Advanced sexual maturation before marine migration of Anguilla bicolor bicolor and Anguilla marmorata at Réunion Island

Maturing sub-adults of two species of anguillid eels (a female Anguilla bicolor bicolor and a male Anguilla marmorata ) were collected for the first time at Réunion Island, western Indian Ocean. Both were silver eels, i.e. maturing eels at the onset of their spawning migration, and characterized by advanced sexual maturation that has been only observed in Anguilla dieffenbachii from New Zealand.     

A century of research on the larval distributions of the Atlantic eels: a re‐examination of the data

The spawning areas of the Atlantic freshwater eels were discovered about a century ago by the Danish scientist Johannes Schmidt who after years of searching found newly hatched larvae of the European eel, Anguilla anguilla, and the American eel, Anguilla rostrata, in the southern Sargasso Sea. The discovery showed that anguillid eels migrate thousands of kilometers to offshore spawning areas for reproduction, and that their larvae, called leptocephali, are transported equally long distances by ocean currents to their continental recruitment areas. The spawning sites were found to be related to oceanographic conditions several decades later by German and American surveys from 1979 to 1989 and by a Danish survey in 2007 and a German survey in 2011. All these later surveys showed that spawning occurred within a restricted latitudinal range, between temperature fronts within the Subtropical Convergence Zone of the Sargasso Sea. New data and re‐examinations of Schmidt's data confirmed his original conclusions about the two species having some overlap in spawning areas. Although there have been additional collections of leptocephali in various parts of the North Atlantic, and both otolith research and transport modelling studies have subsequently been carried out, there is still a range of unresolved questions about the routes of larval transport and durations of migration. This paper reviews the history and basic findings of surveys for anguillid leptocephali in the North Atlantic and analyses a new comprehensive database that includes 22612 A. anguilla and 9634 A. rostrata leptocephali, which provides a detailed view of the spatial and temporal distributions and size of the larvae across the Atlantic basin and in the Mediterranean Sea. The differences in distributions, maximum sizes, and growth rates of the two species of larvae are likely linked to the contrasting migration distances to their recruitment areas on each side of the basin. Anguilla rostrata leptocephali originate from a more western spawning area, grow faster, and metamorphose at smaller sizes of <70 mm than the larvae of A. anguilla, which mostly are spawned further east and can reach sizes of almost 90 mm. The larvae of A. rostrata spread west and northwest from the spawning area as they grow larger, with some being present in the western Caribbean and eastern Gulf of Mexico. Larvae of A. anguilla appear to be able to reach Europe by entering the Gulf Stream system or by being entrained into frontal countercurrents that transport them directly northeastward. The larval duration of A. anguilla is suggested to be quite variable, but gaps in sampling effort prevent firm conclusions. Although knowledge about larval behaviour is lacking, some influences of directional swimming are implicated by the temporal distributions of the largest larvae. Ocean–atmosphere changes have been hypothesized to affect the survival of the larvae and cause reduced recruitment, so even after about a century following the discovery of their spawning areas, mysteries still remain about the marine life histories of the Atlantic eels.     

Effect of lunar periodicity on the locomotor activity of silver-stage Japanese eel,Anguilla japonica

Lunar periodicity has been thought to influence the onset of the spawning migration of anguillid eels. In this study, we measured daily locomotor activity of 8 silver-phase Japanese eels Anguilla japonica in outdoor tanks to examine the effect of lunar periodicity on their activity and the following seaward migration. The activity of silver eels was highest around the new moon during the early part of the experiment, which is the ordinary season of seaward migration in Japan. The observed patterns of activity may reflect the importance of the lunar cycle for the onset of the spawning migration in anguillid eels.     

