Migration of eels tagged in the Baltic Sea and Lake Mälaren—in the context of the stocking question

Eels (Anguilla spp.) are in decline worldwide, and the signs of reduced recruitment have been observed in continental Europe since the early 1970s. To aid recovery of the European eel, stocking is used by many European countries as a management option. In this study, tagging experiments were conducted to follow eel migration from Lake Mälaren and four sites along the Swedish east coast in the Baltic Sea. The recaptured tagged eels were retrieved from fishermen, allowing for the opportunity to investigate their origin (brackish water, stocked in freshwater or a mix in between) by otolith microchemistry and to assess for morphological differences after tagging. Several changes took place; for example, eye index increased while weight and condition decreased with migrated distance and time until recapture. In Lake Mälaren, the majority of tagged eels did not migrate out of the outlets, irrespective of their origin. Most of them were caught in the opposite direction and continued to be caught in the lake 1–3 years after tagging, with significant weight losses. Overall, overwintering is suggested to be an inferior option, but it is uncertain whether this is a natural behaviour or a result of translocation and restocking. For coastal eels, origin had no effect on migratory behaviour; a majority of the tagged eels migrated towards the outlet of the Baltic Sea. Interestingly, a minority of the recaptured eels originated from stocked fish. Instead, recaptures were dominated by natural immigrants that had spent most of their lives in brackish waters.     

A preliminary experiment regarding the natural induction of gonadal development in female Japanese eels without hormone treatment

To test whether gonadal development of female eels could be promoted without any exogenous hormone treatments, we observed the effect of water temperature manipulation. After 3–5 months of water temperature treatments, three silver eels showed higher gonadosomatic indices (GSI). In particular, one eel in the 5–15°C fluctuating temperature treatment group (5°C daytime and 15°C night‐time) had the highest GSI of 8.5 with secondary yolk globule stage oocytes of large diameter (OD; 412 μm) after 3 months, which indicated definite gonadal development compared with those in the initial states (mean GSI, 2.4; OD, 226.7 μm). The 5–15°C fluctuating temperature and constant 5°C groups had low oocyte breakdown (atretic) rates. Because daily temperature fluctuations and cold daytime water are experienced by vertically migrating silver eels in the ocean, these temperature conditions may be key maturation process components that could be useful for hormone‐free artificial maturation.     

Behaviour,escapement and mortality of female European silver eels within a regulated lowland river draining into the Baltic Sea

Migration patterns of 146 female silver eels on their way through fresh and brackish water stretches of the Warnow River into the Baltic Sea were studied using acoustic telemetry. The majority of eels (77%) regained their expected downstream migration behaviour after a median rehabilitation time of 2 days. Positive rheotactic behaviour was observed for 23% of tagged eels after release, probably influenced by experimental procedures. The observed silver eel escapement rate was 63%. Two migratory groups were classified by significantly different migratory speed, time of residence in brackish waters and biometric silvering characteristics. Although the classification of silvering stages is based on these measures, the proportions of these stages did not reflect the observed differences in migratory behaviour. In the context of a permanent silver eel descent in the freshwater part of the Warnow River, a permanent escapement was likewise occurring into the Baltic Sea.     

Estimation of production and long‐term changes in the population of European eel Anguilla anguilla (L.) in an Irish lake

Monitoring of yellow‐phase and silver‐phase Anguilla anguilla during their continental life history is necessary for evaluation of stock recovery measures. Eel population data for an Irish lake (Lough Sheelin) were compiled for the period 1993–2014. Catch data from 2009 to 2014 provided minimum estimates of recent silver eel production ranging annually from 0.79 to 1.84 kg/ha. Long‐term changes in yellow eel abundance and silver eel size structure were assessed as part of a fishery monitoring programme. Yellow eel catch per unit effort (CPUE) in the lake (from longline surveys) was considerably higher in the 1990s (52.2–62.1 eels/100 hooks) than 2002–2008 (1.9–15.8 eels/100 hooks). Conversely, during 1993–2014, the mean size of silver eels migrating from the lake increased significantly (p < .001), from 659 mm to 838 mm. The results suggest that in the absence of direct yellow eel abundance data, interannual variation in silver eel size structure may be a useful monitoring tool for local eel stocks as part of Eel Management Plans (EMP's).     

