European Eel Stock Research Articles

План по сохранению запасов европейского угря в Калининградском (Вислинском) заливе

In 2007, the European Commission initiated the Eel Recovery Plan (Council Regulation No. 1100/2007) to try to bring the European eel stocks back to more sustainable adult levels and the return of the glass eel. Each EU Member State is required to develop a national eel management plan. These plans aim to achieve a return of silver eels to the spawning population equal to or greater than 40% of the potential biomass that could be obtained in the absence of anthropogenic disturbances related to fishing, water quality or barriers to migration. The advantage of this approach is that it offers a standard basis and allows for the integration of stock status data between EU and non-EU countries.
 The purpose of this Plan was to determine for the Vistula Lagoon within the territory of the Kaliningrad region of the Russian Federation:
 1. Management objectives based on an assessment of the potential stock of the silver eel in the absence of anthropogenic mortality and high (before 1980) recruitment levels.
 2. The current level of the stock of silver eel in relation to the planned indicator (ie assessment of compliance with the planned indicator).
 3. Control actions necessary to achieve or maintain this compliance.
 4. A set of data needed to support the steps 1-3 above and to demonstrate whether compliance will be achieved in the future, i.e. that the actions defined in the management plan will lead to the recovery of the eel population.

Identification of virus infections of European eels intended for stocking measures.

The spread of viral diseases in eels is suggested to severely affect the European eel (Anguilla anguilla) panmictic population. The European Commission has initiated the Eel Recovery Plan (Council Regulation No. 1100/2007) to try to return the European eel stock to more sustainable levels within that measures eel restocking. However, scientific evidence evaluating the efficacy of stocking remains scarce. In addition, knowledge about the impact and contribution of eel stocking on the distribution of infectious diseases is insufficient. In this study, we aimed to investigate virus infections in batches of eels intended for restocking. We analysed samples of glass eels from certified fisheries and farmed European eels from different aquaculture farms. All analysed eels were purchased within a North Rhine Westphalian conservation program. Via a combination of cell culture and qPCR-based techniques, we detected infections of glass eels with the rhabdovirus Eel Virus European X and anguillid herpesvirus 1 infections in farmed eels (10-15 cm).

In Memoriam of the Ichthyologist Saulius Stakėnas

‘In Memoriam of Ichthyologist and Scientist Saulius Stakėnas’ presents a scientific biography of the ichthyologist Saulius Stakėnas, doctor of biomedical sciences. Dr Saulius Stakėnas was born in Vilnius on 6 November 1971. He studied biology at the Faculty of Natural Sciences of Vilnius University and pursued his scientific career at the Institute of Ecology of Vilnius University (presently the Nature Research Centre). In 2002, Saulius successfully defended his doctoral dissertation ‘Habitat Selection Strategy of 0+ Fish in the Rivers of Lithuania’. In 2004–2005, he was building his career as a researcher at the Centre for Environment, Fisheries, and Aquaculture Science (CEFAS) in the United Kingdom (under the EU programme Marie Skłodowska-Curie Actions). After traineeship, Saulius took the position of a senior researcher at the Laboratory of Ecology and Physiology of Hydrobionts, Institute of Ecology of Vilnius University, and later the position of a senior researcher at the Nature Research Centre. Dr Saulius Stakėnas deserves to be named the pioneer in the application of the telemetry method for fish migration in Lithuania and an expert of the management and restoration of the stocks of European eel, Atlantic salmon, sea trout, and Atlantic sturgeon in the country. His scientific career encompassed a broad range of activities, including scientific supervision of master and doctoral students and participation in multiple national and international projects; he authored or co-authored over 30 peer-reviewed research articles and wrote multiple popular science publications. The community of the Nature Research Centre lost their colleague Dr Saulius Stakėnas on 16 November 2021.

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.

