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.     

Relationships between locomotor behavior, morphometric characters and thyroid hormone levels give evidence of stage-dependent mechanisms in European eel upstream migration

In order to decipher movements during freshwater eel colonization, we experimentally characterized individual locomotor behavior of two eel life history stages: elvers and yellow eels. A ramp located at the flume tank upstream side required a specific locomotor behavior to be ascended. Placing individually tagged eels in the middle of the tank three times successively tested behavioral consistency. Eels climbing the ramp on each trial were classified as "upstream climbers" whereas eels settling in the tank middle were classified as "inactive". Both stages exhibited these two opposite consistent behaviors. However, elvers were predominantly "upstream climbers" (58.1%) whereas yellow eels were predominantly "inactive" (79.6%). We measured morphometric characters and thyroid hormones to determine if upstream activity was related to body condition and thyroid status. Elver upstream climbers had higher body condition as well as higher thyroxine (T(4)) and triiodothyronine (T(3)) levels compared with inactive elvers. Yellow eel upstream climbers had lower body length as well as higher T(3) and (T(3):T(4)) ratio compared with inactive yellow eels. This indicated that the physiological release factors for eel upstream migration may be stage dependent. For elvers, high thyroid gland activity, together with high body condition, may be the physiological release factors for migration. In contrast, for yellow eels, physiological stress may be the release factor with an increase in T(4) deiodination activity in the smallest eels. Our study revealed inter-stage and intra-stage locomotor behavior plasticity and suggested stage-dependent opposite impacts of physiological condition on eel upstream migration.     

Donor life stage influences juvenile American eel Anguilla rostrata attraction to conspecific chemical cues

The present study investigated the potential role of conspecific chemical cues in inland juvenile American eel Anguilla rostrata migrations by assessing glass eel and 1 year old elver affinities to elver washings, and elver affinity to adult yellow eel washings. In two‐choice maze assays, glass eels were attracted to elver washings, but elvers were neither attracted to nor repulsed by multiple concentrations of elver washings or to yellow eel washings. These results suggest that A. rostrata responses to chemical cues may be life‐stage dependent and that glass eels moving inland may use the odour of the previous year class as information to guide migration. The role of chemical cues and olfaction in eel migrations warrants further investigation as a potential restoration tool.     

Environmental determinism of year-to-year recruitment variability of European eel Anguilla anguilla in a small coastal catchment, the Frémur River, north-west France

The influence of environmental factors (mainly the river flow) on the year-to-year variability of European eel Anguilla anguilla fluvial recruitment in a small coastal catchment, the Frémur River (north-west France) was examined. A comprehensive survey of catches from fixed traps at two weirs located at 4·5 km (Pont es Omnes Dam) and 6·0 km (Bois Joli Dam) above the river mouth was carried out between 1997 and 2004. Young pigmented elvers (mean ± s.d. total length, 133·7 ± 29·6 mm) were recruited in eel-passes from February to October, but the main runs followed a modal curve from April to September with a peak centred in May to June. Catches varied greatly between years, from 381 to 26 765 elvers. For each trap, a positive linear relationship between monthly mean river flow that preceded the maximal intensity of captures and annual total catches was observed. These relationships explained 73·1% ( P < 0.01) and 89·0% ( P < 0.001) of the year-to-year variability of the recruitment observed in the Pont es Omnes and Bois Joli traps respectively. A significant increase in river flow at the beginning of the migration peak would thus trigger a greater proportion of A. anguilla settled in the estuary and in the downstream zone of the Fremur River to begin their freshwater colonization. The physicochemical roles of changes in river discharge in stimulating upstream migration are discussed. It is concluded that fluvial recruitment in the Fremur River is mainly determined by environmental factors.     

Origins of the freshwater attractant(s) of migrating elvers of the American eel,Anguilla rostrata

Synopsis Origins of the freshwater attractant(s) of migrating elvers of the American eel were investigated by assaying elvers' responses to rinses of plants, animals, and inanimate objects collected from a Rhode Island (U.S.A.) brook with a sizable elver run. Odor rinses were tested in a Y-maze at naturally occurring concentrations against both blank and brook water. Many items were attractive, several were repulsive, and some caused a reduction in elvers' rheotactic behavior, suggesting that elvers respond to a bouquet of odors. The odor of abundant decaying leaf detritus was highly attractive as were odors of the surfaces of aquatic plants, submerged stones, and migrating alewives. Conspecific odor was only weakly attractive. Because unattractive leaves became attractive when cultured with stream water, microorganisms responsible for detrital decomposition and present in/on most stream objects are thought to be the major source of the attractant(s). Decaying detritus and its associated microorganisms are abundant in most freshwater streams, where they often constitute the ecosystem's primary energy source; their odor could serve as an index of environmental suitability for migrating eels.     