Salmon ocean migration models suggest a variety of population-specific strategies

Many species of salmon around the world migrate to open ocean environments for multiple years and then return to their natal rivers to spawn. How exactly salmon are able to execute these long distance migrations, and the impact of environmental conditions on migration behavior, is not well understood. Individual based modeling is one tool that has been used to explore salmon migration in the ocean. Although models are usually not able to confirm whether a particular behavior is used, they can rule out some behaviors as unrealistic. An extensive review of published literature suggests that there is no universal migration behavior. Behaviors that fish use to navigate depend on where they are in the ocean relative to where they are going, as well as the ocean flows and conditions along the way. Future models of salmon migration should be flexible and at an appropriate scale to capture variable oceanographic conditions and fish responses.     

Migrating silver eels return from the sea to the river of origin after a false start

The European eel''s singular spawning migration from European waters towards the Sargasso Sea remains elusive, including the early phase of migration at sea. During spawning migration, the movement of freshwater resident eels from river to sea has been thought to be irreversible. We report the first recorded incidents of eels returning to the river of origin after spending up to a year in the marine environment. After migrating to the Baltic Sea, 21% of the silver eels, tagged with acoustic transmitters, returned to the Narva River. Half returned 11–12 months after moving to the sea, with 15 km being the longest upstream movement. The returned eels spent up to 33 days in the river and migrated to the sea again. The fastest specimen migrated to the outlet of the Baltic Sea in 68 days after the second start—roughly 1300 km. The surprising occurrence of returning migrants has implications for sustainable management and protection of this critically endangered species.     

Evidence of local short‐distance spawning migration of tropical freshwater eels,and implications for the evolution of freshwater eel migration

Freshwater eels have fascinated biologists for centuries due to the spectacular long‐distance migrations between the eels’ freshwater habitats and their spawning areas far out in the ocean and the mysteries of their ecology. The spawning areas of Atlantic eels and Japanese eel were located far offshore in the Atlantic Ocean and the Pacific Ocean, respectively, and their reproduction took place thousands of kilometers away from their growth habitats. Phylogenetic studies have revealed that freshwater eels originated in the Indonesian region. However, remarkably little is known about the life histories of tropical freshwater eels despite the fact that tropical eels are key to understanding the nature of primitive forms of catadromous migration. This study found spawning‐condition tropical freshwater eels in Lake Poso, central Sulawesi, Indonesia, with considerably high gonadosomatic index values and with histologically fully developed gonads. This study provides the first evidence that under certain conditions, freshwater eels have conditions that are immediately able to spawn even in river downstream. The results suggest that, in contrast to the migrations made by the Atlantic and Japanese eels, freshwater eels originally migrated only short distances of <100 kilometers to local spawning areas adjacent to their freshwater growth habitats. Ancestral eels most likely underwent a catadromous migration from local short‐distance movements in tropical coastal waters to the long‐distance migrations characteristic of present‐day temperate eels, which has been well established as occurring in subtropical gyres in both hemispheres.     

Deep-ocean origin of the freshwater eels

Of more than 800 species of eels of the order Anguilliformes, only freshwater eels (genus Anguilla with 16 species plus three subspecies) spend most of their lives in freshwater during their catadromous life cycle. Nevertheless, because their spawning areas are located offshore in the open ocean, they migrate back to their specific breeding places in the ocean, often located thousands of kilometres away. The evolutionary origin of such enigmatic behaviour, however, remains elusive because of the uncertain phylogenetic position of freshwater eels within the principally marine anguilliforms. Here, we show strong evidence for a deep oceanic origin of the freshwater eels, based on the phylogenetic analysis of whole mitochondrial genome sequences from 56 species representing all of the 19 anguilliform families. The freshwater eels occupy an apical position within the anguilliforms, forming a highly supported monophyletic group with various oceanic midwater eel species. Moreover, reconstruction of the growth habitats on the resulting tree unequivocally indicates an origination of the freshwater eels from the midwater of the deep ocean. This shows significant concordance with the recent collection of mature adults of the Japanese eel in the upper midwater of the Pacific, suggesting that they have retained their evolutionary origin as a behavioural trait in their spawning areas.     

