Validation of annulus in otolith and estimation of growth rate for Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> in tropical southern Taiwan

The age of Japanese eels (Anguilla japonica) is often estimated from otoliths, but this method has not been fully validated, particularly in tropical areas where the annulus in otolith is considered to be less distinct than in temperate areas. To validate the annuli in Japanese eel otoliths from southern Taiwan, known-age (2 year-old) cultured eels from an eel farm and wild eels from Kao-Ping River were collected. It was found that 26 out of 31 cultured eels (83.9%) showed two clear annuli and the remained 5 eels showed either one or three annuli. The mean (± SD) age of the cultured eels was 1.97 ± 0.4 years. Meanwhile, a clear peak in the mean monthly marginal increment ratio of the otolith in wild yellow and silver eels occurred once a year during winter (November to March). The annual deposition of presumed annuli in otoliths of Japanese eel was validated and the age and growth rate estimation for Japanese eels in the tropical southern Taiwan is deemed feasible. The growth rate of cultured eels was significantly faster than that of wild eels, but it did not differ significantly between sexes for wild silver, yellow or cultured eels. The von Bertalanffy Growth Function parameters (K, and t ₀ ) of the wild eels were estimated as 0.114 ± 0.028 year⁻¹, 1178 ± 171 mm and −0.8 ± 0.2 years, respectively.     

The changing times of Europe's largest remaining commercially harvested population of eel Anguilla anguilla L.

This study quantifies the processes involved in regulating the European eel population of Lough Neagh, a lake in Northern Ireland. The relationship between glass eel input and silver eel output for the 1923–1997 cohorts was best described by a Beverton–Holt stock recruitment model. Glass eel input time series was not complete and was thus derived from the relationship between catches elsewhere in Europe and Lough Neagh, together with the addition of stocked glass eel. Silver eel output was the sum of silver eel escapement, catch and yellow eel catch converted to silver eel equivalents. Natural mortality increased with glass eel density, ranging from 0.017 to 0.142 year⁻¹. The mean carrying capacity increased from ≈3.25 M silver eels (≈26 kg ha⁻¹) for the 1923–1943 cohorts to ≈5.0 M (≈40 kg ha⁻¹) for the 1948–1971 cohorts before regressing back to ≈3.25 M. The total silver eel output was highest during the late 1970s/early 1980s at 35–45 kg ha⁻¹ year⁻¹ and lowest during the early years of the 20th century and is currently at 10–15 kg ha⁻¹ year⁻¹. The findings are discussed in relation to (a) the ecological changes that have occurred within the lough, associated with eutrophication and the introduction of roach (Rutilus rutilus L.), and (b) the decline of the wider European eel stock across its distribution range. The findings from this study have relevance for the wider management of the European eel stock.     

Vital population statistics based on length frequency analysis of the exploited Japanese eel (Anguilla japonica) stock in the Kao‐Ping River,southern Taiwan

Vital statistics such as growth, mortality, and maturity parameters are crucial in understanding the population dynamics of a species. A total of 7 074 Japanese eels (Anguilla japonica) in the lower reach of the Kao‐Ping River, southern Taiwan, were collected with eel tubes in 1998 ～ 2004 and shrimp nets in 2004 ～ 2007. Data from 2004 were excluded due to mixed gear information and escapement of cultured eels; in subsequent years escaped cultured eels were identified and excluded from analyses. The estimated asymptotic length in the von Bertalanffy growth function (84.5–110 cm) was smaller, while the Brody growth parameter (0.30–0.44 year^?1) was higher using electronic length frequency analysis (ELEFAN) than when using Shepherd’s length composition analysis (SLCA). The total instantaneous mortality rate (Z) was around 1 for periods 1998–2003 and 2 year^?1 for 2005–2007 using length‐converted catch curves. The 95% confidence intervals of Z did not overlap for two of the periods, suggesting that the mortality rates were significantly higher during 2005–2007, possibly due to the introduction of shrimp nets. The maturity function differed significantly between sexes, indicating that females become silver eels at a larger size. The Japanese eels in the lower reach of the Kao‐Ping River were likely heavily exploited, thus management and conservation actions are strongly recommended.     