Changes in otolith microchemistry of the Japanese eel, Anguitta japonica, during its migration from the ocean to the rivers of Taiwan

The newly recruited Japanese eel, Anguilla japonica , elvers and 1-year-old eels collected in estuaries and in rivers, respectively, were studied. The microstructure and chemical composition of the sagittal otolith of these eels were examined by SEM and wavelength-dispersive spectrometer (WDS), A transition zone or'elver mark'was observed in the otolith of the young eels. A comparison of the otoliths of elvers with those from the 1-year-old eels suggests that this transition zone was deposited during upstream migration, a change from a marine to freshwater environment. Strontium (Sr) content in the primordium of the otolith of both elvers and young eels was low, probably due to the maternal or freshwater origin of the oocyte. The concentration of Sr in the otolith increased gradually during marine life and reached a peak approximately 1 month before upstream migration. As the elvers entered the estuary, the Src concentration dramatically decreased and remained at a low level thereafter. These findings indicate that the history of the migratory environment of the eel can be reconstructed from a combined study of otolith microstructure and microchemistry analysis.     

Rod visual pigment changes in the elver of the eel Anguilla anguilla L. measured by microspectrophotometry

The visual pigments in the retinal rods of elvers ( Anguilla anguilla L.) were examined by microspectrophotometry and high performance liquid chromatography. The glass elver stage had a mixture of rhodopsin P501₁ and porphyropsin P523₂ with the former predominating (P501₁:P523₂= c. 60:40). More mature pigmented elvers had a predominance of the porphyropsin P523₂ (P501₁:P523₂= c. 10:90). The shift between the proportions of each pigment was monitored over a 7-month period and was shown to occur in individual retinal rods. This shift is a reverse of the pigment shift that occurs in adult eels during their downstream migration.     

Factors that influence cover selection by Japanese eels at elver stage

Adult eels are known to prefer narrow spaces for cover but factors influencing cover selection at the elver stage are unclear. I examined factors likely to be important in cover selection by elvers of the Japanese eel (Anguilla japonica). Pipes with a range of diameters, lengths, internal illumination (brightness), and direction of flow were provided for elvers in small-, medium- and large-sized classes. All sizes chose pipes without preference for diameter. Medium and large elvers preferred longer pipes. All size classes selected darker environments for pipes with similar dimensions. Medium to large elvers chose pipes without preference for orientation to the current up to flow speeds at which they were unable to stay in the pipe. The proportion of small elvers that remained in the pipes was lower than that of medium and large elvers.     

Determination of the freshwater mark in otoliths of Japanese eel elvers using microstructure and Sr/Ca ratios

The microstructure, in particular checks within the otolith edge, of Anguilla japonica glass-eels and elvers and changes in otolith Sr/Ca ratios were examined to ascertain the environmental history of the eels, especially with regard to the time when glass-eels entered the river, and as a benchmark for count daily increments. The percentage of glass eels and elvers with checks and the mean number of checks within the otoliths of glass-eels caught at four localities, Tosa Bay off Tosa City, the mouth of the Gokase River, the mouth of the Saigo River and the dam of the Tsuri River were 0% (0), 15.0% (0.2), 51.6% (1.0) and 100.0% (4.2), respectively. The Sr/Ca ratios and Sr content peaked in the region where checks were formed and the values decreased rapidly towards the edge of the checks; on the other hand, these decreased gradually in the otolith when checks were not formed. These checks were estimated to be formed by stress when the glass-eels were affected by ambient fresh water within the river. The innermost check was called the freshwater mark in the present study and this mark may be useful as a benchmark in studying the growth history of the eel before and after entering freshwater.     