Otolith morphology and microchemistry fingerprints of European eel, Anguilla anguilla (Linnaeus, 1758) stocks from the Adriatic Basin in Croatia and Montenegro

Morphological and elemental composition of otoliths are a promising tool for fish stock identification. In this regard, European eel has not been sufficiently researched. The aim of this study was to determine otolith shape index values and microchemistry composition of 14 elements in otoliths of European eel between riverine and lacustrine stocks from the Adriatic Basin in Croatia and Montenegro. In terms of morphometry, the shape indices, form factor and circularity of the sagittal otolith were statistically different between lacustrine and riverine stocks. The European eel is considered to have a small otolith relative size. Canonical variate analysis indicated no statistical differences among the concentrations of trace elements in sagittal otoliths between riverine and lacustrine stocks. However, constrained redundancy analysis suggested a stronger relationship of width, total length, females and the silver stage with the Sr/Ca, Cr/Ca, Ba/Ca and K/Ca ratios of specimens from riverine than lacustrine habitats. Gender and stage dependent variations showed that females were better indicators of relationships between microchemistry composition and intrinsic factors. In conclusion, form factor and circularity of the sagittal otolith was able to distinguish between European eel stocks from different habitats. Also, it seems that the European eels' otolith fingerprint is homogenous, species specific and independent of ecological conditions.

Economic development in times of population decline—a century of European eel fishing on the Swedish west coast

Abstract The European eel stock is in a multi-decadal decline. When fishing yield diminished throughout Europe, the small-scaled fyke-net fishery for eel on the west coast of Sweden gradually increased. This contrary trend lasted from the early-1900s, until the 1990s when fishing restrictions eventually limited the catch. We identified the processes driving this aberrant historical development. Using data on the fisheries from 1914 to 2006, we analysed the relation of total landed quantities to stock abundance indices, weather conditions, technical development, and market indicators. No relation between landed volumes and abundance indices was found, but market price (inflation-adjusted) was clearly correlated. Weather and technical development had a minor influence on landed volumes. This indicates that the commercial eel fishery on the west coast developed due to increasing demand and increasing eel prices. We found no evidence that the local stock has been fully exploited, though the increasing catch must have gradually reduced the contribution to the international spawning stock. These results stress the importance of considering economic processes when interpreting historical catch data as a source of information on population size in stock assessments, and ultimately, understanding the collapse of the eel stock.

A demographic model for the conservation and management of the European eel: an application to a Mediterranean coastal lagoon

AbstractWe present a general size- and age-structured demographic model of the continental phase of catadromous eels’ life cycle, allowing for the incorporation of (i) fluctuations of juvenile recruitment, (ii) density-dependent settlement and sex determination, (iii) plastic body growth and sexual dimorphism, and (iv) vital rates driven by body size rather than age. The model can be used to assess (i) standing stock, (ii) fishing yield, and (iii) spawner production under different recruitment and management scenarios. We demonstrate the model by applying it to a European eel stock (Camargue lagoons, southern France) that was continuously monitored for 17 years (1993–2009). Results indicate that the upper limit to settlement in the Camargue water system is ∼419 eels ha−1 per year. A balanced sex ratio occurs for a settlement of ∼94 eels ha−1, shifting towards males at higher densities. Catchability is higher in adult silver eels than in sub-adult yellow eels, possibly due to behavioural differences. Estimated standing stock ranged between 7 and 19 kg ha−1. Fishing yield was 5–7 kg ha−1 per year, while spawner escapement ranged between 0 and 13 kg ha−1 per year (<30% of what would occur in the absence of fishing and obstacles to migration).

