Monitoring and advice on managing the eel population
Researchers of Wageningen Marine Research study trends, occurrence and behaviour of the European eel in Dutch lakes, rivers and streams. Market sampling also determines which sizes and ages are extracted by the fishing industry. The various samplings form the basis for (model) calculations, which are used to evaluate eel stocks in the Netherlands. The data are also supplied to ICES (International Council for the Exploration of the Sea), which provides international advice on the entire European stock every year.
Decline in eel stock
The stock of European eel is only a fraction of what it was before the 1980s. In the North Sea, the percentage of glass eel that arrives at the coast is about 0.6% (ICES advice 2021). In November 2021, ICES announced that the eel situation remains critical and, using the precautionary principle, advises that no more eels should be caught and that other mortalities caused by human activities should be stopped or minimised.
One of the causes of the decline is a reduction in the eel's habitat due to an increase in barriers, such as dikes, dams and hydroelectric power stations - in the Netherlands alone there are some 15,000 obstacles that make it difficult for the eel to migrate. Other causes of the decline in the European eel population are fishing, parasites, climate change, pollution of surface waters and predators.
International agreements
Eels are a widespread species with a special life cycle. All European eels spawn in the Sargasso Sea, making the eel stock from North Africa to Norway a single stock. Eel management is therefore an international affair.
To enable the recovery of the population, the European Commission, by means of the Eel Regulation, has obliged Member States to draw up eel management plans, including a description of management measures. The Dutch Eel Management Plan includes protection measures such as the adjustment or removal of physical barriers, the release of glass eel, a restocking obligation for recreational fishers and a closed season (September to November) for eel fishing. Wageningen Marine Research is involved in the triennial evaluation of this Eel Management Plan.
Glass eel monitoring
Newborn glass eels (elvers) return to Europe from the Sargasso Sea. Data on the numbers of glass eels migrating into the Netherlands are of great importance for monitoring the eel population. Wageningen Marine Research has been monitoring this since 1938 at the sluices in Den Oever. In addition, there are 11 other collection points along the coast in the Netherlands where glass eels are sampled.
Read more about glass eel monitoring
How do we monitor?
Traditionally, glass eel sampling is carried out with a lift net in the months of March to May. On average, this is the most important migration period for glass eels in the Netherlands. Per location, dozens of hauls are carried out: three in an evening in the dark. Only at Den Oever are there several hauls every other hour in the evening, which results in several hundred hauls. Monitoring is carried out by volunteers and professional fishermen. The research is coordinated by Wageningen Marine Research.
The glass eel index
The index monitoring indicates the annual variation in glass eel abundance in front of a barrier such as a lock or a pumping station. It is essential that the methodology and the conditions remain the same in order to make a good comparison. Although monitoring is carried out from March to May, the index is determined for April and May. One year the glass eels are early and another year they are late. Practice shows that for one year this gives a higher value of the index, while the other year this gives a lower index. An index is nothing more than the number of glass eels per haul.
Different monitoring techniques
In addition to glass eel monitoring, research is also being conducted into the distribution dynamics and migration success of glass eels. By using multiple techniques during the same period at the same location, a more complete picture of glass eel behaviour is obtained.
We offer
- Long-term monitoring and data sets of eel stocks in the Netherlands and internationally, the recruitment of glass eel and the migration of glass eel and silver eel in Dutch waters.
- Contributions in advising ICES (WGEEL) on eel management in Europe
- Three-yearly EU evaluation of the Dutch eel management plan
- Research on dioxin residues in eels