Long-term effects of hydropower installations and associated river regulation on River Shannon eel populations: mitigation and management

The Shannon, Ireland’s largest river, is used for hydroelectricity generation since 1929. Subsequently, the Electricity Supply Board assumed responsibility for management of its eel stocks, due to the impact of the hydro-dam on recruitment to the commercial fishery. In order to negate a decline in juvenile recruitment resulting from the installation of hydroelectric facilities, management was focused on stocking lakes with elvers and fingerling eels. These were trapped at the hydropower facilities and in estuarine tributaries during their up-stream migrations. Due to the decline of natural recruitment in more recent times, attempts have also been made to develop an estuarine glass eel fishery. Stock levels are then monitored through annual surveys of the population trends of juvenile (glass eel, elver), growing phase (yellow eel) and downstream migrating pre-spawners (silver eels). Survey results and fishery management programmes are reviewed in this article. In addition to the long-term effects the hydroelectric facilities have had on the stock levels, there is also an annual effect on the migratory patterns of downstream migratory silver eels. In the lower reaches of the river system flow rates are regulated by the hydroelectric stations. We review previous work that had highlighted the importance of flow in determining the timing of the silver eels migrations, and assess the relationship between flow and migration in more detail through the use of hydroacoustic and telemetric studies. Current research on seaward migrating silver eel populations, suggests that spawner escapement rates can most effectively be increased by trapping migrating eels at fishing weirs located up-stream of the power station and transporting them towards the estuary. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Freshwater and coastal migration patterns in the silver‐stage eel Anguilla anguilla

The unimpeded downstream movement patterns and migration success of small female and male Anguilla anguilla through a catchment in north‐west Europe were studied using an acoustic hydrophone array along the River Finn and into the Foyle Estuary in Ireland. Twenty silver‐stage A. anguilla (total length, L_T, range: 332–520 mm) were trapped 152 km upstream from a coastal marine sea‐lough outlet and internally tagged with acoustic transmitters of which 19 initiated downstream migration. Migration speed was highly influenced by river flow within the freshwater (FW) compartment. Anguilla anguilla activity patterns were correlated with environmental influences; light, tidal direction and lunar phase all influenced the initiation of migration of tagged individuals. Migration speed varied significantly between upstream and lower river compartments. Individuals migrated at a slower speed in transitional water and sea‐lough compartments compared with the FW compartment. While 88·5% survival was recorded during migration through the upper 121 km of the river and estuary, only 26% of A. anguilla which initiated downstream migration were detected at the outermost end of the acoustic array. Telemetry equipment functioned efficiently, including in the sea‐lough, so this suggests high levels of mortality during sea‐lough migration, or less likely, long‐term sea‐lough residence by silver A. anguilla emigrants. This has important implications for eel management plans.     

基于鳗鱼道诱鱼技术研究花鳗鲡幼鱼光色趋向性

为保护我国花鳗鲡资源,提高鳗鱼道入口吸引力,本研究以3 W水下景观灯为光源,利用自制多功能圆池设计了3种试验工况,探索花鳗鲡幼鱼的光色趋向性.结果表明: 3种试验工况下,鱼类在光区与暗区单位面积出现比例分别为(8.7±0.3)%和(3.6±1.7)%、(8.4±0.2)%和(5.0±1.4)%、(9.0±0.2)%和(1.9±1.3)%,且分布在光区的幼鱼多处于玻璃鳗发育阶段,而暗区的幼鱼多处于线鳗阶段(或向线鳗发育).光色区中鱼类出现次数百分比排在前3的分别为黄光区[(29.4±4.9)%]、红光区[(24.1±3.9)%]和紫光区[(17.2±4.0)%];黄光区[(29.4±3.3)%]、紫光区[(25.2±3.7)%]和红光区[(18.2±2.9)%];红光区[(33.3±2.3)%]、紫光区[(25.6±3.8)%]和黄光区[(20.7±5.7)%].首先进入光区次数排在前3的分别为红、黄、紫光区;红、紫、黄光区;红、紫、黄光区.鱼类在上述3种光区中的总进出次数处于蓝绿光和五彩光之间,且红光和黄光下鱼类会出现视觉疲劳.花鳗鲡幼鱼对光的趋向性由其发育阶段决定,玻璃鳗对光色具有趋向性,尤喜爱红、黄和紫光,在鳗鱼道入口采用灯光诱鱼可帮助玻璃鳗找到生长洄游通道.     

