Eels play an important role in freshwater ecosystems as indiscriminate predators of a wide variety of prey items as scavengers, and as the preferred prey items of otters and piscivorous birds.

A future decline of the UK's freshwater population will, therefore, have a likely detrimental impact on ecosystems and on the conservation status of otters and numerous piscivorous birds, including great cormorants, Phalcrocorax carbo, and the grey heron, Ardea cinerea.
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Ecology & conservation
Despite their current decline, eels play a major part in marine and freshwater ecosystems, both as predators and prey species, as demonstrated by the following excerpt:

The ecological importance of eels and elvers in Highland waters
The West Sutherland Elver Survey(1) reports:
" ...Eels used to represent more than 50% of the standing fish biomass in most European aquatic environments, participating significantly in the food webs and contributing to the functioning of a wide extent of continental and inland hydrosystems ... Due to the eels' multiple interactions with the surrounding ecosystem eels are considered to be an important nutrient flux to and from the sea and a keystone species, maintaining a balanced riverine ecology, both as a predator/scavenger and prey species.

As predators, eels become increasingly piscivorous with increasing length. Most research on the diet of eels has been in nutrient rich basic habitats and concluded that eels <300mm fed mostly on Salmonid ova and parr, Trichoptera sp. larvae,
Simulium sp. larvae, Asellus sp. nymphs, Ephemeroptera, Gammarus pulex, and Diptera larvae, and eels >300mm fed on Gammarus, Asellus, crayfish and fish (Sculpin and Stickleback). Comparisons of eel diets with those of juvenile salmon Salmo salar and trout S. trutta showed that eels preyed more on benthic invertebrates, whereas the salmonids took more mid-water and surface prey. Mann & Blackburn (1991) concluded that in chalk rich salmonid nursery streams, eels do not have a measurable effect on the salmonid population through predation or by utilizing the same food source. However, there is no research on the feeding habits of in acidic, nutrient poor waters such as the highlands, where eels may tend to eat more fish due to lower number of large invertebrates.

Eels as a prey species are an important component of the diet of species listed in the UKBAP and SBAP, such as Otters, Bittern, Osprey and Herons, which preferentially select eels over salmonids due to the higher fat content of eels. In addition, elvers provide a significant source of food for many other species in the spring, at a time when demands of the breeding season are at their height. It is important, therefore, to ensure that eel populations are sufficient to sustain predation by these species".


Decline and re-stocking
It is well-known that eel species worldwide are declining. While fishing (both illegal and legal) undoubtedly plays a major part, the full reasons for their decline are a complex mosaic of different and inter-related factors, not all of which are fully understood. Apart from fishing, these reasons include:

Estimated trends in recruitment and landings of the  
European eel,showing the downward trend in recruitment  
© Willem Dekker, Netherlands Inst. for Fisheries Research  
Re-stocking of catchments using farmed eels (either as glass eels or as 'bootlace' eels) has been carried out frequently across Europe since the nineteenth century, or even earlier. In some European countries, the level of re-stocking has exceeded the natural level of recruitment, although increased prices, lower catches and higher demands for aquaculture have occasionally reduced the levels of re-stocking. It should be remembered that every eel used for re-stocking has been captured from the wild.

Re-stocking is a cost-effective means of restoring or maintaining fishery yields (such as those in Lough Erne and Lough Neagh, which may depend entirely on it), and an important contribution to waters with restricted or blocked access for upstream migration. While in general it is beneficial in augmenting natural stocks on a river or catchment, there are a number of concerns over the effects of re-stocking, particularly when eels are moved from one catchment, or even one country, to another(5): Disease
The most prominent disease to affect eels is Anguillicoloides crassus, a parasitic nematode which infects the fish's swimbladder. Anguillicoloides is parasitic on the Japanese eel, Anguilla japonica, in South-east Asia, and is thought to have been introduced from there to Europe in the 1980s, with the importation of Japanese eels for aquaculture. Partly because of its extremely versatile life cycle, and partly through the prolific trade in live eels, it has spread rapidly in Europe, and an estimated 90% of eel populations are affected by it.


Anguillicoloides crassus
© SERTC
The life cycle of Anguillicoloides starts when the adult nematode releases thousands of eggs in the host eel's swimbladder; these then pass through the digestive system and are release as larvae into the water, settling onto the substrate. They are ingested by their intermediate host - usually a copepod, sometimes a fish. Here the larva reaches its infective stage. The host is eaten by an eel, and the nematode finds its way from the eel's digestive tract to its swimbladder. Once an eel is infected, the parasite attaches to the lining of the swimbladder and draws blood from its host. The infection causes damage to the swimbladder, resulting in partial or complete loss of its vital functions. In addition, it can reduce the metabolic efficiency of the eels, lowering its swimming endurance and its ability to accumulate and store fat during its stay in fresh water. All of this reduces the likelihood of eels' successful journey to their spawning grounds, and their chances of successful reproduction when they get there(2).