Bigger is better in eel stocking measures? Comparison of growth performance, body condition, and benefit-cost ratio of simultaneously stocked glass and farmed eels in a brackish fjord

The recruitment of the European eel stock has collapsed and the stock is in a perilous state compared to the reference period between 1960 and 1979. Despite extensive European Union wide stocking efforts towards a stock recovery and a self-reproducing stock, recruitment stagnates at historical low levels. The aim of this study was to compare the most commonly used stocking forms (glass and farmed eels) in terms of their growth performance, body condition, and benefit-cost ratio to test whether stocking efficiency can be increased by the choice of the stocking form. Therefore, glass eels (117 kg) and farmed eels (1040 kg) were purchased in a cost ratio of 1:1 and then marked chemically with alizarin red S prior to stocking in a brackish Baltic Sea fjord. Two years after stocking, farmed eels (374 ± 36 mm; 86.9 ± 25.8 g) showed a significantly higher total length (TL) and body weight (W) than stocked glass eels (323 ± 39 mm; 56.8 ± 25.0 g). Moreover, within age group 2, no statistically differences in the specific growth rates for length and weight were found between stocking forms indicating that the initial advantage in TL and W of farmed recruits is likely to persist. Derived from the recapture ratio, the mortality of age 2 glass eels was 3.9 times higher than in farmed eels indicating a higher benefit-cost ratio for farmed recruits to refill local eel stocks more efficiently. However, the farmed recruits in this study have been found to be infected with the anguillid herpesvirus 1 which negates the conservation claim specified by the EU regulation.

The Economics of European Eel Management

The European eel stock is endangered. The European Union has, therefore, introduced strict policies to try to reverse the eel’s decline and reduce the threats to its survival. However, the European Union’s eel management policy has been implemented on a ‘one-size-fits-all’ basis, where all the affected countries have been given nearly identical targets, regardless of either the individual country’s costs for reducing damages to eels or its importance for the overall eel stock. In this paper, we draw on data from the different national eel management plans as well as from independent studies to compare the cost of measures to reduce eel mortality imposed in different countries. We compare the overall costs to those that could have been incurred with a union-wide, rather than fragmented, abatement program, and find that such a comprehensive management program would have been substantially cheaper and would have affected poorer member countries less.

Detection of Herpesvirus anguillae (AngHV-1) in European eel Anguilla anguilla (L.) originating from northern Poland-assessment of suitability of selected diagnostic methods.

The Community Action Plan requests EU member states to implement measures that ensure the recovery of the severely depleted European eel stocks. One of the main threats is posed by Anguillid herpesvirus 1 (AngHV-1) leading to increased mortality in both wild and farmed eels. Following recommendations of the OIE to minimize the risk of obtaining false-negative results, the main aim of the study was to optimize diagnostic methods for AngHV-1 detection using conventional PCR, nested PCR and insitu hybridization assay. While 53.3% of the individual organ samples were tested positive for AngHV-1 by PCR, the additional virus analysis via nested PCR revealed that the actual prevalence was 93.3%. In the cell cultivation passages, a cytopathic effect was hardly found in the first two rounds. In the third passage onto cell cultures, a lytic CPE was detected. The identification and confirmation of the viruses obtained from cell cultures as well as directly from the organ tissues were proceeded by PCR, nested PCR and sequencing of the PCR products. While no positive signal was detectable in the first round by PCR using samples from the third cell culture passages, the nested PCR provided weak but visible positive signals.

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

Abstract Several eel species have undergone extensive declines at both local and global level. The aim of this study was to identify the reasons for the collapse of the European eel (Anguilla anguilla) stock in an important area for biodiversity conservation (Comacchio Lagoon, Italy), in order to support the development of eel conservation plans. The records of silver eel catches from Comacchio describe the total migratory population and cover the period 1781–2013. The data are accompanied by information related to habitat loss and other local factors. The role of local factors on the decline of the local stock was investigated, while additional information from the literature was also used to discuss the effects of global factors (including glass eel harvest for aquaculture, climate–oceanographic changes, habitat loss, pollution and parasitism) on the three eel species A. anguilla, Anguilla japonica and Anguilla rostrata. The records from Comacchio provided significant information about the effects of local factors on the local eel populations in the past. However, the current population collapse, which started in the 1970s, could not be explained by local factors. The literature on global factors suggests that the three eel species are under combined threat from various factors. The correlations between European aquaculture production data compared with the Comacchio yields and published data from other European eel and glass eel fisheries were found to be highly significant. Aquaculture, which depends entirely on wild‐caught glass eels, seems to play a key role in the decline of natural stocks. Conservative estimates using FAO data showed that the current numbers of glass eels needed to support aquaculture production in Europe and Asia exceeds 2 × 109 specimens. This requirement, largely supplied by A. anguilla glass eels, can explain the decline of eel populations since the glass eel trade has been expanded at an international level. Copyright © 2016 John Wiley & Sons, Ltd.