Application of otolith microchemistry to estimate the migratory history of Japanese eel Anguilla japonica on the Sanriku Coast of Japan

The age and migratory history of the Japanese eel, Anguilla japonica Temminck & Schlegel, collected in Miyako Bay along the Sanriku coast of Japan, was examined using the otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations conducted with wavelength dispersive X‐ray spectrometry by an electron microprobe. The line analysis of Sr : Ca ratios along the life history transect of each otolith showed a peak (ca. 15–17 × 10^?3) which corresponded with the period of their leptocephalus and early glass eel stages in the ocean. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that there were eels with several general categories of migratory history, including sea eels that never entered freshwater (average Sr : Ca ratios, ≥6.0 × 10^?3), and others that entered freshwater for brief periods but returned to the estuary or bay. This evidence of the occurrence of sea eels in this northern area indicates that Japanese eels of the Sanriku coast do not necessarily migrate into freshwater rivers during recruitment as do glass eels at the beginning of their growth phase; even those that do enter freshwater may later return to the marine environment. Thus, anguillid eel migrations into freshwater are clearly not an obligatory migratory pathway, but rather a facultative catadromy with seawater or estuarine residents as an ecophenotype.     

A long-term study of population characteristics and downstream migrations of the European eel Anguilla anguilla (L.) and the effects of a migration barrier in the Girnock Burn, north-east Scotland

Downstream migrations and population characteristics of eels Anguilla anguilla were studied between 1967–1982 and 2002–2005 using a fish trap and electrofishing in the Girnock Burn, a small oligotrophic upland sub‐catchment of the River Dee, north‐east Scotland, 70 km from the tidal limit. In limited mark‐recapture studies, 9% of eels were recaptured up to three times and 97% of all recaptures were made at the same electrofishing site. The recaptured eels had a low mean growth rate of c. 13 mm year⁻¹. Smaller eels appeared to show preferences for shallower habitats with small boulder and gravel–sand substrata. Trap catches exhibited seasonal modes in total length at 140–180 mm in late spring, and 320–340 mm in early autumn, probably relating to water temperatures and discharges. From other studies, it is inferred that the spring mode comprised sexually undifferentiated nomadic eels and the autumn mode differentiated males beginning their spawning migration. Large female eels were rare. The fish trap appears to have formed a major barrier to upstream migration since its construction in 1966. In‐stream density has decreased significantly since then from 16 to three eels 100 m⁻², biomass from 260 to 78 g 100 m⁻² and emigrants from 700 to 100 individuals year⁻¹. Emigrants have comprised c. 5% of the standing stock year⁻¹ since the 1970s. The proportion of larger differentiated eels in the Girnock Burn has, however, remained relatively constant and escapement has been c. 100–200 (probably male) eels year⁻¹ since the late 1960s. Evidence, including that from other northerly British rivers, is reviewed to assess the possible impacts of Europe‐wide declines in glass eel recruitment since the 1980s. It is recommended that the data series be maintained, plus further sex determination and ageing studies. Installation of an upstream trap to capture immigrants and studies of recolonization are proposed.     

Groundwater discharge from the superficial aquifer into Cockburn Sound Western Australia: estimation by inshore water balance

Submarine groundwater discharge (SGD) into Cockburn Sound Western Australia was quantified by applying a distributed groundwater flow model to estimate the inshore aquifer water balance. Spatially averaged SGD along the coast was estimated to be 2.5–4.8?±?0.9?m³?day^?1?m^?1. The range in estimated average SGD reflected low and high estimates of average groundwater recharge, which ranged from 0.13 to 0.24?m?year^?1 (15–28% of average annual rainfall). The error ±0.9?m³?day^?1?m^?1 was calculated by assuming arbitrary ±20% errors in groundwater pumping and inflow across boundaries. SGD varied spatially along the coastal boundary due to variation in hydraulic connection between the coastal aquifers and ocean, and spatial variability in recharge, transmissivity and pumping. Under assumptions of low and high groundwater recharge, SGD along the coastline varied in the ranges 1.4–4.6?m³?day^?1?m^?1 and 2.4–7.9?m³?day^?1?m^?1, respectively.     

