"Considering these points, I perceived that if the problem were to be solved in anything like a satisfactory manner it would be necessary to ascertain, not only where the youngest larvae were to be found, but also where they were not."

Johannes Schmidt, 1922.

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Porbeagle sharks have a taste for American eels off the coast of Canada

Eight silver eels (American eels) were tagged with minaturized pop-up satellite archival transmitters (PSATs) to document the migratory pathways and environmental conditions encountered as they migrated from St. Lawrence River, Québec, Canada, to the Sargasso Sea. Of the seven which successfully transmitted archived data, six were ingested by warm-blooded predators. Depth profile characteristics and gut temperatures suggested that predation of the silver-stage American eels was by porbeagle sharks. Furthermore, this indicates that, although the tag itself could increase the eels’ susceptibility to predation, eels may represent a reliable, predictable food resource for porbeagle sharks.

Béguer-Pon M, et al., 2012. Shark predation on migrating adult American eels (Anguilla rostrata) in the Gulf of St. Lawrence. PLoS ONE, 7(10): e46830.

Iso-eugenol is a promising stress-reducing anaesthetic for the European eel

Studies showed that for use in common ecology-related procedures, such as capture, tagging and size measuring, iso-eugenol (Aqui-S vet.) is a promising stress-reducing anaesthetic; improving animal welfare and survivability when used properly. The recovery and time to anaesthesia using both Aqui-S vet., a true anaesthetic, and metomidate, a hypnotic agent with no analgesia, were of no difference; yet, metomidate is a potential stressor to European eels during exposure and should not be recommended for use with this species.

Iversen, M. H., et al., 2012. The efficacy of Aqui-S vet. (iso-eugenol) and metomidate as anaesthetics in European eel (Anguilla anguilla L.), and their effects on animal welfare and primary and secondary stress responses. Aquaculture Research, doi: 10.1111/j.1365-2109.2012.03140.x.

A book on the story of eels is reviewed by a leading eeliad scientist

The book entitled "Eels: An Exploration, from New Zealand to the Sargasso, of the World's Most Mysterious Fish" by James Prosek is reviewed

Aarestrup, K., 2010. The mystery of eels. Nature, 468, pp:894-5.

Parasitic infection of European eels can occur in pristine environments

Studies performed in Río Esva, flowing towards the Cantabrian Sea in the north-west of Spain, show that eels in good condition are still at high risk of infection to the parasitic nematode Anguillicoloides crassus (A. Crassus). Due to there being no human influence on the river, such as fish stocking, farming, engineering, obstructions or pollution, the condition of the wild stock was much better than in studies elsewhere yet the prevalence of A. crassus was still equal to or more than 50% in certain seasons of the year. The initial colonisation of the infection can only be speculated. Infection in the uppermost reaches was practically absent however as the stocks recover infected individuals may migrate upstream carrying A. crassus with them.

Costa-Dias, S., et al., 2010. Infection by Anguillicoloides crassus in a riverine stock of European eel, Anguilla anguilla. Fisheries Management and Ecology, 17(6), pp:485-92

Transnational management plans are crucial to conserving the panmictic European eel population

A stringent survey of a representative and highly-inclusive sample of the glass eel population in continental waters was performed and no evidence for geographic or temporal genetic differences was found. An insignificant genetic differentiation between the Sargasso Sea eel larvae and continental glass eels was also determined. With the unlikelihood of an unsampled population existing, the study showed that eels from the whole distributional range from Northern Europe to North Africa could be accounted for by spawning in the Sargasso Sea and that the species is truly panamictic (randomly mating within a single species). With the existence of a distinct marine hybrid zone being disputed, a location where American and European eels are able to hybridize, it stresses the need for transnational management plans to conserve the glass eel populations; as over-exploitation in one region will negatively affect the total population across the whole distribution range.

Als, T.D., et al., 2011. All roads lead to home: panmaxia of European eel in the Sargasso Sea. Molecular Ecology, 20(7), pp:1333-46.

Density-dependent mortality indicates a saturation threshold for male-dominated river stocks

Whilst carrying capacity is a parameter important for population management, evidence for habitat limitations is lacking. Samples in a small coastal catchment in France, confirmed the hypothesis that the eels can effectively estimate habitat suitability and distribute themselves, in space and time, accordingly. In the Frémur, where this research took place, the findings suggest that eel habitats are saturated and the observed mean abundance could be used as a threshold value for similar small, low gradient streams on the north-Atlantic coast of Europe where river stocks are male-dominated; and can be used to derive models which more-realistically feature density-dependent processes. This research indicates that habitat restoration schemes upstream, even where eel passes are put in place to allow migration beyond physical obstructions, could increase the productivity and yield of eels. The findings also suggest that without knowledge of a carrying capacity, a proportion of re-stocked sub-populations, re-stocked as part of programmes adopted to increase the silver eel populations, may be lost due to density-dependent mortality.