Condition and recruitment of Aristeus antennatus at great depths (to 2,300 m) in the Mediterranean: Relationship with environmental factors

Depth and seasonal trends in the biological condition and recruitment of the red shrimp (Aristeus antennatus) have been analyzed over the slope to 2,233 m in the western Mediterranean. The best biological condition of A. antennatus (gonadosomatic index [GSI]) for mating and spawning occurred at 800–1,300 m in summer, in areas deeper than the fishing grounds distributed between 500 and 800 m. Females moved shallower to feed on the upper slope during periods of water‐mass homogeneity (autumn–winter), increasing their hepatic gland weight (hepatosomatic index [HSI]). Females moved downslope (800–1,100 m) to spawn (high GSI) during periods of water mass stratification (late spring–summer). The HSI of females decreased with depth down the slope in autumn, after the reproductive period. Small juveniles were distributed deeper than 1,000 m, associated with high near‐bottom O₂ levels, low turbidity and high C:N in sediments, implying favorable trophic conditions. This confirms the importance in studying the biology of deep‐sea species over their entire depth range. The progressive warming and increasing salinity of deep Mediterranean waters could provoke a decrease of dissolved O₂ that would affect the life cycle of A. antennatus.     

Migration and escapement of silver eel males,Anguilla anguilla,from a southwestern European river

The catadromous life cycle of the European eel encompasses a spawning migration to the Sargasso Sea. Prior to their migration, eels develop a silver coloration being referred to as “silver eels.” Due to the dramatic European eel recruitment decline, it is crucial to quantify silver eel escapement to evaluate the success of measures taken under Eel Management Plans (Regulation EC (1100/2007)). Thus, the percentage of silver eels escaping from the River Mondego (Portugal) was estimated during two consecutive spawning seasons (2014–2016) and their migratory behaviour was studied. Thirty-six male silver eels were tagged with acoustic transmitters and their downstream migration was tracked using automatic receivers deployed at four locations along the river course, from the first impassable obstacle up to the river mouth (67 km). Among the tagged eels, 15 (42%) escaped to the sea. In both years, individuals displayed faster migration speed in the downstream reaches of the river. BRT (“Boosted Regression Trees”) models were used to explain downstream migration and escapement of silver eels. The analyses revealed that the downstream migration is triggered by several environmental variables with water conductivity as the most influential predictor. Escapement of eels from the estuary occurred a few hours after their arrival and was primarily explained by the offshore wind intensity, which promotes a current in the coast that may favour their transport to the open sea. These findings contribute to a better understanding of silver eel migration in southern latitudes and highlight and reinforce the need to monitor spawner escapement.     

Virus infections of the European Eel in North Rhine Westphalian rivers

Viral infections have been suggested to play a role in the decline of the panmictic population of the European eel (Anguilla anguilla). However, despite the importance of knowledge about pathogenic eel viruses, little is known about their spread in the wild European eel population and only a few eel pathogenic viruses have been described so far. In this study, we aimed to investigate the health status of the A. anguilla stock in North Rhine Westphalia (NRW) State of Germany. For this purpose, we examined tissue samples of 16 elvers, 100 yellow eels and 6 silver eels, sampled from the rivers Rhine, Lippe and Ems. Virus detection was performed via a combination of cell culture and PCR. Next to the detection of frequently encountered pathogenic eel viruses (anguillid herpesvirus 1 and eel virus European X (EVEX)), we isolated the eel picornavirus 1 (EPV-1) from tissue of yellow eels and elvers and demonstrate the distribution of EPV-1 in wild eel population in NRW.     

Site fidelity,diel and seasonal activities of yellow‐phase Japanese eels (Anguilla japonica) in a freshwater habitat as inferred from acoustic telemetry

Japanese eels (Anguilla japonica) perform large‐scale oceanic migrations between their spawning ground and growth habitats in continental waters during life history. However, between these migrations, they spend most of time in growth habitats such as rivers. To investigate the diel and seasonal activities, homing behaviour and home range of yellow‐phase Japanese eels in the lower reach of the Tone River, we tracked them throughout a year using a fine‐scale positioning system (VPS) based on acoustic telemetry. The tracked eels were generally nocturnal, but not exclusively. They were mainly mobile from spring to autumn, with little or no activity observed during winter. A transport‐release experiment showed that most eels returned to their original capture area within 13 days after release. The eels had very small home ranges (mean ± SD = 0.085 ± 0.068 km²), core areas (0.014 ± 0.014 km²) and linear home ranges (744 ± 268 m). They also tended to be distributed on one particular side of the river (right or left bank) and in one particular shore type (revetment or vegetation), rarely moving from one to the other. This study provides evidence for nocturnal, dormancy, homing behaviours, limited habitat use and small home range size in Japanese eels. The eels clearly showed strong fidelity to a “familiar” site, which contrasts with the long distances travelled during upstream and downstream migration phases in the river, and during spawning migrations in the ocean.     