Biological parameters and current status of European eel (Anguilla anguilla Linnaeus, 1758) from Asi River,Northeastern Mediterranean region,Turkey

The biological characteristics of eels from the Asi River, Turkey, were assessed between December 2017 and November 2018. Eels were sampled monthly using fyke nets (operated by professional fishermen), yielding a total of 509 specimens. Total length and weight were measured, sex, age and maturity stages (silver or yellow eel) were determined. Catch Per Unit Effort (CPUE) was calculated for both biomass per unit effort and numbers caught on a monthly and annual basis. The length-weight relationships (LWRs) of silver and yellow eel was W = 0.009*TL^3.22 (n = 262) and W = 0.0106*L^3.09 (n = 247), respectively. The age of the sampled fish ranged from year class II to VI. Von Bertalanffy growth parameters were estimated as L_∞=69.25 cm, K = 0.43 1/year, t₀= −0.41 and phi prime index ( ǿ )= 3.31 for all samples. The overall eel fishing mortality rate (F) was 0.31 year^-1, and the exploitation and survival rates of silver stage eels were estimated with 30% and 39%, respectively.     

Evidence for Environmental Sex Determination in the American eel, Anguilla rostrata

Males have predominated among migrating silver eels in the Annaquatucket River, Rhode Island, for at least two decades, with no significant variation in mean total length in either sex. Because the species is panmictic (random breeding), this consistency suggests environmental sex determination (ESD). Most yellow (feeding phase) eels <300mm total length in the Annaquatucket are sexually undifferentiated, and in contrast to all other published sex ratios, males greatly outnumber females (3:1) among differentiated yellow eels. Estimates of yellow eel population densities are 4–10 times greater than published values for other habitats. We propose that this crowding results in a long period of undifferentiation and the suppression of femaleness. Published field and experimental evidence indicates that high population density results in high proportions of males in Atlantic Anguilla, and that low population density results in the predominance of females. This ESD may be adaptive, resulting in vast numbers of small males in coastal habitats, relatively close to the spawing area, and much larger and more fecund females that occupy most of the available eel habitat.     

Growth and Production of Yellow Eels and Glass Eels in Ponds

Two experiments (I and II) were performed in drainable ponds. Yellow eels Anguilla anguilla (L.) were stocked in early June at three biomasses: 10, 20 and 60 kg · ha⁻¹ in experiment I; and 10, 20 and 40 kg · ha⁻¹ in experiment II. The mean body weights were 27.0 and 24.2 g respectively. Glass eels were stocked only in experiment II at equal densities of 1600·ha⁻¹. In both experiments each biomass of yellow eel was combined in a factorial design with three cyprinid communities differing in biomass and in species- and size-composition. The ponds were drained in autumn. The final body weights at draining ranged from 25.9 to 63.6 g for yellow eel and from 3.9 to 8.8 g for glass eel. The final body weights of yellow eel and of glass eel decreased with increasing biomass of yellow eel. No significant relation was found between the bream Abramis brama (L.) biomass and the growth of eel. The growth rates of yellow eel and glass eel were positively correlated in experiment II. At higher biomasses of yellow eel the percentage females decreased slightly. The recapture rates of yellow eel in experiments I and II amounted to 69.4 ± 9.8 % and 92.2 ± 4.9% (mean ± sd) respectively. The lower recapture rates in experiment I were caused by the inappropriate draining technique used. The glass eels were recaptured with 75.0·5.6% efficiency. The maximum net production of yellow eel occurred at a biomass of 20–40kg·ha⁻¹ and amounted to 19 kg·ha⁻¹.     