Regional variation in otolith Sr:Ca ratios of African longfinned eel Anguilla mossambica and mottled eel Anguilla marmorata: a challenge to the classic tool for reconstructing migratory histories of fishes

Otolith Sr:Ca ratios of the African longfinned eel Anguilla mossambica and giant mottled eel Anguilla marmorata from nine freshwater sites in four rivers of South Africa were analysed to reconstruct their migratory life histories between freshwater and saltwater habitats. For A. mossambica, the Sr:Ca ratios in the otolith edge differed significantly among rivers and had large effect sizes, but did not differ among sites within a river. Otolith Sr:Ca ratios did not differ among rivers for A. marmorata. When rivers were pooled, the edge Sr:Ca ratios of A. mossambica were not significantly different from those of A. marmorata. According to the river-specific critical Sr:Ca ratio distinguishing freshwater from saltwater residence, most A. mossambica and A. marmorata had saltwater habitat experience after settlement in fresh water. This was primarily during their elver stage or early in the yellow eel stage. During the middle and late yellow eel stage, freshwater residency was preferred and only sporadic visits were made to saltwater habitats. The data also suggest that regional variations in otolith Sr:Ca ratios affect the critical Sr:Ca value and are a challenge for the reconstruction of migratory life histories that should be explicitly considered to avoid bias and uncertainty.     

Recruitment mechanism of the eel, Anguilla japonica, to the Japanese coast

A total of 149 Anguilla japonica elvers collected at 10 locations along the Japanese coast were aged according to otolith daily increments. The interrelationships among age, birth date, body size, pigmentation stage, sampling location and the timing of recruitment were examined in order to determine the recruitment mechanism of elvers to coastal waters.
First catches of eels were earlier in the locations at lower latitudes. Age at recruitment was roughly constant, 218 ± 29 (mean ± s.d.) days old, disregarding the localities, but a weak positive correlation was obtained between age and sampling date or timing of recruitment. Body length at recruitment was also constant, 56.3 ± 2.3 mm s.l., and showed no significant correlation with either locality or recruitment timing. Estimated birth date ranged from April to November, the mean ± s.d. being 22 July 1982 ± 42 days, suggesting a peak season spawning in summer. Birth date was closely related with both the latitude of location and sampling time. Pigmentation developed more at lower latitude. Recruitment mechanism of the Japanese eel was summarized as follows: the earlier-born individual recruits earlier at lower latitude and at younger age, but at a constant body size.     

Return migration of Atlantic salmon in the River Tana: the role of environmental factors

Multi-sea-winter Atlantic salmon (75–115 cm fork length L _F, 2–4-winter fish) were radio-tagged in the Tanafjord (700 N), Norway, in 1992–1993, and 130 fish entered the large subarctic River Tana (Teno). They entered the fresh water at any time of the tidal cycle but more so during the high and ebbing tides. No diel rhythm was detected in river entry under polar day conditions. There were no differences in the change of flow between days when salmon moved and when they did not, but during active migration increasing discharge was associated with increased swimming activity of salmon, especially later in the summer. Increasing air temperature was also associated with enhanced migration activity. Low river flow was associated with increasing delay in salmon passing the first riffle area of the river, 35 km from the sea.     

Preliminary studies on the conversion of Tubifex tubifex as food by elvers of Anguilla nebulosa (Gray and Hardwicke)

Elvers of Anguilla nebulosa were reared at 23°C in confined waters and their food intake, growth and conversion efficiency (K₁:%) were studied. Elvers weighing 278.33±14.35 mg were fed on an ad libitum diet of the oligochaete worm Tubifex tubifex for 60 days. On an average, the test individuals consumed 99.55±19.81 mg food/elver/day amounting to 35.76% of the initial biomass; during the corresponding period the elvers exhibited a growth rate of 5.03±2.42 mg gain in weight/elver/day which is equivalent to 5.06% of the consumed food. Thus the 'housekeeping' of these elvers may be regarded as established. In 50 days, the food required amounts to 0.358±50 = 17.90 times the initially stocked biomass. That is, 1 kg of initially stocked elvers may need 17.90 kg of T. tubifex to produce 0.912 kg of elver meat. A poor conversion ratio of 20 : 1 may be one reason for the slow growth of the elvers.     

