Eel Management Plan Research Articles

Escapement success of silver eels from a German river system is low compared to management‐based estimates

Summary The European eel (Anguilla anguilla) stock experienced a sharp decline during the last decades and is suffering from massive anthropogenic impacts on inland waters. To evaluate the benefit of management measures and to better understand the contribution of single drainage systems to spawner production, knowledge of the respective silver eel escapement is required. Furthermore, a better understanding of environmental conditions that potentially trigger the onset of spawning migration is needed to reduce anthropogenic mortalities during riverine eel migration. Investigations are also necessary to clarify whether fish protecting devices and bypasses at barriers are functional and truly increase eel survival and escapement rates. In this study, total female silver eel escapement from a northern German drainage system (Schwentine River) was assessed over a period of three consecutive years, and downstream migration patterns were compared to potential environmental triggers. Furthermore, the benefit of two fish bypasses (surface and deep) and a trash rack at the hydropower station for the survival of migrating eels was examined, and the spawner quality of escaping silver eels was determined by analysing lipid content and infection intensities with the swimbladder parasite Anguillicoloides crassus. The results indicate that silver eel escapement from the Schwentine drainage system is far below the estimated values underlying the respective eel management plan, highlighting the necessity of direct migration assessments to validate indirect estimations that include multiple assumptions and uncertainties. Major downstream migration events took place during short time periods in autumn and appear to be influenced by river discharge and water temperatures, suggesting that a precise prediction of escapement events is possible. Regarding spawner quality, fat reserves appear sufficient for escaping silver eels to migrate and spawn. However, high A. crassus prevalence and infection intensities are assumed to further reduce the number of potential spawners. Another matter of concern is the high trash rack mortality at the hydropower station that illustrates the need of fish protecting devices that fulfil eel‐specific requirements.

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

AbstractAlthough 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.

The home range and behaviour of yellow‐stage European eel Anguilla anguilla in an estuarine environment

Abstract The behaviour patterns of yellow‐stage European eels Anguilla anguilla within the tidal waters of the rivers Frome and Piddle, and the neighbouring Poole Harbour estuary (Dorset, England) were examined from July to December using acoustic transmitters and a fixed hydrophone array. Point‐to‐point measures of range varied between 630 m and 4150 m, with no relationship between eel size and range. Eight eels displayed repeated circuits, travelling distances of 440 m to 5060 m over a few hours. Activity was generally, but not exclusively, nocturnal, with the start and end times closely associated with sunset and sunrise, respectively. Neither direction of travel nor average ground speed was influenced by tidal flow direction, and seasonal declines in water temperature did not appear to influence behaviours. These results on distance travelled during regular, nocturnal movements provide valuable insights into the spatial and temporal distribution of yellow eels in an estuarine environment, which can aid the design of survey programmes in terms of the range over which to deploy fishing gears and the number of nights they should be fished. These survey programmes are urgently required to estimate eel production from saline environments for national Eel Management Plans, and the subsequent implementation of effective conservation measures. The strong influence of light regime on the behaviour patterns of eels points towards control of artificial illumination (light pollution) being an important eel conservation measure in this environment, at least. Copyright © 2013 Crown copyright.

Traceability of stocked eels – the Swedish approach

AbstractStocking of eels is one of the management measures in the eel regulation (EC No 1100/2007) to recover the stock. The Swedish Eel Management Plan doubles the numbers stocked to 2.5 million eels per year. Whether stocked eels contribute to the spawning stock or not has been questioned: stocked eels might not migrate as successful as wild recruited eels. The EIFAAC/ICES Working Group on Eel (2011) recommended ‘that all stocked eel should be marked and thereby separable from wild eel in subsequent sampling’. Since 2009, eels stocked in Sweden have been bathed in a strontium (Sr) solution, which gives a detectable mark in their otoliths. So far, 5.5 million eels have been marked in Sweden; Finland imports eels for stocking via Sweden, and these 0.6 million eels were marked by two Sr rings. We present results on marking success and recapture rates and also from marking with alizarin complexone and PIT tags in combination with Sr. If all eels stocked in the Baltic are marked, their contribution to the spawning run can be estimated. Using different combinations of marks in different regions, the relative contribution from separate stocking programmes can be evaluated. To increase the set of suitable marks, barium was tested as an additional tracer.

