Improving management with new data
DCF: Stock assessment of European Eel (Anguilla anguilla) in German inland waters
The status of the European Eel stock is considered critical. Yet, most of the required data are based on rough estimates or model assumptions.
As part of a europe-wide network, we are working on the standardized collection of age-structure and growth in all relevant German inland waters in order to provide a solid data base for eel stock assesment and improve future management measures.
Background and Objective
Since the 1970s, the abundance of glass eels on European coasts drastically declined. In 2011, this trend reached an all-time low of one percent in the north sea region and five percent in other regions, in relation to values from the years 1960-1979. Albeit a slight increase in glass eel recruitment during the past three years, the stock situation of the European eel is still considered critical by the International Council for the Exploration of the Sea (ICES). However, due to a lack of fundamental stock assessment data an evaluation of recently applied managament measures is barely possible.
Eel management is especially challenging, since there is only a single stock, which is scattered across the whole European continent and Northern Africa. To allow for a comprehensive collection of assessment data across the whole distributional range, the European eel was implemented in the EU Data Collection Framework in 2007. In this context, we focus our research on eel growth and maturation in german inland waters, in order to allow for the evaluation and optimization of current management measures.
Eels were sampled from commercial fisheries in all German river basin districts (according to the Water Framework Directive) until 2017. In addition to the mandatory data on length, weight, age and sex, additional aspects, such as the infestation of the swimbladder with the Nematode Anguillicola crassus and the contamination with pollutants, such as heavy metals and PCB's were and are being investigated. These data allow for the identification of differences in habitat quality and will eventually result in recommendations concerning management measures, such as restocking.
Since 2017, the sampling is primarily focused on silver eels, which are particularly important to calibrate escapement models.
Our Research Questions
What are the differences in growth and maturation between different management units (i.e. River Basin Districts)?
Is actual management appropriate and how can it be optimized?
First results linked the contamination level of eels to habitat and life-stage. Furthermore, it was demonstrated that pollutants were transferred from muscle fat to the gonads, thus highlighting their importance for the reproductive success of the species.
Another core aspect of the project - the system specific assessment of vital population characteristics (e.g. growth) - reached a milestone. In 2017, the respective data was made available for management plans across all German eel management units, thus providing a significant improvement to the assessment of local eel stocks.
Links and Downloads
Capoccioni F, Leone C, Giustini F, Brilli M, Buttazzoni L, Hanel R, Ciccotti E (2021) δ13C and δ15N in yellow and silver eels (Anguilla anguilla, 1758) from different Mediterranean local stocks and their variation with body size and growth. Mar Freshwater Res 72(8):1208-1219, DOI:10.1071/MF20144
Righton D, Piper A, Aarestrup K, Amilhat E, Belpaire C, Casselman J, Castonguay M, Diaz E, Dörner H, Faliex E, Feunteun E, Fukuda N, Hanel R, Hanzen C, Jellyman D, Kaifu K, McCarthy K, Miller MJ, Pratt T, Sasal P, et al (2021) Important questions to progress science and sustainable management of anguillid eels. Fish Fisheries 22(4):762-788, DOI:10.1111/faf.12549
O'Leary C, Walker A, Beaulaton L, Boulenger C, Briand C, Diaz E, Drouineau H, Garcia-Alvarez B, van Gemert R, Mateo MM, Mestav B, Ozdilek SY, Pohlmann J-D (2021) Third Workshop on Designing an Eel Data Call (WKEELDATA3). Copenhagen: ICES, 19 p, ICES Sci Rep 3(50), DOI:10.17895/ices.pub.8140
Ensing D, April J, Bardarson H, Ahlbeck Bergendahl I, Bolstad GH, Breau C, Buoro M, Camara K, Chaput G, Cooper A, Dauphin G, Erkinaro J, Fiske P, Freese M, Gillson J, Gregory S, Hanson N, Jepsen N, Kelly N, Maxwell H, et al (2021) Working Group on North Atlantic Salmon (WGNAS). Copenhagen: ICES, 407 p, ICES Sci Rep 3(29), DOI:10.17895/ices.pub.7923
Ensing D, Maxwell H, Walker A, Romakkaniemi A, Briand C, Evans D, de Eyto E, Erkinaro J, Pohlmann J-D, Dannewitz J, Höjesjör J, Mills K, Saunders M, Freese M, Kesler M, Velterop R, Poole R, Palm S, Pakarinen T (2020) Working Group on Science to Support Conservation, Restoration and Management of Diadromous Species (WGDIAD; outputs from 2020 meeting). Copenhagen: ICES, 42 p, Ices Bus Rep 1(3), DOI:10.17895/ices.pub.7693
Freese M, Sühring R, Pohlmann J-D, Wolschke H, Magath V, Ebinghaus R, Hanel R (2016) A question of origin: dioxin-like PCBs and their relevance in stock management of European eels. Ecotoxicol 25(1):41-55, DOI:10.1007/s10646-015-1565-y
Pohlmann J-D, Freese M, Hanel R (2016) Minimum landing size in European eel fisheries management: limitations of simplistic management approaches in a semelparous species. ICES J Mar Sci 73(10):2509-2517, DOI:10.1093/icesjms/fsw090
Brinkmann M, Freese M, Pohlmann J-D, Kammann U, Preuss TG, Buchinger S, Reifferscheid G, Beiermeister A, Hanel R, Hollert H (2015) A physiologically based toxicokinetic (PBTK) model for moderately hydrophobic organic chemicals in the European eel (Anguilla anguilla). Sci Total Environ 536:279-287, DOI:10.1016/j.scitotenv.2015.07.046