen demand.     

Environmental factors affecting migration of the European eel in the Rivers Severn and Avon, England

Studies were conducted during 1991–1993 on environmental factors affecting the upstream migration of eels in the Rivers Severn and Avon, England. Migrants (> 156 000 pigmented elvers and > 189 000 juveniles) were trapped as they attempted to ascend weir or sluice barriers. Multiple regression models were developed to compare catches per trap per night (C) with data for various key environmental parameters at seven sites, from the tidal limit to a maximum of 42.5 km upstream. The key stimulus for migration of both elvers and juveniles at the tidal limit was water temperature, with some weaker monthly influences related to seasonal temperature increases. Smaller annual influences probably related to earlier glass eel recruitment into the lower estuary. A weak early tidal effect was demonstrated only once, in 1993 in the Severn. Temperature also exerted significant effects on C of juvenile eels at the tidal limit and in the non-tidal rivers, although effects weakened with distance upstream. Year, month, river flows and whether traps were mounted on weirs or sluices made only small contributions at a few sites. Distance between traps also contributed to combined data for upper Severn sites. The threshold temperature in all cases was 14–16°C, with low to zero catches below 10–11°C, catch maxima being achieved above 18–20°C. The implications of strong temperature-dependence of migration in relation to stock recruitment and management are discussed. Special reference is made to recent decreases in recruitment of eels to Europe and N. America and possible long-term effects of global warming.     

Artificial hybrid between Anguilla anguilla and A. japonica

Interspecific hybrids were obtained by artificial insemination between male European eels Anguilla anguilla and female Japanese eels Anguilla japonica . The hybrid larvae developed normally and survived up to 30 days post‐hatching similarly to normal A. japonica larvae.     

Isolation and characterization of expressed sequence tag-linked microsatellite loci for the European eel (Anguilla anguilla)

A European eel (Anguilla anguilla) expressed sequence tag database consisting of 795 contigs and 4008 singletons was screened for microsatellites sequences. Primers were designed to amplify 96 repeats, of which 86 gave good quality amplification products. Twenty-eight microsatellites were selected for further microsatellite genotyping. Only two loci were found to be monomorphic; out of the 26 polymorphic loci, number of alleles per locus ranged from two to 14, while the observed and expected heterozygosities ranged from 0.05 to 0.93, and from 0.05 to 0.95, respectively. All 28 primer sets tested revealed positive amplification in American eel (Anguilla rostrata).     

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.     

常压低氧小鼠氧耗量测定新装置的研究及应用

目的：建立一种实时记录常压低氧环境中动物氧耗量的方法。方法：本实验装置由动物舱、补水控制系统、天平、软管、装有体重记录软件的电脑等组成。为了实现常压低氧,用水补充动物消耗的氧气以保持动物舱内压力恒定,这个过程由气液联动装置控制;补充的水量由天平测量并同步输出信号至excel文档中。用注射器抽气校准方法检测了装置的准确性和精度。利用该装置观察了6只急性重复低氧小鼠（处理组）和6只未经低氧处理的小鼠（对照组）的常压低氧过程的氧耗量特征。结果：不同体积抽气量与相应补水量两组数据配对t检验P=1;重复抽1 ml氧气6次的补水量变异系数为4%。处理组小鼠的存活时间为（58.8±6.8）min,显著高于对照组（46.0±8.7）min（P〈0.05）。处理组小鼠的总氧耗量为（85.1±8.5）ml,显著高于对照组（73.6±5.4）ml（P〈0.05）。结论：处理组小鼠摄取氧总量增多从而显著延长其存活时间。氧耗量测定装置准确度和精密度较高,可用于低氧研究中氧耗量的测定。     

Intraseasonal characterization of glass eel migration in the River Tiber: space and time dynamics

Glass eel ( Anguilla anguilla ) upstream migration was studied in the River Tiber estuary to obtain a better understanding of spatial and temporal migration dynamics within the season of ascent. Using data from glass eel fisheries, time series analysis of daily catches per unit of effort revealed a fortnightly cycle that can be related to invasion waves possibly corresponding to tidal currents. The amplitude of these waves appeared to correspond to the tidal area of the estuary. Furthermore, glass eels apparently had a delay in this area before resuming upstream migration.     

