Silver eel escapement in the Lower Saxonian River Ems

Background and Objective

Due to a variety of factors, the European eel population has been declining for decades and is now in a critical state. Accordingly, the species is classified as "outside of safe biological limits" by the International Council for the Exploration of the Sea (ICES). In response to this development, the European Union adopted eel management plans (EMPs) to rebuild the stock under the common fisheries policy. The primary aim of this regulation is to increase the escapement of mature eels (“silver eels”), from coastal and inland waters. In order to achieve the management target, at least 40% of the biomass that would have existed without anthropogenic influences should be able to escape from each river basin in the EU. This is to ensure the recovery of the stock in the long-term by assuring sufficient numbers of potential spawning fish.

Due to the extensive spatial distribution of the eel across Europe and North Africa, monitoring of the entire stock is not feasible. In addition, quantitative surveys are complex and costly. Therefore, population models are frequently used to evaluate whether the escapement targets are met in a given river system. However, the input parameters of the German model-in-use (“German Eel Model”) are based on assumptions. The assumptions directly influence the model outcome, yet they cannot always be examined specifically for any river catchment.

Thus, it is necessary to verify the model predictions of silver eel escapement numbers through empirical studies to evaluate the success of management measures. A comparison of the model prediction with observed numbers of escaping silver eels allows conclusions about the quality of the German Eel Model and potential need for revision.

In addition, the migration behavior of eels in relation to their developmental stage (i.e., degree of silvering) and to environmental conditions will be investigated. An acoustic telemetry system will provide insights about possible migration obstacles (e.g., navigation locks) in the River Ems. The results promise a better understanding of the river-specific migration dynamics and help to improve the conservation and sustainable use of European eel in the River Ems.

Approach

Mark-recapture studies are commonly used to calculate the total number of silver eels escaping from a specific river system. Stow nets will be operated by a local fisherman at one specific location throughout the year and the catch will be documented. In the course of this work, the stage of maturity, body length and mass will be recorded and a growth profile of migrating silver eels will be determined from a sub-sample. A given number of migrating silver eels will be tagged both externally for recognition and internally with an acoustic transmitter, and will be released upstream of the fishing gear. The recapture rate of tagged fish will then be used to calculate the total number of migrating eels.

The use of acoustic telemetry will refine the absolute quantification of escaping silver eels by accounting for the proportion of tagged eels that interrupt their migration or remain in the water system after passing the monitoring station. In addition, receiver stations along the river provide insights into the migratory behavior depending on the degree of maturation of the silver eels and allow the identification of local migration obstacles. Moreover, the influence of environmental parameters (e.g., temperature, moon phase and river discharge) on the migration behavior will be analyzed in order to be able to predict not only the quantity but also the timing of eel migration using environmental data.

Our Research Questions

• Do predictions of silver eel escapement by the "German Eel Model" match the empirically estimated population size?
• How can the model be refined to improve its prediction accuracy?
• Does the migration behavior of eels differ across stages of maturity?
• What role do river-specific obstructions such as navigation locks play in causing migration bottlenecks?
• Can migration activity be predicted by environmental conditions?

Results

In the BALANCE project, eel escapement from the Ems was empirically surveyed over a period of two years by means of a mark-recapture experiment and a telemetry study and compared with the results of GEM modelling for the same area. In addition, studies were conducted on the migration behaviour of silver eels depending on different environmental factors.

The main migration period of silver eels extended from mid-September to the end of January in the 2020/2021 migration season and from mid-September to February in 2021/22. The start of the migration correlated with falling temperatures in autumn, whereby the flow velocity in particular played an important role in the migration dynamics. The results also show that silver eels caught in spring often interrupted their migration compared to individuals caught in autumn. This was especially true for eels of the pre-migration stage SI-III, which is why they should not be classified as migrating silver eels per se.

The project results show that the actual total escapement of silver eels from the Ems is significantly lower than assumed. The surveyed escapement figures reach only about 17 % of the values calculated by the GEM for the same area. The modelled estimate thus exceeds the real silver eel biomass by a factor of about six. In previous evaluations of the GEM, the silver eel escapement of eels was also frequently overestimated. A systematic error in the selection of input parameters (overly optimistic consideration) and/or faulty assumptions of the model itself are possible explanations for this, which is why a non-river-specific validated application of the GEM results for the implementation of German eel management must be advised against.