Accumulation of neurotoxic organochlorines and trace elements in brain of female European eel ( Anguilla anguilla )

Xenobiotics such as organochlorine compounds (OCs) and metals have been suggested to play a significant role in the collapse of European eel stocks in the last decades. Several of these pollutants could affect functioning of the nervous system. Still, no information is so far available on levels of potentially neurotoxic pollutants in eel brain. In present study, carried out on female eels caught in Belgian rivers and canals, we analyzed brain levels of potentially-neurotoxic trace elements (Ag, Al, As, Cd, Co, Cr, Cu, Fe, Hg, MeHg, Mn, Ni, Pb, Sn, Sb, Zn) and OCs (Polychlorinated biphenyls, PCBs; Hexachlorocyclohexanes, HCHs; Dichlorodiphenyltrichloroethane and its metabolites, DDTs). Data were compared to levels in liver and muscle tissues. Eel brain contained very high amounts of OCs, superior to those found in the two other tissues. Interestingly, the relative abundance of PCB congeners markedly differed between tissues. In brain, a predominance of low chlorinated PCBs was noted, whereas highly chlorinated congeners prevailed in muscle and liver. HCHs were particularly abundant in brain, which contains the highest amounts of β-HCH and ϒ-HCH. p,p'-DDTs concentration was similar between brain and muscle (i.e., about twice that of liver). A higher proportion of p,p'-DDT was noticed in brain. Except for Cr and inorganic Hg, all potentially neurotoxic metals accumulated in brain to levels equal to or lower than hepatic levels. Altogether, results indicate that eel brain is an important target for organic and, to a lesser extent, for inorganic neurotoxic pollutants.

Management of the eel is slipping through our hands! Distribute control and orchestrate national protection

Following a multi-decadal decline of the European eel stock all across the continent, the EU adopted a protection and recovery plan in 2007, known as the Eel Regulation. Implementation, however, has come to a standstill: in 2015, the agreed goals had not been realised, the required protection had not been achieved, and from 2012 to 2015, no further reduction in mortality has been accomplished-while the stock is at a historical minimum. To analyse this manifest impasse, this article characterises the steering framework of the Eel Regulation as a governance problem. The Eel Problem is found to be extremely complex, due to many knowledge uncertainties and countless societal forces having an influence. The Eel Regulation divides this complexity along geographical lines, obliging national governments to implement national protection plans. This deliberate distribution of control has improved communication between countrymen-stakeholders, and has stimulated protective action in most EU Member States and elsewhere. In the absence of adequate international coordination and feedback on national plans, however, coherence is lacking and the common goals are not met. Actions and achievements have been assessed at the national level, but these assessments have not been evaluated internationally. Full geographical coverage has not been attained, nor is that plausible in future. Meanwhile, ICES' advice remained focused on whole-stock management, a conservative approach not matching the structure of the Eel Problem or the approach of the Eel Regulation. Hence, essentially localised problems (non-reporting, insufficient action) now lead to a hard fail, paralysing the whole European eel recovery plan. Here, I argue that immediate re-focusing protective actions, assessments, evaluations and advice on mortality goals and indicators, for each management area individually, will enable feedback on national protection plans, and in that way, will break the impasse.

