Stocking is essential to meet the silver eel escapement target in a river system with currently low natural recruitment

Abstract

Under the European Eel Regulation EG 1100/2007, Member States exhibiting natural habitats for the European eel (Anguilla anguilla) on their territory are obliged to prepare Eel Management Plans (EMP) containing appropriate measures to safeguard the escapement of a river system specific silver eel target biomass. Stocking is one management option to reach this target. We used various methodical approaches to study population parameters in a large lowland river under the application of a multi-annual intense stocking programme. The approaches were used to further enhance modelling of stock dynamics and silver eel escapement, in particular. Parameterizing the German Eel Model III (GEM III) with values and functions obtained for recruitment, growth, and mortality resulted in an annual escapement estimate of roughly 32 000–64 000 silver eels from 2010 to 2012. Escapement estimates based on a mark-recapture study conducted in parallel revealed somewhat lower values (11 000–25 000) for the same years. In view of the small number of natural recruits, such values are only contingent if stocking had a profound effect on silver eel production. Results from modelling annual silver eel escapement values indicate that escapement targets set in the EMP for this tributary cannot be reached without stocking. This constellation is likely to apply to other Eel Management Units with low current natural immigration values as well, and might be considered a key dilemma in eel management in such catchments due to the current confusion whether translocation of recruits yields a net benefit to the panmictic stock of the European eel.

Evaluation from otolith Sr stable isotope ratios of possible juvenile growth areas of Japanese eels collected from the West Mariana Ridge spawning area

“…Chen et al (2014) also documented eel habitat loss in various areas in East Asia, which would have reduced the number of silver eels produced each year, and Itakura et al (2014Itakura et al ( , 2015 presented data showing the negative effects of revetment shorelines in rivers on eel growth habitats. On the other hand, various attempts, such as (1) regulations of glass, yellow and silver eel fisheries, (2) management of migration obstacles (chemical or physical), in particular fish passes, and (3) restocking, have been conducted to manage and restore local stocks of Japanese eels, as has been done for European eel stocks (Moriarty and Dekker 1997;Knights and White 1998;Feunteun 2002;McCarthy et al 2014;Brämick et al 2016). Furthermore, monitoring of glass eel recruitment all year-round has been started on several rivers in each East Asian country (Japan, Taiwan, China and Korea) with an attempt to make those rivers sanctuaries or model rivers in near future (Kuroki et al 2014).…”

Decreased stock entering the Belgian Meuse is associated with the loss of colonisation behaviour in yellow-phase European eels

“…This outcome suggests that the upper part of the basin will be progressively emptied by the progressive departure of the oldest individuals at the silver stage. To repopulate the upper part of the basin, a well-targeted stocking, using glass eels and very young eels 300 mm, might succeed (Ovidio et al, 2015;Brämick et al, 2016;Josset et al, 2016;Pedersen and Rasmussen, 2016), but the success of downstream migration of these restocked eels remains an important topic.…”

Dynamics of upstream movements of the European eel Anguilla anguilla in an inland area of the River Meuse over the last 20 years

“…Particularly in the River Meuse, the upstream habitats are no longer colonized by eels and will therefore be emptied of this fish species when the last resident eels migrate to spawn. In this context, eel conservation actions such as well-targeted glass eel stocking to enhance yellow eel abundance may be necessary conservation measures, if adequately carried out (Ovidio et al 2015;Brämick et al 2015).…”