Pollution
Eels are especially vulnerable to the effects of chemical pollution in their habitat, particularly heavy metals, polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and brominated flame retardants (BFRs). This is primarily because the species is long-lived, dwelling at the bottom of lakes and rivers, and carnivorous. In addition, once a yellow eel has completed its upstream migration, it tends to remain in the same locality for a long time, perhaps even decades. Their position towards the top of the food chain means they bio-accumulate pollutants present in their food sources; they can also absorb high levels of lipophilic (fat-loving) chemicals through their gills and skin. Because eels' flesh has a high fat content, lipophilic chemicals such as PCBs, OCPs and BFRs tend to be absorbed and stored in the eels' bodies. As they don't have a seasonal reproductive cycle, and do not migrate for years, if not decades, their sub-adult stage does not usually metabolise body fats, and so they continue to bioaccumulate pollutants, whose levels increase with age, and reach a maximum prior to their sexual maturity and downstream migration.

Different classes of pollutants, as one might expect, elicit different physiological effects. The table below summarises a few of these:

Pollutants Potential physiological effects
Polychlorinated biphenyls (PCBs)
  • Histopathological changes in spleen, liver & kidney
  • Disruption of enzymes responsible for detoxification
  • Disturbance of hormonal regulatory systems governing metabolism, growth, reproduction and osmoregulation
  • Immunosuppressive effects facilitating parasite infection
  • Disruption of lipid storage and metabolism
  • DNA damage leading to reduced fertility
Pesticides
(organochlorines, organophosphates, DDT, pentachlorophenol)
  • Carcinogenesis
  • Liver toxicity, changes in function, and enlargement
  • Damage to gills, reducing their ability to absorb oxygen
  • Hyperglycaemia and disruption to metabolism
  • Increased free radical levels, leading to cell damage
  • Disturbance to lipid storage
  • Direct toxicity to nervous system
  • Abnormal ovarian and egge development
Heavy metals
(Copper, mercury, cadmium, lead, chromium, zinc, nickel, arsenic, selenium)
  • Immunosuppression
  • Gill, liver and kidney damage
  • Changes in enzyme function
  • Disturbance of the osmoregulatory mechanism
  • Increased free radical levels, leading to cell damage
  • Reduction in lipid storage and general condition
  • Reduced genetic variability
Brominated flame retardants (BFRs)
  • Acute direct toxicity
  • Neurological effects
  • Liver damage
  • Reproductive toxicity
  • Hormonal disruption
Perfluorooctane sulfonic acid (PFOS)
  • Liver damage
  • Disrupted lipid metabolism
Polyaromatic hydrocarbons (PAHs)
  • Carcinogenesis
  • DNA and cell damage
  • Mutation

This tendency to absorb and store contaminants, combined with their general lack of movement, means that eels can be used as an indicator of pollutant levels in the environment, and can even be used to pinpoint individual sources of pollution at very specific locations. Unfortunately, their effectiveness at bio-accumulation of toxins can detrimentally affect the eels' health, either through acute toxicity or through the long-term effects of pollutants. These can include, as shown in the table above, a wide range of effects including decrease in growth rate, immunodeficiency, and a reduction in body condition and fat reserves, decreasing the chances of successful migration.

The induced changes in fat reserves are perhaps crucial to understanding the eels' problem. Claude Belpaire(3) reports:
"Lipid reserves are essential to cover energetic requirements for migration and reproduction. Two large and independent data sets from Belgium and The Netherlands show a one-third decrease in fat contents of yellow eels over the past 15 years ... On the basis of the somatic energy reserves, reproductive potential of female eels from various latitudes were estimated, indicating the poor status of eels throughout Europe. Only large individuals, females as well as males, with high lipid content seem to be able to contribute to the spawning stock. The decrease in fat content may be a key element in the stock decline and raises serious concerns about the chances of the stock to recover."

As well as direct physical effects, high levels of pollutants reduce genetic variability, and affect embryonic development, hatching or growth. Additionally, of course, the accumulation of toxic substances can constitute a risk for human health from the consumption of eel - particularly from extreme toxins such as PCBs. They are known to bioaccumulate in the human body, just as they do in eels, and once absorbed into the body can be extremely persistent. Their physiological effects include acute direct toxicity, persistent acne, immune system damage, thyroid disorder, hormonal disruption, neurotoxicity and carcinogenesis. They can also be transmitted to infants via breast-feeding.

In a study in Flanders, in 80% of all sampled localities, the Belgian PCB standard for fish was exceeded. The intake of PCBs from fishermen's consumption of their own eel catches was shown to be at a level of high concern(3).

Climate change
While the early stages of eel larvae are still mysterious, their survival and development is dependent upon food availability and quality. There is evidence that changes in climate have affected primary production - the production of organic compounds from atmospheric or aquatic carbon dioxide; in the oceans mostly carried out by microscopic plankton. The effects of sea temperature and current changes are still poorly understood, but it is clear that they can and do have profound effects on the global production of oceanic phytoplankton, which in turn are correlated with the declines in recruitment of populations of the European, American and Japanese eels (Anguilla anguilla, A. rostrata and A. japonica)(4).

