A caution for conservation stocking as an approach for recovering Atlantic eels

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Several possible spawning sites of the Japanese eel determined from collections of their eggs and preleptocephali

The spawning area of the Japanese eel is located at the southern part of the West Mariana Ridge in the western North Pacific, but their spawning events have not been observed. To further understand Japanese eel spawning ecology, an interdisciplinary research survey by the R/V NATSUSHIMA (NT14-09, 14 May–4 June 2014) was conducted to detect spawning sites based on the seamount, salinity front, new moon and third quadrant (spawning south of front, west of ridge) hypotheses. Attempts were made to film spawning events with underwater camera systems and to consider if eels might be detected in hydroacoustic observations. Although no Japanese eels or spawning events were video-recorded and no eel aggregations could be clearly identified acoustically, three eggs were collected at two stations in the third quadrant region at or just south of 13° N on 26 and 27 May. Three or four days later, newly hatched preleptocephali were collected at two stations far to the south, including 224 at a station > 160 km southwest of the egg catches, and a few preleptocephali were caught at two stations closer to the egg stations. The eggs and southern preleptocephali were from discrete spawning events, which indicated that at least two spawning sites occurred in May 2014.

     

Long‐term records (1781–2013) of European eel (Anguilla anguilla L.) production in the Comacchio Lagoon (Italy): evaluation of local and global factors as causes of the population collapse

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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).     

Occurrence of estuarine and sea eels <Emphasis Type="Italic">Anguilla japonica</Emphasis> and a migrating silver eel <Emphasis Type="Italic">Anguilla anguilla</Emphasis> in the Tokyo Bay area,Japan

Otolith microchemical analyses of the strontium (Sr) and calcium (Ca) concentrations in the eels Anguilla japonica and A. anguilla caught in Tokyo Bay were undertaken to reconstruct the eels’ migratory histories. A. japonica in the yellow stage (immature stage) were caught in a bay without any adjacent rivers or streams. A. anguilla was in the silver stage (early maturing stage), and the eel was confirmed to have just begun spawning migration to the open ocean from Tokyo Bay based on the otolith Sr:Ca ratios, which showed a typical catadromous life history with low Sr:Ca ratio values throughout the eel’s life after recruitment. The mean Sr:Ca ratios in A. japonica from the elver mark to the otolith edge indicated the eels belonged to several general categories of migratory histories, including sea eels (average Sr:Ca ratio ≥6.0 × 10⁻³) and estuarine eels (average Sr:Ca ratio 2.5 to 6.0 × 10⁻³) based on the criteria reported previously in A. japonica. All eels had a certain freshwater life period, although the period was highly variable among fish. These results indicate that A. japonica has a flexible pattern of migration, with the ability to adapt to various habitats and salinities.     

Video observation of an eel in the Anguilla japonica spawning area along the West Mariana Ridge

Some anguillid spawning areas are known based on collections of small larvae, but recently for the Japanese eel Anguilla japonica, adult spawners have been caught in trawls and their eggs and preleptocephali collected. The spawning area of A. japonica is located along the western side of the West Mariana Ridge, but the natural spawning behavior of this species or that of any other anguillid species has never been observed. This study reports on the first effort to observe spawning aggregations of anguillid eels that was conducted by the R/V Yokosuka using the Shinkai 6500 submersible and a Deep-Tow camera system in the A. japonica spawning area in July 2012. The submersible was deployed mostly at 200–800 m during daytime and the Deep-Tow was deployed mostly at 130–250 m during nighttime, both in multiple oblique depth tracks along linear transects. Various fishes and invertebrates were seen in the pelagic environment during day and night, but no spawning aggregations were observed. One eel was briefly recorded by a Deep-Tow camera at 20:13 on 17 July (2 days before new moon) at a depth of 179 m. The eel was recorded for <1 s as it passed in front of the camera. Its anterior body and head shape were consistent with a male A. japonica, or possibly a Derichthys serpentinus eel, but not with other mesopelagic eels. Because the tail region of the eel was not visible, species identification was not possible.     