Freshwater habitat use by a moray eel species,Gymnothorax polyuranodon,in Fiji shown by otolith microchemistry

Freshwater eels of the Anguillidae are diadromous because they migrate between ocean and freshwater environments, but other anguilliform fishes are generally considered to be strictly marine species. A few marine eels of the Muraenidae and Ophichthidae have occasionally been found in freshwater or estuaries, indicating that anguillids are not the only anguilliform eels that can use freshwater in some parts of the world. The moray eel Gymnothorax polyuranodon is one species that is known to be present in freshwater in the Indo-Pacific, but its life history is unknown. One way to evaluate what types of habitats are used by fishes is to determine the ratio of strontium (Sr) to calcium (Ca) in their otoliths, because this can show if they have used freshwater or saltwater environments. To evaluate the patterns of freshwater use by this unusual species of marine eel, the otolith Sr/Ca ratios of four G. polyuranodon (275–344 mm) caught in a freshwater stream of Fiji were analyzed. The consistently low Sr/Ca values (0–4) indicated upstream movement after settlement and freshwater or estuarine residence of all four individuals. These eels did not appear to have entered freshwater just for a short time period, which is consistent with other reports that this species is present in estuarine and freshwater habitats. This suggests that G. polyuranodon may be a catadromous species of marine eel. The similarities and differences between the life histories of anguillid eels and the few marine eels that have evolved the ability to invade freshwater habitats is discussed in relation to the evolutionary origin of diadromy in anguilliform fishes that originated in the marine environment.     

Sympatric spawning of Anguilla marmorata and Anguilla japonica in the western North Pacific Ocean

Extensive collections were made of the larvae of the temperate Japanese eel Anguilla japonica and the tropical giant mottled eel Anguilla marmorata in an overlapping area of the North Equatorial Current region of the western North Pacific Ocean. Collections of 189 A. marmorata and > 2500 A. japonica larvae during nine surveys from 1991 to 2007 showed that these two anguillid eels have similar spawning areas just west of the southern West Mariana Ridge. In July to August 2006 and August 2007, morphologically and genetically identified A. marmorata preleptocephali were mainly collected between 14·5–15° N and 142–142·5° E, where A. japonica preleptocephali were also caught in some of the same net tows. Fewer A. marmorata preleptocephali, however, were collected ( n = 31) compared to those of A. japonica ( n = c . 165), and fewer small larvae of A. marmorata were collected per tow than A. japonica ( n = 1–10 and 1–294, respectively), suggesting relatively smaller spawning aggregations of A. marmorata . The distribution of preleptocephali and small larvae was wider in longitude in A. marmorata (131– 143° E) than in A. japonica (137–143° E), while the latitudinal range was almost the same (12–17° N). Although spawning by these two species overlaps both spatially and temporally, the tropical eels of the North Pacific population of A. marmorata probably have a much longer spawning season with fewer spawners, at least in summer, and recruit to a much wider latitudinal range of growth habitats.     

Thermal preferenda and behavior of Atlantic eels (genus Anguilla) in relation to their spawning migration

Synopsis The final preferred temperatures (FPTs) of adult premigratory and migratory life-history phases of American eels, Anguilla rostrata, were determined by chronic tests in a horizontal thermal gradient. Mean FPTs were between 17 and 20°C and were not significantly different between life-history phases, acclimation temperatures, illumination regimes, photoperiods or sexual maturation states. Thermal behavior of eels was highly variable, both among individuals of the various test groups and among repeated tests of single individuals. Light inhibited behavioral thermoregulation by promoting shelter-seeking. The following inferences are drawn from the laboratory findings and observations of migrating A. rostrata and A. anguilla (European eels) in the North Atlantic: (1) decreasing temperatures may initiate downstream migration of silver eels, (2) eels may select temperatures close to their FPT in thermally stratified environments, but will tolerate higher and lower temperatures depending on illumination or other physical constraints, (3) the oceanic phase of the migration to the Sargasso Sea may take place at relatively shallow depths in the open ocean, probably within the upper 1000 meters. The strong eurythermality observed in eels may facilitate their occupation of and migration through thermally diverse and unpredictable habitats.     