Habitat carrying capacity is reached for the European eel in a small coastal catchment: evidence and implications for managing eel stocks

1. Despite carrying capacity being one of the most important parameters in population management and modelling, we lack substantial evidence for habitat limitations on freshwater species. Here we tested the ideal free distribution (IFD) hypothesis using an indirect behaviour‐based method for small closed populations assuming that animals can effectively estimate habitat suitability and distribute themselves accordingly in time and space. 2. We analysed spatiotemporal variations in the density of the European eel Anguilla, a catadromous species with good colonisation abilities in a small coastal catchment in France. The general linear model used enabled us to test simultaneously the effect of temporal, macro‐ and meso‐scale habitat factors on the presence and abundance of eels at 30 sites over an 8‐year period. 3. Almost every site sampled had eels, whatever its location on the catchment and its habitat characteristics. Density estimates (overall mean ± SD of 0.40 ± 0.48 m^?2) were at the upper range of other values for European catchments. Moreover, eel densities were mainly influenced by the availability of suitable habitats (rocky substratum and instream cover), which suggests that their distribution reflects an IFD. 4. Despite marked variability in recruitment, the density of the oldest size‐class remained stable over the study, suggesting that density‐dependent mortality occurred, probably due to intraspecific competition for space and food and to predation. 5. These findings suggest that eel habitats are saturated in the Frémur. Therefore, we suggest that the mean abundance of eels observed could serve as a threshold value for other male‐dominated river stocks (provided they have a similar overall percentage of suitable habitats) that are common in small, low gradient streams on the north‐Atlantic coast of Europe.     

Migration and habitat use of the tropical eels Anguilla marmorata and A. bicolor pacifica in Vietnam

The patterns of use of marine and freshwater habitats by the tropical anguillid eels Anguilla marmorata and A. bicolor pacifica were examined by analysing the otolith strontium (Sr) and calcium (Ca) concentrations of yellow (immature) and silver (mature) stage eels collected in Vietnamese waters. In A. marmorata, the change in the Sr:Ca ratios outside the high Sr:Ca core was generally divided into three patterns: (1) typical catadromous life history pattern; (2) constant residence in brackish water; and (3) habitat shifting between sea and brackish waters with no freshwater life. In A. bicolor pacifica, no eels had a general life history as freshwater residents. The eels were also divided into three patterns: (1) constant residence in sea water; (2) constantly living in brackish water; and (3) habitat shifting from brackish to sea water with no freshwater residence. The mean Sr:Ca ratio value after recruitment to coastal waters ranged from 1.73 to 5.67 × 10^?3 (mean 3.2 × 10^?3) in A. marmorata and from 2.53 to 6.32 × 10^?3 (mean 4.3 × 10^?3) in A. bicolor pacifica. The wide range of otolith Sr:Ca ratios in both species indicated that the habitat use of these tropical eels was facultative among fresh, brackish, and marine waters during their growth phases after recruitment to coastal areas. Tropical eel species may have the same behavioural plasticity as temperate anguillid species regarding whether to enter freshwater or to remain in estuarine and marine environments.     

Migratory patterns and contribution of stocking to the population of European eel in Lithuanian waters as indicated by otolith Sr:Ca ratios

Otolith Sr:Ca ratios were examined to evaluate the contribution of the stocked eel Anguilla anguilla elvers, which have been stocked in Lithuanian waters and mixed with naturally recruited eels for several decades, to the native eel population. Stocked eels were identified by the freshwater signature (Sr:Ca ratios <2·24 × 10⁻³) on the otolith after the glass eel stage. Naturally recruited eels, that had migrated through the North and Baltic Seas, were characterized by an extended seawater and brackish-water signature (Sr:Ca ratios >3·23 × 10⁻³) after the glass eel stage. Of 108 eels analysed, 21 eels had otolith Sr:Ca ratio profiles consistent with stocking while 87 showed patterns of natural recruitment. The ages of naturally recruited eels arriving in Lithuanian fresh waters varied from 1 to 10 years, with a mean ±  s.d . age of 5·2 ± 2·1 years. Eels from the inland Lake Baluošai were all freshwater residents of stocked origin. Stocked eels, however, accounted for only 20% of the eels from the Curonian Lagoon and 2% of eels sampled in Baltic coastal waters. This finding does not support the hypothesis that the eel fishery in the Curonian Lagoon depends mostly on stocking.     