The development of stocked eels (Anguilla anguilla) in previously eel‐free Austrian Alpine lakes

Over a period of 34 years (1954–1987), a total of 11.9 million glass eels were stocked all over Austria to improve the yields of commercial fishermen and to make the lakes more attractive for anglers. Nearly half (48%) of the imported glass eels were introduced into natural, previously eel‐free Alpine lakes until stocking ceased in 1988. Twenty‐five years later, their occurrence could be verified in 56% (N = 22) of the Austrian Alpine lakes larger than 50 ha and mean body length had increased to 66 cm. Eels from seven selected Austrian Alpine lakes exhibited a yearly mean increase in length between 0.6 and 1.5 cm, indicating a comparatively poor growth compared with the majority of eel populations from within their natural distribution range. At the same time, high expectations in developing a flourishing eel fishery were not met. In all but one lake, the harvest remained below 3% of the total yearly catch and salmonids remained the dominant target species. During peak harvest in the 1980s, it exceeded 8% in mesotrophic Wallersee (2600 kg). Simulations of stock development indicated that the yield could have been improved, but contrary to earlier expectations eels never became important for local cuisine and catches remained too low for the development of a profitable export business. On the other hand, eels may have contributed to the loss of small‐bodied fish taxa.     

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.     

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.     

The impact of Anguillicola crassus on European eels

The impact of the parasitic swimbladder nematode, Anguillicola crassus Kuwahara, Niimi & Itagaki, on European eel populations is assessed with reference to published research on its origin and rapid dissemination, life cycle and transmission dynamics, and its pathogenic effect. The parasite was originally endemic to East Asia, but has transferred from its native host, the Japanese eel, Anguilla japonica Temminck & Schlegel, to the European eel, Anguilla anguilla (L.) and American eel, Anguilla rostrata (Le Seur). Anguillicola crassus is a very successful colonizer and is now known to occur in four continents (Asia, Europe, Africa and America). The nematode can severely impair swimbladder function and has caused mortalities in both farmed and wild populations in the presence of other stressors. Anguillicola crassus may impair the capacity of European eels to complete the spawning migration, although direct evidence is not available to support this hypothesis. Areas for future research are recommended.     

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.     

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.     

First capture of post-spawning female of the Japanese eel Anguilla japonica at the southern West Mariana Ridge

Two adult male freshwater eels, Anguilla japonica, were captured in June 2008 in the West Mariana Ridge (13°N, 142°E) in the North Pacific, but collections of females have yet to be reported. In September 2008, we successfully caught two adult female A. japonica, 55.5 and 66.2 cm in total length, in the adjacent but northern area (14°N, 143°E). Six newly hatched eel larvae (pre-leptocephali) were also collected by subsequent plankton sampling conducted near the female catch area. Female adults appeared to be in the post-spawning state, probably a considerable time after spawning, since a small number of remarkably regressed oocytes (50–250 μm in diameter) were observed in the ovaries. Capture of post-spawning female eels and newly hatched larvae near the Suruga Seamount (14°N) together with the previous collection of mature males in the southern area (13°N) corroborates that the area along the West Mariana Ridge is the spawning area of this species, but suggests that the eel spawning may occur over a wider area than previously expected.     

Synthesis and possible function of 11-ketotestosterone during oogenesis in eel (Anguilla spp.)

The control of 11-ketotestosterone (11-KT) biosynthesis and its physiological roles were examined in female Japanese eel (Anguilla japonica) and New Zealand longfinned eel (Anguilla dieffenbachii). 11-KT was detected in serum of female eels of both species. Among various tissues from Japanese eel, the ovary had the greatest capacity to synthesize 11-KT in vitro. In addition, the oocyte diameters of eels treated with 11-KT had increased significantly. Furthermore, these oocytes were found to have an increased number of oil droplets. These findings suggest that 11-KT in female eels may be mostly of ovarian origin and that this androgen appears to play an important role in controlling pre-vitellogenic oocyte growth.     

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.     

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.     

Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area

Due to a recruitment decline of more than 90% in 30 years, the European eel (Anguilla anguilla L.) has been classified by IUCN as critically endangered. Although the species has been studied intensively to obtain knowledge to improve management, studies about the resident yellow stage are relatively scarce. In this study, 52 large female yellow eels were tagged with acoustic transmitters in a Belgian polder system and tracked by a network of 23 automatic listening stations. We studied both circadian and seasonal movement patterns and the effect of environmental variables on these patterns. Large female yellow eels were most active at night in late summer and early autumn. A generalised linear mixed model showed that their movement is only slightly influenced by environmental variables. Moreover, as yellow eels show high site fidelity (i.e., the majority was detected only in the habitat type of their catch‐release location), they do not encounter many human‐induced connectivity problems in polder systems, which makes these systems highly suitable as eel growth habitat. These results can contribute to an effective eel management regarding habitat protection and restoration.     

