- Muraena anguilla Linnaeus, 1758
IUCN: СR; Bo (II); RDBUkrCarp: NT
Value of species
Other Anguilla species have occasionally been stocked in Europe, but none have established a self-sustaining population. Pure A. rostrata (American Eel) have been recorded, likely due to stocking attempts, but are relatively rare (Böetius 1976). DNA analysis is the best tool to distinguish between European Eels and other species, and A. rostrata have fewer vertebrae than A. anguilla (102-112, usually 106-108, vs. 111-119, usually 114-116). Hybrids are known to have been found in Iceland where pure A. rostrata and A. anguilla also exist (Albert et al. 2006).
Anguilla anguilla has been shown to be distributed from North Cape in Northern Norway, southwards along the coast of Europe, all coasts of the Mediterranean and on the North African Coast (Schmidt 1909, Dekker 2003). It very rarely enters the White and Barents seas, but it has been recorded eastward to the Pechora River in northwest Russia. The species occurs in low abundance in the Black Sea where it migrates east to the Kuban drainage (occasional individuals reach the Volga drainage through canals), in northern Scandinavia and eastern Europe. A report by the ICES Study Group on Anguillid Eels in Saline Waters (SGAESAW) indicates that eel populations typically contain a mix of freshwater residents, saline water residents, and inter‐habitat migrants (ICES/SGAESAW 2009). It also widely occurs in most inland waters of Europe (e.g. lakes). It is thought that the continental distribution of the European Eel is over an area of approximately 90,000 km² in Europe and parts of North Africa (Moriarty and Dekker 1997), with a substantially larger range if their marine distribution is considered. For example, in England and Wales, there are thought to be a total of 2,694 km² of transitional waters, which account for approximately 68% of the potential eel producing habitat across all 11 River Basin Districts (Defra 2010).
For several decades prior to an EU-wide ban on export in 2010, A. anguilla was also exported to Asia for seed stock in eel farms (Ringuet et al. 2002). This species may well have been introduced in some parts of Asia (through escape or release from farms), however these are not thought to contribute to the population and therefore areas of introduction have been excluded in the range information. Anguilla anguilla are thought to spawn in the Sargasso Sea in the West Central Atlantic between late winter and early spring, before eggs hatch and leptocephalus larvae migrate back across the Atlantic to begin the continental phase of their life history (Schmidt 1912, Aarestrup et al. 2009).
A subset of the recruitment index data in the ICES WGEEL 2012 report was used for analysis i.e., data collected as part of fisheries independent scientific monitoring or fisheries dependent data with an associated metric of effort (e.g. catch per unit effort (CPUE)). Other data were kindly provided by Dr Brian Knights, and this was included as part of the assessment process. Catch effort can be variable in fishing data, and under-reporting and, in some cases, an absence of reporting of landings is a serious problem in most European countries. Thus landings data cannot be accepted as a precise measure of stock status. However, trends in the reported catch data will, to some extent, reflect true changes in fishing yields. All the data were analysed to assess trends in recruitment, population and spawner escapement; however, it was primarily used to guide discussions that resulted in the broad agreement of the Critically Endangered listing by the assessment team, which was in turn supported by the majority of reviewers.
There are more data available for A. anguilla in northern, central and southern European countries compared to North Africa, however there is considerably less freshwater habitat available to the European Eel in this part of its range. For analytical purposes the North Sea subpopulation is often referred to in separate terms to the rest of Europe by ICES. This is because the decline in abundance of A. anguilla has been shown to be substantially greater for this area compared to anywhere else (ICES WGEEL 2012), although this declining trend can be mostly accounted for during the period between 1980 and 1985.
Determining changes in the international stock in anguillid eels is difficult due to limited data and the poor understanding of the relationship between recruitment, freshwater populations, and escapement. Not only is there a huge time lag between the recruitment of glass eels to fresh and brackish water and the subsequent escapement of silver eels, but given that A. anguilla are panmictic, escapement from one area does not translate directly into returning larval recruitment at the same locality. Indeed for all intents and purposes it is assumed that practically nothing is known about the dynamics of the oceanic phase of A. anguilla (ICES WGEEL 2013). It has been proposed that due to the relatively short time-span between spawning and recruitment that the latter is a good indicator of the past spawning stock that produced the juvenile cohort; this will depend, to an extent, on the significance of oceanic factors on larval transport.
Assessment of these datasets using the IUCN Red List Categories and Criteria took into account, where this information was available, consistency of sampling; longevity of the data set; whether they were eel-specific or multi-species; whether the collection methods were active or passive; whether the watershed the data related to was subject to restocking activity; and/or whether, in the case of fisheries independent data, there was exploitation in the region.