Acou, A., et al., 2010. Habitat carrying capacity is reached for the European eel in a small coastal catchment: evidence and implications for managing eel stocks. Freshwater Biology, 5, pp:952-68

Is there more than one spawning site in the Sargasso Sea?

It is understood that eels are born at a single spawning site, the Sargasso Sea, and randomly migrate to fill the continental distribution range of the species.

Genetic studies show no evidence for a geographical subdivision of glass eels. Changes in the chemical (isotopic) composition of part of the inner ear (the otolith) of a number of eels and elvers, collected from eleven different study sites over three years, showed that they were all born at a single spawning site. 20-30 days after hatching, the chemical fingerprint of the otolith was compared to adult eels and a difference was found. These differences could be caused by a change in water composition and temperature, which tends to vary only slightly within the Sargasso Sea (widely-accepted as the only location where spawning occurs), as well as diet. The implications of these finding require further research to determine the cause of the observed change in chemical signature.

Martin, J., et al., 2010, An otolith microchemistry study of possible relationships between the origins of leptocephali of European eels in the Sargasso Sea and the continental destinations and relative migration success of glass eels. Ecology of Freshwater Fish, 19, pp:627-37.

Fishing may be a major cause of observed mortality in the early phase of marine migration

Fifty large, female eels, migrating in the lower part of the River Gudenaa and during the first phase of the marine migration in the Rangers Fjord in Denmark, were equipped with acoustic transmitters; used to monitor their progression rate and mortality. 60% were lost in the inner and outer fjord but, of those who survived, progression rate increased as they proceeded downriver and out of the fjord; where migration was predominantly nocturnal. The study also indicated that individuals at a lesser speed or larger size were not more prone to fishing mortality.

The escapement goals of silver eels out of foraging areas set by the recent EU recovery plan of 40%, in this study, was not met, with only 38% survival in the lower river and fjord which is combined with the additional pressure of a 23% survival rate relating to hydropower production in the middle part of the river.

Aarestrup, K., et al., 2010. Survival and progression rates of large European silver eel Anguilla anguilla in late freshwater and early marine phases. Aquat Biol, 9, pp:263-70.

The early eel is at an advantage for upstream migration

As the migration season progresses, so does the pigmentation of European eels. Individuals with more pigmentation were of lesser mass and condition. As larger eels in better physical condition may boast greater energy content, it is implied that those migration at an earlier stage within the migratory season will be more successful in upstream migration than their later counterparts.

Iglesias, T., et al., 2010. Variation of life traits of glass eels of Anguilla anguilla (L.) during the colonization of Ríos Nalón and Minho estuaries (northwestern Iberian Peninsula). Hydrobiologia, 651(1), pp:213-23(11).

Modelling exploitation rates using abundance indicators

Minimising human-induced impacts on eel fisheries and habitats is crucial to restore eel abundance to the level observed during the seventies. The INDICANG project aimed to elaborate abundance indicators of the European eel in the central part of its distribution area. Research performed in the Adour estuary, south-western France, have developed a method to establish daily and seasonal density (biomass) estimations in the water column during various flood tides, characterised by various environmental (hydrodynamic) conditions. The relationship between the fishing (exploitation) rate, catches and hydrodynamic conditions allows calculations of exploitation rates and nocturnal (night-time) biomass fluctuations during the fishing season. Understanding rates, and indicators, of exploitation can contribute to the development of more sustainable management plans aiming to restore eel abundance.

Bru, N., et al., 2009. Daily and seasonal estimates of the recruitment and biomass of glass eels runs (Anguilla anguilla) and exploitation rates in the Adour open estuary (Southwestern France). Aquat. Living Resour., 22(4), pp:509–23

Spawning migration passage highlighted by minaturized pop-up satellite archival transmitters

Minaturized pop-up satellite archival transmitters (PSATs) have been utilised to track the spawning migration pattern of 14 tagged silver eels released on the west coast of Ireland in October and November 2006. Of the ~5000 km route from Europe to the Sargasso Sea, the eels were tracked for 1300 km. With a mean migration speed of 13.8 km per day, where a speed of 35 km per day would be required to reach the Sargasso Sea in time for spawning in April, the research agreed with the hypothesis that eels gain speed and increase travel efficiency as the join the subtropical gyre system to the Caribbean. The PSATs also revealed that they undertook daily vertical migrations between depths of 200 m to 1000 m; occupying shallow, warm water at night and dived steeply into cooler, deeper waters at dawn. This process allows organisms to avoid predators during the day and maximise feeding at night; however, as some research suggests, eels do not feed during the spawning migration and thus this behaviour may be performed to regulate their body temperatures.

