Skip to main content

Restoration of the European eel population; pilot studies for a scientific framework in support of sustainable management

Final Report Summary - SLIME (Restoration of the European eel population; pilot studies for a scientific framework in support of sustainable management)

The aims of 'Study leading to informed management of eels' (SLIME) project were to test quantitative approaches to evaluate the status of national eel stocks at a river basin level, to derive reference points for sustainability, and to model the potential effect of legal and technical measures aimed at stock recovery. A total of six different models were tested, using 10 case study data sets from all over Europe, dealing with fisheries for glass eels and for yellow / silver eels, and covering habitat quantity, quality, productivity and connectivity issues, recruitment variation, and other aspects of the biology of eels and anthropogenic impacts on their populations.

The eel stock in Europe is in rapid decline, and both EIFAC / ICES and the European Commission have advised urgent management action to protect and restore the stock. Given the scattered distribution of the eel in inland and coastal waters across Europe, sustainable management and restoration of the common spawning stock in the Sargasso Sea can only be achieved through local management measures, integrated on a European scale. These measures must address all continental life stages: glass eel (young, unpigmented eel, recruiting from the sea into continental waters), yellow eel (resident growth stage) and silver eel (emigrating spawners), with the specific objective of overall protection of the spawning stock. Individual river basins have been identified as the primary management units for implementation of the Water Framework Directive. While protective measures for eel must also be river basin-specific, a common approach and an equitable balance between countries are required. Joint development of targets and tools will provide a cost-effective and consistent approach to management of these widespread fish. The EIFAC/ICES Working Group on Eels (ICES 2005), therefore, suggested an initiative to coordinate ongoing national development of models, and to stimulate further development by applying them to real data.

The precarious state of the eel stock has given rise to several national / regional initiatives to document local eel stocks and to develop modelling tools. The SLIME-project did not initiate these developments, but collated the results from separate initiatives and compared and contrasted them at the international level. The aim of the SLIME project was to coordinate and stimulate further development of national eel-related models by application to real data for case studies. The case studies were:
1. the Swedish West (mainly yellow eel) and East Coast (mainly silver eel) fisheries;
2. lake IJsselmeer yellow eel fishery in the Netherlands;
3. lough Neagh yellow and silver eel fisheries in Northern Ireland;
4. the Burrishoole catchment, an unexploited river in Western Ireland;
5. the Shannon catchment in South-western Ireland, with small-scale test fishing;
6. the Severn in the South of Wales, with an intense glass eel fishery;
7. the Piddle and Frome catchment in Southern England, based on detailed stock surveys;
8. the glass eel fishery Vilaine estuary, southern Brittany;
9. the Garonne catchment in South-western France, having glass eel and yellow eel fisheries, and major dams; and
10. The Camargue in Southern France showing the typical Mediterranean lagoon fisheries.

Six different models have been reviewed within the SLIME project:
1. Scenario-based model for eel populations (SMEP). SMEP models the freshwater phase of the eel population within a river catchment, considering both the biological characteristics and potential anthropogenic influences. The user may vary anthropogenic influences and levels of recruitment in order to create 'what-if' management scenarios, relative to the given reference point. Also, SMEP can be used to reconstruct the historic sizes of the eel population, either under pristine conditions and recruitment or to find the initial pre-exploitation level of equilibrium recruitment that will produce a simulated population structure in a user-specified reach and year that best matches the observed data.
2. Glass eel model to assess compliance (GEMAC). GEMAC was developed to investigate how glass eel fisheries and intake pumping affect the number of settled glass eels per area in a specified estuary. It computes the proportion of settled glass eels relatively to un-fished conditions, with or without pristine recruitment, and under different management scenarios. It is also helps to derive proxies, and can be run in data-poor situations.
3. Demographic Camargue model (DemCam). DemCam provides a realistic description of the status of the stock in a homogeneous water body considering the main aspects (both natural and anthropogenic) that affect eel population dynamics. The model evaluates the consequences of fisheries (separately for glass, yellow and silver eels), restocking, maturation, growth and natural mortality on the yellow and silver eel population. The model is designed to simulate the condition of the stock in actual, pristine and future conditions under different scenarios.
4. Globang, a model of eel population dynamics within a hydrographical network. Globang is designed to perform simulations of eel population dynamics within a hydrographical network. After calibration with real field data, the model is able to evaluate the putative pristine silver eel escapement and that in response to a variety of management scenarios, especially when the spatial (reach) dimension is important.
5. Swedish analytical models (SWAM). SWAM was developed to investigate how yellow and silver eel fisheries, fishery restrictions and glass eel restocking affect the present and future spawner escapement (and catch) in a specified homogenous water body.
6. Length-based virtual population assessment (LVPA). The LVPA model quantifies the population state and the impact of fishing, based on total landings in numbers by length class in recent years. The model provides a critical post-evaluation of management measures implemented during the data years.

There is a complementarity of stage between GEMAC (glass eel) and SMEP or Globang (elvers to silver eels), and of use between SWAM and DemCam (prospective scenarios) or LPVA (post-evaluation).

Ten cases selected for their data availability have been studied within this project, which constitutes a very small subset of the eel's total distribution. Application of models to other cases will therefore often suffer from reduced data availability, both to assess the local situations and to tune the model. The available models differ in the amount of data they require, and in their applicability to specific situations. Thus, case-specific considerations will determine what model to choose, and what data to collect. Further development of the models will be required.

The project CD and web-site (see online) provide detailed reports on each model and case study, as well as presenting a more detailed comparison of models and their applications.

Related documents