Timing and pattern of annual silver eel migration in two European watersheds are determined by similar cues

Many animals perform long‐distance migrations in order to maximize lifetime reproductive success. The European eel migrates several thousand kilometers between their feeding habitats in continental waters (fresh‐, brackish, and sea water) and their spawning area in the Sargasso Sea. Eels residing in freshwaters usually initiate their spawning migration as silver eels during autumn, triggered by diverse environmental cues. We analyzed the time series of silver eel downstream migration in Burrishoole, Ireland (1971–2015), and Imsa, Norway (1975–2015), to examine factors regulating the silver eel migration from freshwater to the sea. The migration season (90% of the run) generally lasted from 1 August to 30 November. Environmental factors acting in the months before migration impacted timing and duration of migration, likely through influencing the internal processes preparing the fish for migration. Once the migration had started, environmental factors impacted the day‐to‐day variation in number of migrants, apparently stimulating migration among those eels ready for migration. Both the day‐to‐day variation in the number of migrants and the onset of migration were described by nearly identical models in the two rivers. Variables explaining day‐to‐day variation were all associated with conditions that may minimize predation risk; number of migrants was reduced under a strong moon and short nights and increased during high and increasing water levels. Presence of other migrants stimulated migration, which further indicates that silver eel migration has evolved to minimize predation risk. The onset of migration was explained mainly by water levels in August. The models for duration of the migration season were less similar between the sites. Thus, the overall migration season seems governed by the need to reach the spawning areas in a synchronized manner, while during the actual seaward migration, antipredator behavior seems of overriding importance.     

A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)

The European eel (Anguilla anguilla) is a catadromous fish that spawns in the Sargasso Sea. As larvae, eels cross the Atlantic Ocean and reach the continental slope of Europe, where they metamorphose into post‐larval glass eels. These reach the continent, where some enter fresh water, some remain in marine waters, and others move between fresh and marine waters. After 5–25 years, as adult silver eels, they migrate back from fresh water to the Sargasso Sea to spawn and die. The glass eel stage is a critical step during which the eels cross the continental shelf and recruit to estuaries, where they facultatively transition to fresh water. Extensive research has been conducted to understand the behavioural mechanisms and environmental cues that aid and guide glass eels' migration. Glass eels follow odours and salinity gradients, they avoid light, and they change orientation and depth according to the tides. Recent work revealed that European glass eels also use Earth's magnetic field and lunar cues to orient. However, while we understand many aspects of their orientation behaviour, a unifying theory describing how glass eels migrate from the continental slope to fresh water is lacking. The goal of this review is to develop a comprehensive hypothesis on the migration of European glass eels, integrating previous knowledge on their orientation behaviour with recent findings on magnetic and celestial orientation. This review follows the journey of a hypothetical glass eel, describing the nature and the role of orientation cues involved at each step. I propose that, although glass eels have the sensory capacity to use multiple cues at any given time, their migration is based on a hierarchical succession of orientation mechanisms dictated by the physical properties of the environments that they occupy: (i) lunar and magnetic cues in pelagic water; (ii) chemical and magnetic cues in coastal areas; and (iii) odours, salinity, water current and magnetic cues in estuaries.     

Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassus

Infection with the swim-bladder parasite Anguillicola crassus is suggested as one of the principal causes of the collapse of the European eel population. This nematode has been introduced in Europe from Asia in the 80s and parasitized in a short time Anguilla eel species in different geographical regions across the globe. The parasites drain energy due to their sanguivorous feeding and they cause mechanical damage on the swim-bladder wall. These two effects are hypothesized to impair the spawning migration of the European eel. In this study, we have investigated both effects on swimming performance. We hypothesized that parasitic sanguivorous activities - related to parasite weight - reduce swimming endurance, while mechanical damage of the swim-bladder impairs buoyancy control. Eighty eels suffering various degrees of infection were introduced in swim-tunnels and subjected to a swimming fitness test. The relation between A. crassus infection and swimming efficiency was measured for large female silver eels swimming at various speeds. Infected eels had lower cruising speeds and a higher cost of transport. Eels without parasites, but with a damaged swim-bladder showed similar effects. Almost half of the eels that contained damaged swim-bladders (43%) stopped swimming at low aerobic swimming speeds (< 0.7 m/s). Simulated migration trials in a recent related study have confirmed that eels with a high parasite level or with damaged swim-bladder show early migration failure (< 1000-km). Reduced swimming performance appears to be associated with swim-bladder dysfunction. As we found that especially silver eels have much higher infection levels than yellow eels, it is concluded that migrating silver eels with severely infected or damaged swim-bladders are unable to reach the spawning grounds.     

When Does this Fish Spawn? Fishermen’s Local Knowledge of Migration and Reproduction of Brazilian Coastal Fishes

Fishermen’s local knowledge of fishing resources may be an important source of information to improve artisanal tropical fisheries management, such as those found in Brazil, where most data on fish biology is lacking. We aim to study the local ecological knowledge that Brazilian coastal fishers have about reproductive aspects (season, places and migration) of 13 coastal fish species of commercial importance. We selected fishermen with more than 30 years of fishing practice and we interviewed a total of 67 fishermen: 29 from the southeastern coast, from the communities of Puruba, Almada, Picinguaba and Bertioga, and 38 from the northeastern coast, from the communities of Valença, Arembepe and Porto Sauípe. In the interviews, we used standardized questionnaires and showed photos of fish species. Our results indicate some general patterns in fishes’ reproduction according to fishermen knowledge: fish species spawn in open ocean, near reefs or in coastal rivers (estuaries); some fishes reproduce during the summer and others in winter, while some have more defined spawning months. The main fish migratory patterns mentioned by interviewees were: long migrations along the coast, usually in the South to North direction, short migrations among reefs, fishes that do not migrate, migrations between the shore and open ocean and migrations between the sea and coastal rivers. Fishermen’s knowledge differed among fish species: most fishermen did not know spawning places or seasons of large pelagic fishes, which raised concerns of their possible depletion. We compared such ethnoichthyological information with available scientific data, indicating promising insights about reproduction and migration of Brazilian coastal fishes. Data gathered from local fishermen may provide inexpensive and prompt information, potentially applicable to fisheries management. Our approach might be useful to several other small-scale fisheries, especially the tropical ones, where there is a high diversity of target species and a low biological and ecological knowledge about these species.     

Taxonomic revision,ecology and endangerment categorization of the Andean catfish Astroblepus ubidiai (Teleostei: Astroblepidae)