The ultrastructure of the integument of the American eel,Anguilla rostrata

Summary The morphology and ultrastructure of the lateral body integument of the leptocephalus, glass eel, pigmented elver, and adult stages of the American eel, Anguilla rostrata, were examined with light and electron microscopy. The integument consists of an epidermis separated by a basal lamina from the underlying dermis. Three cell types are present in the epidermis in all stages. Filament-containing cells, which are the principal structural cell type, are increasingly numerous at each stage. Mucous cells, which secrete the mucous that compose the mucous surface coat, are also more numerous in each subsequent stage and are more numerous in the anterior lateral body epidermis than in the posterior lateral body epidermis of the adult. Club cells, whose function is unknown, are most numerous in the glass eel and pigmented elver. Chloride cells are common in the leptocephalus which is marine and infrequent in the glass eel. They are not present in the pigmented elver and adult which inhabit estuaries and fresh-water. Lymphocytes and melanocytes are also present in some stages. The dermis comprises two layers: a layer of collagenous lamellae, the stratum compactum, and an underlying layer of loose connective tissue, the stratum spongiosum.There is a progressive increase in epidermal thickness at each stage which is paralleled by an increase in the thickness of the stratum compactum. Rudimentary scales are present in the dermis of the adult. The increase in the number of epidermal filament-containing cells, epidermal thickness and stratum compactum thickness is correlated with an increased need for protection from abrasion and mechanical damage as the eel moves from a pelagic, oceanic habitat to a benthic, freshwater habitat. The increase in mucous cell numbers is likewise correlated with an increased need for the protective and anti-bacterial action of the mucous surface coat in the freshwater environment.This investigation was supported by NIH research grant NS-11276 from National Institute of Neurological Diseases and Stroke to Dr. J.D. McCleave and by N.S.F. Grant GD 38933 to the Bermuda Biological Station, St. Georges West, Bermuda. Bermuda Biological Station Contribution No. 668     

Swimming performance of European eel (Anguilla anguilla (L.)) elvers

To determine the relation between swimming endurance time and burst swimming speed, elvers of the European eel, Anguilla anguilla (L.), were made to swim at speeds from 3.6 to 7.2 L (body lengths) s⁻¹ in both fresh and sea water. Swimming endurance time of elvers averaging 7.2 cm total length decreased logarithmically with increased swimming speed from 3.0 min at 3.5 L s⁻¹ to 0.7 min at 5.0 L s⁻¹, and again logarithmically but with a lesser slope to 0.27 min at 7.5 L s⁻¹. No differences were found between fresh and sea water elvers. In still water, elvers could swim at high speeds for about 10–45m before exhaustion, depending upon speed. Elvers would be able to make virtually no progress against water currents >50 cm s⁻¹. Drift in coastal water currents and selective tidal transport probably involve swimming speeds below those tested in this study. Migration into freshwater streams undoubtedly involves avoidance of free stream speeds and a combination of burst and sustained swimming.     

Behaviour and swimming performance of elvers of the American eel, Anguitta rostrata, in an experimental flume

Migrating elvers of the American eel, Anguilla rosirata , were examined in an experimental flume to determine the effect of water speed and presence of substrate on swimming performance. Use of the substrate as a resting area between movements increased with increasing water speed. The number of elvers moving in the boundary layer above the substrate was significantly greater than those moving in the water column. Swimming speed in the water column and along the substrate increased with water speed. Rate of progress over ground in the water column and boundary layer increased slightly with water speed initially, but subsequently declined in the boundary layer. Overall, swimming speed increased linearly with increasing water speed and net upstream progress remained constant with increasing water speed. Use of the boundary layer probably was restricted by the thickness of the layer as a function of water speed. The boundary layer should be used to maximize distance and minimize energy expense when migrating upstream. Elvers appear to use substrate to avoid free stream velocities and not as a medium for movement. Fifty per cent of all elvers tested in a chamber without substrate, which had no resting places, were unable to swim; thus migrations may be limited by the need to rest. The potential migration rate was estimated to approach 100m per day, based on movements in the substrate chamber.     

Compilation of Japanese fisheries statistics for the Japanese eel, <Emphasis Type="Italic">Anguilla japonica</Emphasis>, since 1894: a historical dataset for stock assessment

Fishery sustainability and the extinction risk of the Japanese eel, Anguilla japonica, are of global concern. The landings of the Japanese eel in Japan comprise a large part of the landings in East Asia. This study provides a compiled dataset of the annual fisheries statistics of the Japanese eel in Japan for stock assessment. The Japanese government has been recording Japanese eel statistics annually since 1984 in five series of annual reports by conducting systematic questionnaire surveys of fisheries managers and associations; however, most of these data are stored in analog format. The key variables in the dataset include the harvest weight of eels, the harvest weight and number of seeds for aquaculture, the number of eels stocked, and the number of management entities engaged in the eel fishery. The levels of spatial aggregation of the variables include the site (river and lake), prefecture, inland and coastal waters, and total in Japan. We also incorporated location data (latitude and longitude) of the site and prefecture into the dataset. Eel harvest includes primarily yellow eels (late juvenile stage) and silver eels (mature stage). Seed harvest in inland waters includes glass eels (intermediate stage between leptocephalus and elver) and elvers (early juvenile stage). Seed harvest from coastal waters comprises glass eels. This dataset provides information to assess long-term trends in the Japanese eel population.