Some population parameters of Ruditapes philippinarum (Bivalvia,Veneridae) on the southern coast of the Marmara Sea,Turkey

Ruditapes philippinarum, a venerid clam, is a dominant species in the sandy and muddy areas in the coastal waters of the Marmara Sea. Intensive commercial harvesting of this species is conducted in these regions. We studied the population dynamics of R. philippinarum on the southern coast of the Marmara Sea (Band?rma). Samples were collected on a monthly basis between September 2012 and August 2013. Seasonal von Bertalanffy growth parameters using the length–frequency distribution of R. philippinarum were estimated at L _∞ = 67.50 mm and K = 0.33 year^?1, and the seasonal oscillation in growth rate was 0.53. The slowest growth period was in January. The growth performance index and potential lifespan were 3.182 and 8.06 years, respectively. The growth relationship was confirmed to have a positive allometric pattern. The average total mortality rate was estimated to be 0.777 year^?1, whereas the natural and fishing mortality rates were 0.539 and 0.238 year^?1, respectively. The current exploitation rate of R. philippinarum was 0.306. The recruitment pattern peaked during June–August, and spawning occurred between May and August. The results of this study provide valuable information on the status of R. philippinarum stocks.     

Burrowing behaviour of the European eel (Anguilla anguilla): Effects of life stage

The European eel (Anguilla anguilla) is a fascinating species, exhibiting a complex life cycle. The species is, however, listed as critically endangered on the IUCN Red List due to an amalgam of factors, including habitat loss. This study investigated the burrowing behaviour and substrate preference of glass, elver and yellow stages of A. anguilla. Preference was determined by introducing eels in aquaria with different substrates and evaluating the chosen substrate for burrowing. In addition, burrowing was recorded using a camera in all substrate types and analysed for kinematics. The experiments showed that all of these life stages sought refuge in the sediments with particle sizes ranging from sand to coarse gravel. Starting from a resting position, they shook their head horizontally in combination with rapid body undulations until half of their body was within the substrate. High-speed X-ray videography revealed that once partly in the sediment, eels used only horizontal head sweeps to penetrate further, without the use of their tail. Of the substrates tested, burrowing performance was highest in fine gravel (diameter 1–2 mm; lower burrowing duration, less body movements and/or lower frequency of movements), and all eels readily selected this substrate for burrowing. However, glass eels and elvers were able to use coarse gravel (diameter >8 mm) because their smaller size allowed manoeuvring through the spaces between the grains. Further, burrowing performance increased with body size: glass eels required more body undulations compared to yellow eels. Interestingly, the urge to hide within the sediment was highest for glass eels and elvers. Documentation of substrate preference and burrowing behaviour of A. anguilla provides new information about their potential habitat use. Considering that habitat alterations and deteriorations are partly responsible for the decline of the eel, this information can contribute to the development of more effective conservation measures.     