The European eel (Anguilla anguilla Linnaeus 1758) is a species typical for waters of Western Europe. Thanks to early expeditions on the Atlantic Ocean by the Danish biologist Johannes Schmidt who found small (<10mm) leptocephali larvae in the Sargasso Sea about 100 years ago, we have now a strong indication where the spawning site for this species is located. The American eel (Anguilla rostrata, LeSueur) also spawns in the Sargasso Sea. The spawning time and location of both species have been supported and refined in recent analyses of the available historical data. Subsequent ichthyoplankton surveys conducted by McCleave (USA) and Tesch (Germany) in the 1980s indicated an increase in the number of leptocephali <10 mm , confirming and refining the Sargasso Sea theory of Johannes Schmidt. Distinctions between the European and American eel are based on morphological characteristics (number of vertebrae) as well as molecular markers (allozymes, mitochondrial DNA and anonymous genomic-DNA. Although recognised as two distinct species, it remains unclear which mechanisms play a role in species separation during larval drift, and what orientation mechanism eels use during migration in the open sea. The current status of knowledge on these issues will be presented. The hypothesis that all European eel migrate to the Sargasso Sea for reproduction and comprise a single randomly mating population, the so called panmixia theory, was until recently broadly accepted. However, based on field observations, morphological parameters and molecular studies there are some indications that Schmidt’s claim of complete homogeneity of the European eel population and a unique spawning location may be an overstatement. Recent molecular work on European eel indicated a genetic mosaic consisting of several isolated groups, leading to a rejection of the panmixia theory. Nevertheless, the latest extensive genetic survey indicated that the geographical component of genetic structure lacked temporal stability, emphasising the need for temporal replication in the study of highly vagile marine species. Induced spawning of hormone treated eels in the aquarium was collective and simultaneous. In this work for the first time group spawning behaviour has ever been observed and recorded in eels. Studies in swim-tunnels indicate that eels can swim four to six times more efficiently than non-anguilliform fish such as trout. After a laboratory swim trial of eels over 5,500 km, the body composition did not change and fat, protein and carbohydrate were used in the same proportion. This study demonstrated for the first time that European eel are physiologically able of reaching the Sargasso Sea without feeding. Based on catches of newly hatched larvae, temperature preference tests and telemetry tracking of mature hormone treated animals, it can be hypothesised that spawning in the Sargasso Sea is collective and simultaneous, while presumably taking place in the upper 200 m of the ocean. Successful satellite tracking of longfin female eels in New Zealand has been performed to monitor migration pathways. Implementation of this new technology is possible in this species because it is three times larger than the European eel. In the future, miniaturisation of tagging technology may allow European eels to be tracked in time by satellite. The most interesting potential contribution of telemetry tracking of silver eels is additional knowledge about migration routes, rates, and depths. In combination with catches of larvae in the Sargasso Sea, it may elucidate the precise spawning locations of different eel species or groups. Only then, we will be able to define sustainable management issues by integrating this novel knowledge into spawners escapement and juvenile fishing quota.     

Magnetic Compass Orientation in the European Eel

European eel migrate from freshwater or coastal habitats throughout Europe to their spawning grounds in the Sargasso Sea. However, their route (∼ 6000 km) and orientation mechanisms are unknown. Several attempts have been made to prove the existence of magnetoreception in Anguilla sp., but none of these studies have demonstrated magnetic compass orientation in earth-strength magnetic field intensities. We tested eels in four altered magnetic field conditions where magnetic North was set at geographic North, South, East, or West. Eels oriented in a manner that was related to the tank in which they were housed before the test. At lower temperature (under 12°C), their orientation relative to magnetic North corresponded to the direction of their displacement from the holding tank. At higher temperatures (12–17°C), eels showed bimodal orientation along an axis perpendicular to the axis of their displacement. These temperature-related shifts in orientation may be linked to the changes in behavior that occur between the warm season (during which eels are foraging) and the colder fall and winter (during which eels undertake their migrations). These observations support the conclusion that 1. eels have a magnetic compass, and 2. they use this sense to orient in a direction that they have registered moments before they are displaced. The adaptive advantage of having a magnetic compass and learning the direction in which they have been displaced becomes clear when set in the context of the eel’s seaward migration. For example, if their migration is halted or blocked, as it is the case when environmental conditions become unfavorable or when they encounter a barrier, eels would be able to resume their movements along their old bearing when conditions become favorable again or when they pass by the barrier.     