Fine‐scale swimming movement and behaviour of female silver eels,Anguilla dieffenbachii,within a lake affected by hydro‐power generation

Lake Manapouri (South Island, New Zealand) maintains the country’s largest hydroelectric scheme (700 MW). During their seaward migration, silver eel Anguilla dieffenbachii Gray in this lake are attracted to the power station intake, as well as the natural outlet. To understand the behaviour of such eels, 210 female eels were tagged with acoustic transmitters between 2005 and 2010. Swimming speeds of tagged eels averaged 2.0 km/hr, and swimming depths were deeper during daytime than at night. Silver eels from sites upstream of Lake Manapouri tended to spend more time exploring Lake Manapouri than locally caught eels, and a higher proportion entered the power station intake. Almost 20% of tagged eels did not leave the lake during the year they were tagged. Eels typically showed extensive and complex patterns of movement before emigrating, including multiple passes across the power station intake or the natural outlet.     

Using a telemetry study to assess the boundary net efficiency of an enclosure system used for yellow eel density monitoring

To assess yellow eel (Anguilla anguilla L.) densities in non‐tidal waters, 1‐ha enclosure approach was developed. For unbiased density and biomass calculations, the catchability of the system needs to be evaluated. A telemetry study with 49 tagged eels was conducted in 2015 and 2016 to verify the effectiveness of the 1.8‐m high boundary net. Excluding recaptured eels (N = 10), 42.8% of the released eels (N = 15) escaped the enclosure within 48 hr. Regarding the movement frequency, similar movement patterns were observed for enclosed and escaped eels within 48 hr after release. Based on the telemetry study, it is concluded that the boundary net is recognised as an obstacle by yellow eels, and reduces the escapement of enclosed eels.     

Aeromonas veronii caused disease and pathological changes in Asian swamp eel Monopterus albus

More and more diseases were occurred in the aquaculture of Asian swamp eel. Bacterial isolates were collected from the pathogenic organs of diseased Asian swamp eels. A dominant strain, Aeromonas veronii JL‐01, was isolated and identified from the liver and intestine tissue of diseased Asian swamp eels. The symptoms of fish infected artificially with A. veronii were the same as those observed under natural infection. The mortality rate was 40%–80% and infected Asian swamp eels died 2–3 days after infection. Drug sensitivity tests showed that the isolated bacterium was highly sensitive to seven kinds of antibiotics, including cephalosporins, aminoglycoside, quinolones, tetracyclines (except Tetracycline), pheniclos, nitrofurans and sulfonamides. The bacterium was resistant to penicillins (except Piperacillin), macrolides, polypeptides and lincomycin. The histological changes in Asian Swamp Eel induced by A. veronii infection mainly occurred in the liver, intestines and kidney. A. veronii showed strong pathogenicity towards the Asian swamp eel, causing tissue damage and death. Gentamycin, doxycycline and sulfamethoxazole could be used for preventing and treat this disease.     

A size‐age model based on bootstrapping and Bayesian approaches to assess population dynamics of Anguilla anguilla L. in semi‐closed lagoons

A size‐age modelling technique is presented for assessing the vital rates, stock and recruitment of eel populations in semi‐closed lagoons with fully monitored migration of silver eels. Data for yellow and silver European eels (Anguilla anguilla L.) were obtained in 2011 from the Comacchio lagoon (Italy). The analysis was performed in three steps: (i) correction of yellow eel data, which are affected by the fyke nets selectivity during samplings, (ii) estimation of survival curve, stock, recruitment and metamorphosis rates of the population (calibration using data from 2011) and (iii) validation of the model using the observed amount of silver eel migrating population of the next year. A bootstrap procedure was used to assess the level of uncertainty for each parameter using the 95% intervals of the highest posterior density distribution HPDD (Bayesian approach). The measured abundance of silver eels was 0.56 ind·ha^?1, while the yellow eel abundance and recruitment were estimated by the model for 2011 at 8.77 ind·ha^?1 and 5.99 ind. ha^?1 respectively. The model performance during validation was satisfactory as the observed total mass of migrating population of 2012 (3777 kg) was inside the 95% HPDD intervals (3197–3839 kg) of model's predictions. The estimated stocks and recruitment were at least ten times lower from the respective estimations of previous studies of 1989 highlighting the crucial conditions of the population. The proposed modelling approach can provide significant information about eel population conditions, facilitating the evaluation of a range of management options in the context of eel conservation plans.     