Habitat loss and alteration
"Loss of habitat" encompasses quite a range of problems, including the destruction of the habitat itself, through development or road-building, pollution, natural events, and the blocking of access to headwaters of rivers by dams and weirs.

For centuries, rivers and other aquatic systems have been heavily modified by man. Development of agriculture, navigation, industry and urban areas has affected wetlands and river channels; in some European countries over 50% of original wetlands have been lost. Moreover, river channels have been dredged, widened, straightened or canalised to accommodate shipping, power generation and other industrial uses, and for flood control; all these reduce the suitability of the habitat for eels. Marshland has been converted to agriculture, or historical drainage and regulation practices have been discontinued.

The upstream migration of glass eels and elvers can be blocked by dams, weirs, and other structures, restricting or entirely preventing their access to headwaters. If eels' progress is not entirely cut off, dams can modify habitat quality, by converting shallow running waters into deep eutrophic habitats, and generally act as a delaying influence on migration. These conditions are assumed to increase mortality. While eel passes do exist - and are increasingly being fitted on new constructions as a legal requirement - most European river systems remain highly obstructed. (5).

Water abstraction
Water abstraction, for industrial cooling water, drinking water supply or irrigation, also takes its share of migrating eels, both up- and down-stream. Coastal power stations, which may abstract as much as 50 cubic metres of cooling water per second, 24 hours a day, catch numbers of glass eels, elvers, and adult eels, which vary with the seasons, tidal cycle, and other factors. Many eels thus caught will die, unless the station has a system for returning captured fish to the sea. Even then, injuries sustained dring the process, through mechanical damage, stress, exhaustion or extreme changes in pressure, may later result in death, either as a direct result, or through infection, or predation of weakened and confused individuals. Other abstraction sites, for instance for public water supply or irrigation of crops, also result in eel mortality, both in upstream and downstream migration.

On their downstream journey, eel mortality in hydroelectric turbines can be considerable. Death caused by passage through a turbine does vary according to the design and layout of the turbine (on average, 28%, according to one study), but in general long thin fish like eels are more vulnerable than other fish. Furthermore, many rivers feature a number of hydroelectric stations along their length, so the cumulative mortality rate along the entire length of a river can be high.

While dams and weirs occasionally have eel passes installed to allow upstream migration of glass eels and elvers, most have not been designed to allow downstream migration. Downstream passage from a dammed reservoir is often only available through a bypass tube, which can have a very high mortality rate (100% in one case study in northern France). If eels are not killed, their downstream migration may also be delayed for several months by reservoirs, until high water levels allow overflowing of the dam(6).

In short, dams and turbines may result in considerable eel mortality and obstruction to their migration, and thus reduce silver eel escapement considerably. The overall impact is thought to be of the same order of magnitude as fishing exploitation. (5).

On a positive note, in Sweden a collaboration with the energy company Statkraft and other companies has been capturing and transporting migrating eels down-river, to get them past areas where they are in danger from power station turbine damage. Meanwhile, on the River Weser in Germany, Statkraft have been employing a much more high-tech approach. A collection of eels, fitted with transponder devices, is held in a series of facilities, known as Migromats, fed by water from the Weser river. The transponders can detect when the eels exhibit behaviour associated with their downstream migration. When such activity is observed, it is assumed that the other eels living in the river are also preparing to move downstream. Triggered by this, at dusk when the eels begin moving, the turbines of the power plants downstream are optimised to maximise eel-friendliness: the turbine nearest the bank is switched off (to force the eels out into mid-river), and the pitch of the blades on the other turbines is altered to the maximum setting, to decrease the likelihood of damaging the fish on their passage through the plant.

This operational policy causes a small loss (about 1.5%) of power production overall, and obviously is dependent on a suitable power plant layout, but it represents an excellent example of how industry can operate in an environmentally sensitive way. More details are shown in the Statkraft 2011 newsletter.



Sources
(1). West Sutherland Fisheries Trust - West Sutherland Elver Survey
(2). Eeliad - Anguillicoloides crassus: The 'deadly' nematode?
(3). Claude Belpaire - Pollution in eel. A cause of their decline?. Instituut voor Natuur - en Bosonderzoek - INBO (2008). (PDF - 86 MB)
(4). Sylvain Bonhommeau, et al., - Impact of climate on eel populations of the Northern Hemisphere.
(5). European Inland Fisheries Advisory Commission / ICES, - Report of the thirteenth session of the Joint EIFAC/ICES Working Group on Eels. Copenhagen, Denmark, 28-31 August 2001.
(6). Acou, Anthony, Laffaille, Pascal and Legault, Antoine, - Migration pattern of silver eel (Anguilla anguilla, L.) in an obstructed river system. (2008) Ecology of Freshwater Fish, Vol. 17 (n° 3). pp. 432-442. ISSN 1600-0633



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