Simulating the Oceanic Migration of Silver Japanese Eels

The oceanic migration of silver Japanese eels starts from their continental growth habitats in East Asia and ends at the spawning area near the West Mariana Ridge seamount chain. However, the actual migration routes remain unknown. In this study, we examined the possible oceanic migration routes and strategies of silver Japanese eels using a particle tracking method in which virtual eels (v-eels) were programmed to move vertically and horizontally in an ocean circulation model (Japan Coastal Ocean Predictability Experiment 2, JCOPE2). Four horizontal swimming strategies were tested: random heading, true navigation (readjusted heading), orientation toward the spawning area (fixed heading), and swimming against the Kuroshio. We found that all strategies, except random swimming, allowed v-eels swimming at 0.65 m s⁻¹ to reach the spawning area within eight months after their departure from the south coast of Japan (end of the spawning season). The estimated minimum swimming speed required to reach the area spawning within eight months was 0.1 m s⁻¹ for true navigation, 0.12 m s⁻¹ for constant compass heading, and 0.35 m s⁻¹ for swimming against the Kuroshio. The lowest swimming speed estimated from tracked Japanese eels at sea was 0.03 m.s⁻¹, which would not allow them to reach the spawning area within eight months, through any of the tested orientation strategies. Our numerical experiments also showed that ocean circulation significantly affected the migration of Japanese v-eels. A strong Kuroshio could advect v-eels further eastward. In addition, western Pacific ocean currents accelerated the migration of navigating v-eels. The migration duration was shortened in years with a stronger southward flow, contributed by a stronger recirculation south of Japan, an enhanced subtropical gyre, or a higher southward Kuroshio velocity.     

The Anguilla spp. migration problem: 40 million years of evolution and two millennia of speculation

Anguillid eels Anguilla spp. evolved between 20 and 40 million years ago and possess a number of remarkable migratory traits that have fascinated scientists for millennia. Despite centuries of effort, the spawning areas and migrations are known only for a few species. Even for these species, information on migratory behaviour is remarkably sketchy. The latest knowledge on the requirements for successful migration and field data on the migrations of adults and larvae are presented, how experiments on swimming efficiency have progressed the understanding of migration are highlighted and the challenges of swimming at depth considered. The decline of Anguilla spp. across the world is an ongoing concern for fisheries and environmental managers. New developments in the knowledge of eel migration will, in addition to solving a centuries old mystery, probably help to identify how this decline might be halted or even reversed.     

Glycolytic fluxes in European silver eel, Anguilla anguilla: sex differences and temperature sensitivity

European silver eels migrate 6000 km to their supposed spawning area in the Sargasso sea. As the eel is fasting, this intense swimming activity is realised only with fat stores, involving mainly red muscle i.e. aerobic metabolism. However, eel migration is performed at depth and thus in cold water, both being known to induce changes in muscle energy metabolism. During migration, white and red muscles can operate together or separately in order to counteract the eventual effects of low temperatures and/or high pressures. We have studied the temperature sensitivity (5, 15, and 25 °C) of aerobic and anaerobic metabolism in both sexes. At the same temperature, migrating eels have a higher basal glycolytic flux. Moreover, there are temperature and sex effects: anaerobic glycolysis (JB) is more sensitive to cold water whereas aerobic (JA) is more affected by warm. Males, which are less sensitive to cold water, also have higher aerobic fluxes than females. As depth corresponds to low temperature, the possibility that males migrate more deeply than females is discussed. In an ecophysiological context, it is interesting to suppose that males and female eels migrate at different depths in order to optimize their energy utilization by aerobic and / or anaerobic pathways.     

Hydrometric and Meteorological Factors Affecting the Seaward Migration of Silver eels (Anguilla anguilla, L.) in the Lower River Shannon

European eels, Anguilla anguilla, L., are captured in coghill nets at three commercial weirs on the Irish River Shannon during their autumn/winter seaward migration. The variations in daily silver eel catches were analysed in relation to environmental conditions, especially hydrometric and meteorological factors. Three multivariate environmental axes were distinguished with which daily eel catches could be correlated. The relative importance of various hydrometric, meteorological and temporal (seasonality, lunar phase) factors was identified, showing how hydrological and meteorological factors generally obscure the underlying lunar periodicity of the silver eel migrations at Killaloe.