Ecology and conservation of the American eel in the Caribbean region

The majority of American eel, Anguilla rostrata LeSueur, knowledge is derived from temperate regions in the United States and Canada, with little known from its tropical Caribbean distribution. Findings of original research on American eel distribution, abundance, population biology, habitat ecology and threats from the Caribbean island of Puerto Rico were synthesised. American eel were captured from 48 of 116 sites (41.4%) in 26 of 49 river basins (53.1%) during 2005–2016, and it was extirpated upstream of dams and migration barriers >3.0 m high (38.9% of habitat). Mean density and biomass were 438.9 fish/ha and 23.44 kg/ha, respectively. Upstream habitats favoured larger individuals, and females were larger than males. The swim‐bladder parasite Anguillicoloides crassus Kuwahara, Niimi & Hagaki was not found in 120 eels examined. Realised threats include dams and other migratory barriers, habitat loss and alteration and pollution; exotic species and commercial fishing are impending threats; and the least understood is climate change.     

Direct and indirect impacts of pumping station operation on downstream migration of critically endangered European eel

Downstream passage of European eel Anguilla anguilla (L.) in catchments with pump(s) for water level management is a major concern. Catchment‐wide acoustic telemetry revealed silver eels quickly migrated downstream through unobstructed reaches (n = 12; mean ± SD = 17.9 ± 1.9 km/day). Fourteen of 17 acoustic‐tagged eels detected at the pumping station (82.1%) retreated back upstream and ten (58.8%) passed through pumps after delays (9.5 ± 11.0 days). Multi‐beam sonar imaging across the intake screen (55‐mm gaps) revealed that peaks in migration occurred during the nights preceding the new moon but 76.7% retreated back upstream. All passive integrated transponder (PIT)‐tagged eels recaptured (n = 56) downstream of a large (2.23‐m diameter) mixed flow pump survived but 96.5% had minor injuries, reduced physical condition and/or abnormal behaviour. By contrast, 64.7% of PIT‐tagged eels recaptured (n = 17) downstream of a small (0.8‐m diameter) axial flow pump died. No acoustic‐tagged eels that passed through the small axial flow pump (n = 10) performed onward migration at sea, unlike “control” eels released downstream (n = 11). This evidence may help develop effective remediation measures, such as operational changes, to maximise escapement of catadromous eel species at pumping stations.     

Large‐scale,seasonal habitat use and movements of yellow American eels in the St. Lawrence River revealed by acoustic telemetry

Large‐scale habitat use and movements of yellow American eels (Anguilla rostrata) from the St. Lawrence River were examined using acoustic telemetry from early summer to late fall in 2010 and 2011. Sixty‐seven eels were tagged, and their passage or presence was recorded using fixed acoustic arrays covering a 400 km distance along the St. Lawrence River and Estuary. Sixty‐four per cent of the 67 tagged eels were detected. Most eels were detected at only one array; the closest to their release location and at several occasions during the tracking period, suggesting a high proportion of freshwater residency in the upstream part of the St. Lawrence River. Downstream movements towards the brackish estuary (63–418 km distance) were demonstrated for 16.4% of the eels, particularly for those caught at the most downstream site that is close to the brackish estuary. Our results strongly suggest a lower activity of freshwater resident yellow eels during summer, a behaviour that may be related to day length, which defines time available for their nocturnal foraging. Indeed, yellow eels were detected primarily at night; no effect of moon phase was revealed. Movements in the vicinity of arrays (up to 116 km in the fluvial estuary) were suggested and smaller‐scale movements within Lac St. Louis were demonstrated, highlighting a yellow‐eel home range far more extensive than previously reported in smaller systems. Evidence for within‐season homing and site fidelity is also reported.     