Age and growth rates of North Atlantic eel larvae (Anguilla spp.), based on published length data

The age of glass eels arriving at the European coasts is estimated by different authors to range from 3/4 to 3+years; some of these estimates are based on counts of “daily rings”. The present study reviews published data on North Atlantic length frequency distributions. The only years for which larvae samples are available from the spawning area to the European continental slope are 1979 and, to a certain extent, also 1922. In the spring of both years, length frequency distributions exhibited two distinct maxima which are considered to belong to the AG 0 (mean lengths 12 and 18 mm for 1979 and 1922, respectively) and the AG I (47 and 44 mm). In addition, a third, less distinct maximum is visible and the existence of an AG II must be considered. Growth during the first year of life is calculated using the means of total lengths of sufficiently large samples, collected with progressing season, of the years 1920, 21, 22, 79, 81. From the resulting linear regressions forAnguilla anguilla until the beginning of winter, a daily length increase of 0.15 mm was estimated. Growth ofA. rostrata was faster (0.22 mm·day⁻¹) Daily length increase ofA. anguilla in winter was only 0.03 mm; during a possible second summer 0.06–0.09 mm·day⁻¹ and during a possible third summer considerably less. There is a length increase of the European eel larvae from south to north, known also from glass eels, which makes evaluation of length increase from west to east even more difficult. A higher age of glass eels in the north than in the south is therefore likely.     

Diving activity of migrating silver eel with and without Anguillicola crassus infection

Infection with the swim bladder nematode Anguillicola crassus has been hypothesised to threaten the spawning migration success of the endangered European eel (Anguilla anguilla). To examine this assumption, we compared the swimming behaviour of one Anguillicola crassus infested eel in the North Sea and three parasite‐free eels in the Baltic using data recovered from data storage tags attached to migrating silver eels. In both areas, eel activity was characterized by frequent diving behaviour throughout the water column during the night, with reduced activity during the day. Despite substantial damage of the swim bladder, the behaviour of the infested eel from the North Sea was within the same range of migrating and diving activity parameters as the three parasite‐free eels from the Baltic Sea. All eels had a similar frequency distribution of descent or ascent speeds and a similar average horizontal migration speed. The diving speeds and dive ranges exclude the possibility that the eels were in continuous hydrostatic equilibrium during their migrations and suggests therefore that the role of the swim bladder in vertical migration is likely to be more complex than currently thought. Our results suggest that eels infested by Anguillicola crassus are capable of diving in a similar manner to uninfested eels during the first stretch of their spawning migration.     

Distribution, standing stock, growth, mortality and production of Strongylocentrotus pallidus (Echinodermata: Echinoidea) in the northern Barents Sea

The regular sea urchin, Strongylocentrotus pallidus (G.O. Sars, 1871), is a widespread epibenthic species in high-Arctic waters. However, little is known about its distribution, standing stock, population dynamics and production. In the northern Barents Sea, S. pallidus was recorded on seabed still photographs at 10 out of 11 stations in water depths of 80–360?m. Mean abundances along photographic transects of 150–300 m length ranged between <0.1 and 14.7?ind. m^?2 yielding a grand average of 3.6?ind.?m^?2. The small-scale distribution along the transects was patchy, with densities varying from nil to an overall maximum of 25.5 ind. m^?2, and exhibited a significant relation to the number of stones present. Sea urchin test diameters, measured on scaled photographs, extended from 7 to 90?mm. Median values at single stations varied from 14 to 46?mm, showing a significant inverse relationship to water depth. Biomass, estimated by combining photographic abundances, size frequencies and a size-mass function established with trawled specimens, ranged between <0.1 and 3.0?g ash-free dry mass m^?2, averaging about 1.0?g ash free dry mass m^?2. An analysis of skeletal growth bands in genital plates was carried out with 143 trawled individuals ranging in test diameter (D) from 4 to 48?mm. Assuming these bands to represent annual growth marks, the ages of the specimens analysed ranged between 3 and 42 years. A von Bertalanffy function was fitted to size-at-age data to model individual growth pattern (D_∞?=?102.3?mm, k?=?0.011 year^?1, t₀?=?0.633?year). The annual mortality rate Z of the population in the northern Barents Sea was estimated from a size-converted catch curve to be 0.08 year^?1. Applying the weight-specific growth rate method, the average P/B ratio and the mean annual production of this population were estimated as 0.07 year^?1 and 0.076?g AFDM m^?2 year^?1, respectively. In conclusion, S. pallidus is characterized by slow growth, low mortality, high longevity and low productivity. Because of its relatively high biomass, it is considered to contribute significantly to total benthic standing stock and carbon flux in the study area.     