Model prediction vs. reality—testing the predictions of a European eel (Anguilla anguilla) stock dynamics model against the in situ observation of silver eel escapement in compliance with the European eel regulation

Abstract Prigge, E., Marohn, L., Oeberst R., and Hanel, R. 2013. Model prediction vs. reality—testing the predictions of a European eel (Anguilla anguilla) stock dynamics model against the in situ observation of silver eel escapement in compliance with the European eel regulation – ICES Journal of Marine Science, 70: 309–318. A direct monitoring of European silver eel (Anguilla anguilla, L) escapement from rivers and estuaries has been proven to be challenging, and a Europe-wide documentation of escaping silver eel numbers therefore hardly seems realistic. To reinforce management decisions, policy-makers are thus widely reliant on the accuracy of escapement models. A 3-year programme of silver eel escapement monitoring was undertaken to compile model input data and revise an eel population model (German Eel Model II; GEM II) already used in the decision-making process of management authorities. By compiling necessary input data and analysing vital system-specific population characteristics, it was possible to compare the documented silver eel escapement with the modelled potential silver eel escapement. Resulting model predictions were close to actually monitored escapement numbers, which were distinctly lower than reference escapement values for the same freshwater system given in the implementation report of the German Eel Management Plans. Applying different commercial and recreational catch scenarios revealed the sensitivity of the model. The results show the potential of the GEM II and highlight the importance of high-quality input data to use model predictions as the basis for management measures.

Will regionalisation improve fisheries management in the EU? An analysis of the Swedish eel management plan reflects difficulties

Abstract In response to the shortcomings of the EU's Common Fisheries Policy, the European Commission has suggested regionalising fisheries management. Examples already exist of the more decentralised management of certain fisheries. Since 2007, the Commission has requested national eel management plans (EMPs) from all EU eel-fishing nations, giving national management bodies considerable freedom to develop their own EMPs. To examine the prerequisites for decentralisation, the Swedish EMP was chosen as a case. The European eel is critically endangered due to overfishing and environmental degradation. Analysis of the Swedish EMP reveals serious flaws: the conflict between the objectives of species and fishery preservation has not been clarified nor is the prioritisation clear. The plan has not been critically reviewed and alternative options are not considered. Though the basic data are uncertain, this uncertainty is viewed as support for not adopting any safety margin. Management is therefore directed towards mitigating the negative effects of fishing and other human activities rather than realising the conservation objective. The efficiency of the various protection measures is also disputable; for example, translocation is problematic, as translocated (i.e., stocked) eels display impaired navigational abilities. Another problematic aspect of this conservation strategy is the slow implementation of the EMP. In conclusion, this study emphasises the necessity of legal and social science as well as natural science research to evaluate the efficiency and implementation of fishery management.

On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: the River Severn

Abstract Aprahamian, M. W., Walker, A. M., Williams, B., Bark, A., and Knights, B. 2007. On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: the River Severn. – ICES Journal of Marine Science, 64: 1472–1482. The European eel stock has declined significantly since the 1980s, and the Eel Recovery Plan of the European Commission requires Member States to develop river basin Eel Management Plans (EMPs) that will achieve an escapement of silver eels that equals or exceeds 40% of the escapement biomass that would be produced in the absence of human activities. However, because silver eel escapement is not quantified within the UK, a modelling approach is required to estimate potential and actual escapement, and to assess the likely effects of management measures. We focus on two approaches developed in the UK, the Reference Condition Model (RCM) and the Scenario-based Model for Eel Populations (SMEP), and illustrate how such approaches can be used in EMPs using selected data from the River Severn. The RCM results indicate that the yellow eel population in the River Severn basin may be just 30–40% of the potential density indicated by reference conditions derived from a selection of rivers between the late 1970s and the early 1980s. The challenges of applying a model designed to be as realistic of eel production as possible, and the limited data on natural eel habitat and eel production in the Severn, preclude a SMEP analysis similar to that of the RCM, but simulations based on a simplified basin description and eel survey data from the early 1980s illustrate the potential of this model to assess compliance and test management scenarios.

Multi-objective assessment of conservation measures for the European eel (Anguilla anguilla): an application to the Camargue lagoons

Abstract Bevacqua, D., Melià, P., Crivelli, A. J., Gatto, M., and De Leo, G. A. 2007. Multi-objective assessment of conservation measures for the European eel (Anguilla anguilla): an application to the Carmargue lagoons. – ICES Journal of Marine Science, 64: 000–000: –. The European eel (Anguilla anguilla) stock has declined since the early 1970s and is currently considered to be outside safe biological limits. The European Commission has proposed a regulation (COM 2005/472 final) to establish measures for the recovery of the stock, with the aim of achieving an escapement to the sea of 40% of the adult eel biomass (with respect to undisturbed conditions) from each river basin. The proposed regulation imposes an effective reduction of fishing activities until implementation of an approved eel management plan. We use a demographic model, explicitly accounting for age, length, and sex structure, and important features of the continental phase of eel life cycle, to assess the effectiveness of the proposed regulation. We explore alternative management options with reference to the Camargue (southern France) eel population. Using multi-criteria methods, we compare different fishing policies with respect to two potentially conflicting objectives: preserving a sufficient spawner escapement and guaranteeing an acceptable harvest to fishers. We show that the current fishery is inefficient and that appropriate management policies (such as limiting the fishing season and increasing the mesh size of fishing gear) are likely to have a doubly positive effect, by achieving the conservation target of the regulation and increasing fisher revenues.