Assessment of the Entering Stock, Migration Dynamics and Fish Pass Fidelity of European Eel in the Belgian Meuse River

AbstractMigration dynamics of incoming eels in Belgium via Lixhe in the Meuse River were investigated using two fish passes with different configurations—net traps and automatic detection stations—as tools to distinguish resident and migrating eels. From April to September 2013, 435 eels (P50 length, 403 mm; range, 196–836 mm) were caught (daily maxima catch, 90 eels per day), 90% between 13 June and 1 August (50 days) and P50 on 19 July. Eels migrated mostly at 19–26 °C (P50, 24.4 °C), river discharge 65–314 m3 s−1 (P50, 84 m3 s−1), during the dark at 00:00–05:00 h and during both the waxing and waning phases of moonlight. From 396 eels tagged and released 0.3 km downstream of the Lixhe dam, 6.8% of them were recaptured, and 37.4% were detected. Migration flux was estimated at 7184 eels (0.863 t) using the mark‐recapture method and decreased to 1156 eels (0.139 t) using automatic transponder detection. Most eels probably migrated through a sluice located downstream of Lixhe to reach the upper Meuse via the Albert Canal. Eels moved almost independently to the configuration of the fish passes and their location, but most eels displayed fidelity to the fish pass where they were captured. Migrant eels showed a wide range of size and life stages, with a higher proportion of eels (80%) belonging to the yellow eel stage. A lower proportion of eels (6%) had a larger size and presented an advanced continental silvering process corresponding to the migrating stage before their transatlantic migration. Copyright © 2016 John Wiley & Sons, Ltd.

First evidence of European eels exiting the Mediterranean Sea during their spawning migration.

The migration route and the spawning site of the European eel Anguilla anguilla are still uncertain. It has been suggested that the Mediterranean eel stock does not contribute to spawning because there is no evidence of eels leaving the Mediterranean Sea. To test this hypothesis, we equipped eight female silver eels from the south of France with pop-up satellite tags during escapement from coastal waters. Once in deeper water, the eels quickly established diel vertical migration (DVM) between the upper and lower mesopelagic zone. Five tagged eels were taken by predators within the Mediterranean, but two eels reached the Atlantic Ocean after six months and at distances greater than 2000 km from release. These eels ceased their DVM while they negotiated the Gibraltar Strait, and remained in deep water until they reached the Atlantic Ocean, when they recommenced DVM. Our results are the first to show that eels from Mediterranean can cross the Strait of Gibraltar and continue their migration into the Atlantic Ocean. This finding suggests that Mediterranean countries, as for other EU states, have an important role to play in contributing to conservation efforts for the recovery of the European eel stock.

Estimates of escapement, exploitation rate, and number of downstream migrating European eels Anguilla anguilla in Ichkeul Lake (northern Tunisia)

Abstract The European eel (Anguilla anguilla) stock has been declining for the last three decades and today, it is currently listed as endangered. The objective of this study was to quantify the escapement rate of silver eels to obtain an estimate of the future spawners migrating from the southern part of its distribution area in Tunisia. A mark–recapture experiment was conducted in Ichkeul Lake (Tunisia) between December 2013 and February 2014, covering the winter downstream run of eels. The size range of the downstream migrating eels was between 33 and 79 cm with 585.3 ± 156.77 g mean weight. The number of migrating silver eels was estimated to 342 221 (297 956–386 486), corresponding to a biomass of 200.2 (174.3–226.1) t, with a density of 23.55 (20.51–26.6) kg ha−1. The commercial fishing rate was estimated to be 18.8% (16.6–21.5%), which translates into an escapement rate of 81.2% (78.5–83.4%). The ratio current/pristine escapement was 0.69 for the entire migration period.

A question of origin: dioxin-like PCBs and their relevance in stock management of European eels