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.     

Temporal changes in digestive enzyme activities in the gastrointestinal tract of European eel (Anguilla anguilla) (Linneo 1758) following feeding

Changes occurring after feeding in the digestive enzyme activities of European eel were investigated to provide some insights into the digestive physiology of this fish. Total and specific proteases, amylase and lipase activities were measured using standard biochemical assays over a 24 h cycle in fed eels, compared to starved ones, under the same rearing conditions. In the gastrointestinal tract of fed eels quantitative changes started 4 h after feeding and continued later on; conversely, in starved eels enzyme activities remained unchanged over time. In fed eels, total and specific protease activities showed an overall increasing trend in the intestine, while in the stomach they progressively decreased to values 22–50% lower than those measured at the pre-feeding time; this behaviour probably reflected the progression of digesta along the intestinal tract. The prolonged secretory response of European eel to food ingestion proved its extended activity in the digestive process.     

Implications of individual growth status on the future sex of the European eel

Sex-related differences in growth status was demonstrated in eels Anguilla anguilla reared indoors at 17, 20 or 26° C, from the elver stage. Growth status was defined as length increase, weight increase and length–weight relationship. Eels attaining at least 10 g body weight (180–220 mm body length) were tagged with Passive Integrated Transponders (PIT). Length and weight were measured at 6-week intervals, until individuals stopped growing or had attained 150 g weight (380–450 mm). Sex-specific data from potentially undifferentiated eels were provided by retrospective classification of sex. Comparisions between sexes were made within groups graded by length or weight data from the beginning of each 6-week period. There was no consistent difference in absolute length increase between small males and females, but below 40–60 g initial body weight, males displayed on average a higher weight increase than females. Males also had lower length at weight than females, even in the smallest weight groups. Early growth status may influence the future sex of undifferentiated eels, but other approaches are needed for distinction between cause and effect.     

The effects of intermittent exposure to hypoxia during endurance exercise training on the ventilatory responses to hypoxia and hypercapnia in humans

The present study was performed to investigate the effects of a combination of intermittent exposure to hypoxia during exercise training for short periods on ventilatory responses to hypoxia and hypercapnia (HVR and HCVR respectively) in humans. In a hypobaric chamber at a simulated altitude of 4,500 m (barometric pressure 432 mmHg), seven subjects (training group) performed exercise training for 6 consecutive days (30 min · day⁻¹), while six subjects (control group) were inactive during the same period. The HVR, HCVR and maximal oxygen uptake (V˙O_{2 max}) for each subject were measured at sea level before (pre) and after exposure to intermittent hypoxia. The post exposure test was carried out twice, i.e. on the 1st day and 1 week post exposure. It was found that HVR, as an index of peripheral chemosensitivity to hypoxia, was increased significantly (P < 0.05) in the control group after intermittent exposure to hypoxia. In contrast, there was no significant increase in HVR in the training group after exposure. The HCVR in both groups was not changed by intermittent exposure to hypoxia, while V˙O_{2 max} increased significantly in the training group. These results would suggest that endurance training during intermittent exposure to hypoxia depresses the increment of chemosensitivity to hypoxia, and that intermittent exposure to hypoxia in the presence or absence of exercise training does not induce an increase in the chemosensitivity to hypercapnia in humans. Accepted: 18 March 1998     

Occurrence of Phyllodistomum folium (Olfers) (Digenea) in the eel, Anguilla anguilla (Linnaeus), in a small river

The occurrence of Phyllodistomum folium in a population of eels was studied throughout one year. Abundance exhibited a summer peak. Gravid adults were found in all months, indicating that eels can serve as a definitive host for this parasite.