Effect of aromatable androgen (17‐methyltestosterone) on induced maturation of silver European eels (Anguilla Anguilla): Oocyte performance and synchronization

The reproductive performances of silver European eel in term of gonad development and egg production, employing slow‐release implants with the androgen 17‐MT (1 mg) in combination with traditional weekly injection of carp pituitary extract (CPE) was evaluated. Wild female European eels (Anguilla anguilla) underwent a standard induction protocol with CPE and were randomly divided into three groups (N‐group, no implant; Y‐group, with implant; and control, C‐group, no treatment). The results showed that 17‐MT‐treated females (Y‐group) reproduced spontaneously about 6 weeks earlier than the N‐group females with a saving of almost 40% in CPE and time of induction. Concerning artificial induction of maturation in female silver eels, our study demonstrated that they positively respond to androgen exposure also in terms of eggs productivity. Indeed, Y‐group was more productive than N‐group: in Y‐group, 11 eels ensured an eggs production that exceeded 50% of initial body weight (BW), whereas in N‐group only three eels have exceeded this value. The results suggest that 17‐MT should be considered in future protocols for the improvement of the artificial reproduction of female silver European eels.     

Exotic silver eels Anguilla anguilla in Japanese waters: seaward migration and environmental factors

To understand the ecology of the exotic silver European eel Anguilla anguilla introduced into Japanese waters, the migratory behavior of 106 specimens captured in the coastal waters of Japan between April 1997 and March 2002 was analyzed. Their migratory behavior was apparently correlated with various environmental factors, particularly photoperiod, water temperature, lunar phase, and passage of atmospheric depressions, and was similar to the behavior of the species in European waters. These findings suggest that transplanted European eels retain their ability to respond to environmental cues for seaward migration in similar temperate habitats. The timing of the migration of silver European eels coincided with that of the native Japanese eels A. japonica, suggesting that the silver European eel was synchronized physiologically with the native eel by the same environmental factors.     

Detection of Herpesvirus anguillae during two mortality investigations of wild European eel in England: implications for fishery management

Herpesvirus anguillae (HVA) was detected during disease investigations of European eel, Anguilla anguilla L. at two stillwater fisheries in central England. These represent the first records of HVA from UK eels. Both mortalities were eel‐specific and took place during August 2009 and July 2010 at water temperatures between 17 and 19.4 °C. Pathological changes consistent with HVA infection included haemorrhaging in the fins, skin lesions and necrosis within the gills and liver. Transmission electron microscopy revealed active virion replication within the gill tissue. An initial assessment of risk is presented, indicating that HVA represents a high disease risk to UK eel stocks. However, further studies are required to establish the distribution of HVA before a reliable assessment of impact may be obtained. Until then, the detection of HVA holds important implications for eel conservation and management, in particular eel stocking activity.     

Mortality of European eel after downstream migration through two types of pumping stations

Although numerous pumping stations (PS) have been used by water managers for numerous applications on rivers, canals and other water bodies, their impact on fish populations is poorly understood. This study investigates European eel, Anguilla anguilla (L.), mortality after natural downstream passage through a propeller pump and two Archimedes screw pumps at two PSs on two lowland canals in Belgium. Fyke nets were mounted permanently on the outflow of the pumps during the silver eel migration periods. Based on the condition and injuries, maximum eel mortality rates were assessed. Mortality rates ranged from 97 ± 5% for the propeller pump to 17 ± 7% for the large Archimedes screw pump and 19 ± 11% for the small Archimedes screw pump. Most injuries were caused by striking or grinding. The results demonstrate that PSs may significantly threaten escapement targets set in eel management plans.     

Migration of silver eel,Anguilla anguilla,through three water pumping stations in The Netherlands

European eel, Anguilla anguilla (L.), migrating to sea encounter many man-made structures that can hamper and delay migration or induce mortality. Three pumping stations in Friesland, the Netherlands, were covered with acoustic receivers. Ninety-three silver eels tagged with acoustic transmitters were released in the polders upstream of the stations and 89% were detected passing a pumping station. The majority of silver eels passed the stations within a day after arriving at the station. Four silver eels stayed for longer than 2 weeks before passage, and 18 were detected at the receiver downstream the pumping station for more than one day, with detections up to several weeks. These detections probably indicated a dead eel, but could also indicate a live eel remaining at the site. Most of the silver eels passed the pumping stations within a day after release, so fish-friendly pumps will benefit the migrating population most. In the Netherlands, there are several thousand pumping stations. Installing fish passages near these stations is not feasible due to high costs. Prioritising all these sites in relation to the degree of blockage, mortality rates and its relative importance for migratory fish, can maximise the effectiveness of measures and mitigation taken.