The stock of European Eel (Anguilla anguilla L.) has reached an all-time low in 2011. Spawner quality of mature eels in terms of health status and fitness is considered one of the key elements for successful migration and reproduction. Dioxin-like Polychlorinated Biphenyls (dl-PCBs) are known persistent organic pollutants potentially affecting the reproductive capability and health status of eels throughout their entire lifetime. In this study, muscle tissue samples of 192 European eels of all continental life stages from 6 different water bodies and 13 sampling sites were analyzed for contamination with lipophilic dl-PCBs to investigate the potential relevance of the respective habitat in light of eel stock management. Results of this study reveal habitat-dependent and life history stage-related accumulation of targeted PCBs. Sum concentrations of targeted PCBs differed significantly between life stages and inter-habitat variability in dl-PCB levels and -profiles was observed. Among all investigated life stages, migrant silver eels were found to be the most suitable life history stage to represent their particular water system due to habitat dwell-time and their terminal contamination status. With reference to a possible negative impact of dl-PCBs on health and the reproductive capability of eels, it was hypothesized that those growing up in less polluted habitats have a better chance to produce healthy offspring than those growing up in highly polluted habitats. We suggest that the contamination status of water systems is fundamental for the life cycle of eels and needs to be considered in stock management and restocking programs.

Climbing back up what slippery slope? Dynamics of the European eel stock and its management in historical perspective

Abstract Few fish stocks are as influenced by (intentional and inadvertent) human impacts as the European eel, all across the continent. The dynamics of this stock, however, are poorly understood—neither the causes of the historically low abundance, nor minimal protection levels, are beyond discussion. Rather than analysing contemporary processes, this article turns back in time—two centuries or more—unravelling historical abundances and distribution patterns; reviewing historical actions and objectives; and discussing technical developments and scientific advice—picturing the slippery slope the eel stock has come down from. The first claim, that the continental stock was in decline, dates from the early 1800s; stock-enhancement actions were initiated shortly after. Diffuse objectives, technical innovations, eternal optimism, and—above all—no quantification impede the exact evaluation of historical reports. After 1950, when quantification improved, a slow but consistent decline was observed, but it is only two decades after the crash in glass eel recruitment (in 1980), that protection plans addressed the bad status of the stock. A slippery slope, full of pitfalls—yet, we now observe several years of increasing recruitment.

A global viability assessment of the European eel.

The global European eel (Anguilla anguilla) stock is critically endangered according to the IUCN, and the European Commission has urged the development of conservation plans aimed to ensure its viability. However, the complex life cycle of this panmictic species, which reproduces in the open ocean but spends most of its prereproductive life in continental waters (thus embracing a huge geographic range and a variety of habitat types), makes it difficult to assess the long-term effectiveness of conservation measures. The interplay between local and global stressors raises intriguing cross-scale conservation challenges that require a comprehensive modelling approach to be addressed. We developed a full life cycle model of the global European eel stock, encompassing both the oceanic and the continental phases of eel's life, and explicitly allowing for spatial heterogeneity in vital rates, availability of suitable habitat and settlement potential via a metapopulation approach. We calibrated the model against a long-term time series of global European eel catches and used it to hindcast the dynamics of the stock in the past and project it over the 21st century under different management scenarios. Although our analysis relies on a number of inevitable simplifying assumptions and on data that may not embrace the whole range of variation in population dynamics at the small spatiotemporal scale, our hindcast is consistent with the general pattern of decline of the stock over recent decades. The results of our projections suggest that (i) habitat loss played a major role in the European eel decline; (ii) the viability of the global stock is at risk if appropriate protection measures are not implemented; (iii) the recovery of spawner escapement requires that fishing mortality is significantly reduced; and (iv) the recovery of recruitment might not be feasible if reproductive output is not enhanced.

A user-friendly tool to assess management plans for European eel fishery and conservation

The European eel Regulation EC 1100/2007 establishes measures to recover the European eel stock. The Regulation requires Member States to guarantee a spawner escapement ≥40% of pristine levels by reducing eel mortality. The complexity and plasticity of eel life history make it difficult to assess the effectiveness of alternative management options, and tools allowing decision makers and fishermen to quickly assess the effectiveness of proposed management scenarios are urgently needed. We used state-of-the-art knowledge to develop a user-friendly simulation software allowing users to evaluate if current management policies meet the conservation target and evaluate the expected performances (spawner escapement and fishing yield) of alternative management scenarios. The software relies upon a demographic model explicitly accounting for the most relevant features of eel demography and has default settings for specific geographical areas and water systems. We demonstrate the software by exploring a variety of management plans in three European water systems.