Tracking the migratory success of stocked European eels Anguilla anguilla in the Baltic Sea

Slower growth of farmed eels stocked into rivers with higher wild eel density

Farmed anguillid eels are frequently stocked into natural fresh waters to enhance eel resources, but little is known about what happens to these eels or their interactions with wild eels after stocking. A recent study observed a depressed survival and growth rate of farmed Japanese eels when they were reared with wild eels, which indicated that wild eels might interfere with the survival and growth of farmed-and-stocked eels through intraspecific competition. To contribute to improving eel stocking efficiency, the growth of farmed-and-stocked Japanese eels was compared among four rivers with different wild eel densities using mark-and-recapture studies. Based on the 2-year recapture survey after stocking, it was found that the density of the farmed-and-stocked eels was not significantly different among rivers. The daily growth rates of farmed-and-stocked eels in the rivers with lower wild eel density were significantly higher than those of the eels stocked into the rivers with higher wild eel density. The farmed-and-stocked eels moved significantly greater distances downstream than wild eels that showed sedentary behaviour. This and previous studies indicate that significant questions remain about the effectiveness of stocking farmed eels into water bodies where naturally recruited wild eels are present.

Agonistic behaviour of wild eels and depressed survival and growth of farmed eels in mixed rearing experiments

To contribute to improving eel stocking procedures, the survival, growth and behaviour of farmed Japanese eels reared together with wild individuals were evaluated to learn about possible encounters of farmed- and stocked eels with wild eels. Two experiments were conducted to evaluate the (a) effect of the presence of wild eels on survival and growth of farmed eels in experimental ponds and (b) behavioural differences between wild and farmed eels placed in a small tank. In the survival and growth experiment, significantly lower survival and growth rates of farmed eels reared with wild eels were observed compared with farmed eels reared without wild eels (control). In the behavioural observations, the authors observed significantly higher occupation rates of a refuge and more frequent biting for wild eels, indicating a dominance of wild eels over farmed eels. Because wild and farmed eels used in the current experiments belong to the same genetic population, these differences resulted within one generation through the aquaculture rearing process.

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.

What are the best upland river characteristics for glass eel restocking practice?

The fitness of restocked European eel (Anguilla anguilla), an endangered fish species, was studied in relation to the environmental variables of habitats in six upland rivers that are typologically different in terms of their hydromorphological and physicochemical characteristics, food resources and fish communities. These rivers received a total of 76,370 imported glass eels in 2017. During a three-year period, we monitored eels with respect to total length, annual growth rate, condition factor and density using capture-mark-recapture experiments to understand the effects of the characteristics of receiving rivers on restocking success levels. Our results showed the survival of the restocked eels in the six rivers and revealed significant differences between them in terms of total length, condition factor and density. Better performance in eel yield variables was observed in a eutrophic alkaline river with greater roughness of riverbed substrates, dominant pool- and riffle-type flow facies and lower brown trout density. The variables conductivity and total hardness had higher explanatory power and were strongly associated with increased eel density. This study suggests that a well-selected habitat/river in a restocking programme can be beneficial for the species and recommends restocking practice as a management tool to achieve eel conservation goals.

An Evaluation of Restocking Practice and Demographic Stock Assessment Methods for Cryptic Juvenile European Eel in Upland Rivers

Restocking of the critically endangered European eel Anguilla anguilla is widespread, but it is rarely scientifically evaluated. Methods used to assess its associated performance by estimating the survival rate and implement restocking for maximum recruitment in rivers have not yet been investigated. Based on two glass eel restocking events using a single release site/point and multiple sites per river performed in upland rivers (>340 km from the North Sea), the recruitment success of stocked eels was scientifically evaluated during a 3-year study using multiple capture-mark-recapture methods and mobile telemetry. We compared the observed data with the data estimated from the Telemetry, De Lury and Jolly-Seber stock assessment methods. For recruitment data, Telemetry was very close to Jolly-Seber, an appropriate stock assessment method for open populations. Using the best model of Jolly-Seber, survival probability was higher (>95%) in both restocking practices, but recruitment yields were higher and densities of stocked eels were lower in multiple sites compared to a single site. Our results suggest that Telemetry can help to rapidly assess cryptic juvenile eel stocks with good accuracy under a limited number of capture-mark-recapture sessions. Artificial dispersal of glass eels on several productive habitats/sites per river appears to be the better-suited practice for restocking.

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.

Introduced American eels Anguilla rostrata in European waters: life-history traits in a non-native environment

This study investigated growth, condition and development of American eels Anguilla rostrata that were introduced into a European river to estimate their competitive potential in a non-native habitat. Results demonstrate that A. rostrata develops normally in European waters and successfully competes with the native European eel Anguilla anguilla. In addition, A. rostrata appears to be more susceptible to the Asian swimbladder nematode Anguillicola crassus than A. anguilla and could support the further propagation of this parasite. Detected differences in fat content and gonad mass between Anguilla species are assumed to reflect species-specific adaptations to spawning migration distances. This study indicates that A. rostrata is a potential competitor for the native fauna in European fresh waters and suggests strict import regulations to prevent additional pressure on A. anguilla and a potential further deterioration of its stock situation.