Isolation and characterization of bacteria causing mortality in early stage larvae of captive‐bred Japanese eels (Anguilla japonica Temminck & Schlegel)

Bacteria with lethal effect on eel larvae were isolated from moribund captive‐bred eel larvae and their 16S rRNA gene sequences were analysed. Nucleotide sequence of 10 isolates showed highest similarity with Lacinutrix algicola, Crocinitomix catalasitica and Pseudoalteromonas rubra. Age‐dependent changes in the susceptibility of eel larvae were observed in response to challenge by a highly lethal isolate. Compared with 10 and 11 days after hatching (DAH), larval susceptibility to the highly lethal isolate was lower at 18 DAH. We found that the bacterial isolates have lethal effect on the captive‐bred eel larvae, especially at the early developmental stage. These results will be useful to establish appropriate culture practices for eel larvae that will improve the success of mass production of glass eels for aquaculture.     

Trophic overlap between non‐native brown bullhead (Ameiurus nebulosus) and native shortfin eel (Anguilla australis) in shallow lakes

We quantified trophic overlap between the invasive, non‐native catfish brown bullhead (Ameiurus nebulosus) and the New Zealand native shortfin eel (Anguilla australis) in four peat and riverine lakes using stable isotope (δ¹³C and δ¹⁵N) and gut content analyses. Across all lakes and fish sizes over the austral spring–summer period, shortfin eel guts were dominated numerically by fish prey (57% occurrence cf 42% in brown bullhead), while Diptera larvae were most commonly encountered in guts of brown bullhead (45% cf 14% in eels). Significant differences in % composition of animal contents in guts were detected between fish species and sampling occasions (n = 4) but not between lakes. In contrast, stable isotope signatures of brown bullhead and shortfin eel did differ significantly between lakes but not between sampling occasions, indicating enduring sources of nutrition despite apparently differing ingestion patterns over time. The R mixing model MixSIAR indicated that shortfins likely assimilated higher proportions of fish prey carbon compared to brown bullheads, which appeared to show greater assimilation of invertebrates, consistent with the results of gut content analyses. Isotopic niche regions, calculated in nicheROVER using probabilistic ellipses, indicated that shortfin eels occupied at least c.60% of brown bullhead trophic niche, which occupied less than 30% of eel trophic niche in all but one lake. These estimates suggest that brown bullhead has higher potential to influence shortfin eel nutrition than vice versa, or that a broad trophic niche occupied by eels provides resilience to the effects of overlapping consumption patterns with invasive omnivores.     

Species composition and diversity of fish larvae in the Subtropical Convergence Zone of the Sargasso Sea from morphology and DNA barcoding

Specific regions of otherwise oligotrophic oceans seem to attract fish spawning and sustain significant abundances of fish larvae. The Sargasso Sea in the North Atlantic subtropical gyre is known as the spawning area of the Atlantic eels, but numerous other fish species also spawn in the area. In order to evaluate spatial variability of larval fish in the region, we examined species diversity, composition and abundances at eight stations in the Subtropical Convergence Zone (STCZ) using morphological identification and DNA barcoding. From a total of approximately 3500 specimens collected, at least 154 species from 50 families could be identified. The family Myctophidae had the highest species richness, with at least 32 species represented. The myctophids Lepidophanes gaussi, Bolinichthys indicus, Notolychnus valdiviae and Ceratoscopelus warmingii were the four most abundant species. Other common species included the three eels: Nemichthys scolopaceus, Ariosoma balearicum and Anguilla anguilla. Larval fish species composition differed substantially between the relatively closely spaced stations on either side of prominent hydrographic fronts in the study area, presumably because of the strong environmental gradients. Common eel species were concentrated between the fronts whereas common myctophids were of highest abundance at the outer edges of the fronts. The abundances of most species were generally enhanced in the vicinity of the fronts. The use of combined morphological and DNA‐barcoding identification methods facilitated species identification, and we could document substantially higher levels and a larger degree of spatial variability in species diversity of fish larvae than previously shown for oligotrophic ocean areas.