Habitat: The species is found in a range of habitats from small streams to large rivers and lakes, and in estuaries, lagoons and coastal waters. Under natural conditions, it only occurs in water bodies that are connected to the sea; it is stocked elsewhere.
Ecology: The species is facultatively catadromous, living in fresh, brackish and coastal waters but migrating to pelagic marine waters to breed. While there is some understanding of the eel’s continental life history, relatively little is known about its marine phase. The migrations in the European Eel’s life cycle are the longest and most oceanographically complex of the anguillid species (Tsukamoto et al. 2002). There are a number of phases in an eel’s life that have specific terminology; the leaf-shaped marine larval stage is referred to as leptocephalus; these become glass eels as they reach brackish water, before developing into the pigmented, growth phase: the yellow eel. The final stage is the marine-migratory silver eel which is characterised by silvery counter-shading and large eyes.
There are no exact data about specific spawning sites, however, it is proposed that spawning takes place in an elliptic zone, about 2,000 km wide in the Sargasso Sea, in the West Central Atlantic (about 26°N 60°W). Survey catches of leptocephalus larvae suggest that spawning peaks at the beginning of March continuing through until July (McCleave 1993). The adults are assumed to die after spawning. Oceanic migration of leptocephali is estimated to take about two years on average before they arrive at the continental shelf (Bonhommeau et al. 2008, Zenimoto et al. 2011). The mechanisms by which leptocephali reach the European and N. African coasts are also not well understood. The main migrations occur in late-autumn to early-spring in Iberian and Bay of Biscay waters, and they are delayed in more northerly sites until temperatures rise in the spring. By the time the leptocephali reach the continental slope they are as large as 100 mm in size and metamorphose into glass eels which are elongate and have a transparent body. These glass eels are observed in the summer and autumn on Portuguese coasts, and in winter and spring in the North Sea.
Glass eels enter freshwater as sexually undifferentiated individuals. Development and differentiation of the sexual organs are thought to be closely correlated with body size and associated with the yellowing phase of the eels life history. Sex determination is principally driven by environmental factors with density dependence producing more males at high densities (Davey and Jellyman 2005). Male European Eels initially grow faster than females, however, females achieve a greater age and size than males when sexually mature. Furthermore, the mean length increases significantly with latitude in females but not males, whereas age increases significantly in both (Durif et al. 2009, M. Aprahamian unpub. data). Male fitness is maximised by maturing at the smallest size that allows a successful spawning migration (a time minimising strategy) such that males tend to emigrate at a length of <450 mm. Conversely, females adopt a more flexible size-maximising strategy prior to migration that trades off pre-reproductive mortality against fecundity (Davey and Jellyman 2005). There is considerable geographic variation in mean length at metamorphosis of male and female European Eels (Vøllestad 1992). Dekker et al. (1998) produced a paper describing the extreme sizes in each of the life stages of the European Eel from data at a long term capture locality in the Netherlands (Sizes (cm): Min – Max, Glass eels: 5.4 – 9.2, Yellow: 6.9 – 133.0, Silver (M): 21.2 – 44.4, Silver (F): 26.4 – 101.0). Driven by density dependence, there are often skewed sex ratios at individual localities as well as geographic bias associated with latitude.
Eel growth increases with temperature and growth rate is generally faster in saline water than fresh. Furthermore those individuals produced in saline waters usually contain lower loads of the swim bladder parasite, Anguillicola crassus and thus may have improved chances of reaching their spawning grounds (ICES/SGAESAW 2009). During maturation, dependent on size, European Eels feed off a variety of organisms including fish, amphipods and decapod crustaceans. In saline muddy-bottomed habitats eels forage on bivalves, shrimp and polychaete worms.
The age at which silver eels mature and undertake their spawning migration is hugely variable and dependent on latitude and temperature of the environment in which they have grown, physical barriers that block migration routes, growth rate and sex differences. From the data available, lower bound estimates for average length of the continental growth phase are approximately 3-8 years for males and 4-5 years for females and upper bound estimates approximately 12-15 years for males and 18-20 for females (Acou et al. 2003, Froese and Pauly 2005, Durif et al. 2009). Assessment of available data on generation length during the IUCN Red List process highlighted that defining a single figure for species such as eels was extremely difficult. Factors that can significantly affect this parameter include longitude and latitude, sex and habitat quality. Fifteen years was agreed upon after this analysis and it is important to indicate that this is inclusive of an estimated two year larval migration and 0.5 year spawning migration of silver eels.
- Соколов Н.Ю. Каталог колекції круглоротих і риб Державного природознавчого музею НАН України // Наукові записки Державного природознавчого музею. – Львів, 2004. – Т.19. – С. 15-28.