Aarestrup, K., et al. 2009, Oceanic spawning migration of the European Eel (Anguilla anguilla). Science, 325, pp:1660.

Estimates of migration speed calculated from swimming behaviour and environmental conditions

Observations of characteristic swimming behaviour have been studies to find that individuals migrate passively, behind the dynamic tide front, with the flood tide current. Their moments are also linked to the surrounding water turbidity and luminosity. Modellers have combined this information with physical data, including river flow, cloud cover, moon phases and tide alternation between day and night and have been able to calculate the migration speed of a group of glass eels within the Adour estuary, south-western France. This research can then allow a better understanding of what environmental (hydroclimatic) conditions could stop or slow down their migration into the estuary.

Prouzet, P., 2009. Analysis and visualization of the glass eel behaviour (Anguilla anguilla) in the Adour estuary and estimate of its upstream migration speed. Aquat. Living Resour., 22(4), pp:525-34.

No genetic difference between maturing adults from southern and northern Europe

Despite previous studies showing the small but statistically significant differences in the genetic structure of the European eel, this research found that, using a large sample size (1210 individuals), no signs of genetic differences between eels from Italy and the Baltic Sea were observed.

Palm, S., et al., 2009, Panmixia in European eel revisited: no genetic difference between maturing adults from southern and northern Europe. Heredity, 103, pp:82-9.

Response to Palm, S., et al., 2009: Vasemägi, A., 2009. Eel mystery: time makes a difference. Heredity, doi:10.1038/hdy.2009.38.

Lowestoft scientists research future of eels, Evening News 24

A £2.5m Europe-wide research programme is under way to find out more about the breeding habits of eels and slow their decline.

The Centre for Environment, Fisheries and Aquaculture Science (Cefas) in Lowestoft has been taking part in the project, launched in April, to monitor eels as they head for breeding grounds 5,500km away in the Sargasso Sea, south of Bermuda. Scientists from Cefas and the Environment Agency have spent the year tagging elvers and eels with five-inch satellite transmitters which float away after the fish dies and will then be returned to the Pakefield-based laboratories for analysis. They spent July and August tracking yellow eels near Poole, in Dorset, and scientists across Europe have also been involved in the project, from trapping silver eels in Ireland to monitoring the migration of eels in Spanish river basins. A spokesman for Cefas said that all of the tagging has been completed for this year and that findings should be published in spring 2009. The number of eels being recorded has fallen by about 95% in the last 20 years so it is hoped that the results of the research will provide information about migration routes and the physical conditions which the eels experience on their epic journey to breed.

Feeding activity in eels can be dependent on food availability and not just seasonal variability

Research performed in the Río LLorín, a coastal stream flowing towards the Cantabrian Sea in the north-west of Spain, showed that eels remain active all year round at day and night, with a slight increase in feeding activity and intensity in warmer months.

Previous studies at other locations have suggested that European eels cease feeding in autumn and winter; actively and intensively feeding during the late spring and summer with a predominantly nocturnal feeding pattern. This disagreement in findings could be due to the combined effects of mild temperatures (less temporal variability within the region than those previously studied) and greater food availability. Another study, performed locally, noted the observation of continual feeding and concluded that this may be characteristic of smaller eels where larger eels exhibit nocturnal feeding activity.

Costa-Dias, S. and Lobón-Cerviá, 2008. Diel feeding activity and intensity in the European eel Anguilla anguilla (L.) during an annual cycle in a Cantabrian stream. Knowledge and Management of Aquatic Ecosystems, pp:390-1(1).

In-stream abundance of glass eels can increase when estuarine abundance persists to decline

Observational studies using a 21-year data set in Río Esva, flowing towards the Cantabrian Sea in the north-west of Spain, showed a simultaneous decline of estuarine and in-stream densities of glass eels over a 15-year period. After 2000, the abundance of glass eels reached a historical minimum and since the estuarine abundance has continued to decline whereas in-stream densities began an “increase that has continued to the present”. This response is speculated to be due to compensatory changes responding to density variations, due to persistently low glass eel abundance, supporting the observed increase in in-stream density.

Lobón-Cerviá, J. and Iglesias, T., 2008, Long-term numerical changes and regulation in a river stock of European eel Anguilla anguilla. Freshwater Biology, 53(9), pp:1832-44.

line drawing of an eel