Population structure, growth and production of Thoracophelia furcifera (Polychaeta: Opheliidae) on a sandy beach in Southern Brazil

This study investigates aspects of the life history of the polychaete Thoracophelia furcifera on a sandy beach in southern Brazil. Two fixed transects perpendicular to the shoreline in the intertidal zone were sampled fortnightly from May 2008 to April 2009 at low tide. Five T. furcifera samples were collected along each transect and sediment temperature and the salinity of interstitial water were recorded. The material was washed over 0.5- and 0.088-mm sieves, and the width of setiger 8 of each specimen was measured. A total of 5,870 organisms were examined and the estimated parameters of the von Bertalanffy growth curve were L _∞ 3.60?mm (Wd8S), K 0.63?year^?1, C 0.3 and WP 0.97 (Rn 0.132). Life span was 2.6?years, instantaneous mortality rate Z was 3.8?year^?1 and the growth index φ′ 0.91. Mean density ranged from 644.44?±?191.77 to 2,783.33?±?453.64 ind m^?2 and mean biomass ranged from 2.52?±?0.55 to 9.52?±?1.83?g?m^?2. Recruitment occurred from April to July and ovigerous females were found from June to November. Annual secondary production was 6.582?g?m^?2?year^?1, mean biomass was 5.638?g?m^?2 and turnover rate was 1.167. The high values for density, secondary production and biomass suggest that T. furcifera constitute an important food source. These features of T. furcifera’ life strategy demonstrate the significant role this species plays in ecosystem dynamics.     

Validation of the occurrence of the American eel Anguilla rostrata (Le Sueur, 1817) in free-draining European inland waters

The screening of 2,735 eels from European waters and aquaculture farms was conducted using mitochondrial Cytochrome b and 16S rRNA gene fragments amplified by polymerase chain reaction. Reaction products were either sequenced directly or subjected to analysis using restriction fragment length polymorphism which resulted in species-specific restriction patterns. Beside the expected European eel, Anguilla anguilla (Linnaeus, 1758), the American eel, Anguilla rostrata (Le Sueur, 1817), was also identified in samples from both aquaculture (N = 40 out of 1,025) and from natural waters (N = 44 out of 1,710). The life stages of American eels identified from several German waters draining to either the Baltic Sea and the North Sea ranged from elver to silver eels. This indicates that stocking with glass eels or elvers must have occurred several times most likely in the period from 1998 to 2002. The application of a fast and precise method for species identification and genetic monitoring of eels delivered for stocking is therefore essential for maintaining the autochthonous species composition in future. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Glass eel recruitment, Anguilla anguilla (L.), in a Mediterranean lagoon assessed by a glass eel trap: factors explaining the catches

Although the colonisation of coastal rivers on the Atlantic and Mediterranean coast by glass eels, Anguilla anguilla, has been well studied and understood, the colonisation of lagoons by glass eels is much less known. For the first time in the Mediterranean region, the installation of a glass eel fish-pass in Grau de la Fourcade channels in the Rhône delta enabled us to determine which factors could explain the variations in the catches of glass eel entering the Vaccarès coastal lagoon system. Whatever be the procedure chosen, the results of the model were the same: the temperature, the cumulative water discharge from the channel in the 5 nights before the catch (freshwater lure) and time that the drainage pumps were working explained the glass eel catches in the fish-pass in the Grau de la Fourcade. The tide and the cumulative discharge from the channel for only 3 nights before the catch did not seem to have a significant role in explaining catches. These results show that it is important that the lagoons should continue to receive rainfall runoff from their watersheds so that their water levels are high in winter, and that there is a good colonisation by glass eels as a result of a freshwater lure effect, when strong north winds expel low salinity water to the sea.