Making a mark with eels

Photo: Ben Griffioen


Eel populations in the Netherlands and the rest of Europe have declined substantially since the 1980s. To enable the populations to recover, the European Commission has made it mandatory for member states to take population management measures. Researchers at Wageningen University & Research are monitoring the effects of the measures taken in the Netherlands.

The European eel (Anguilla anguilla) is a mysterious fish. The eel spawns once in its entire life, doing so in the Sargasso Sea, a kilometres-deep region of the northern Atlantic near the Bermuda islands. The Atlantic Gulf Stream then carries the larvae towards the coast of Europe. After about 10 to 30 years, the adult fish return to spawn, a journey of almost 6000 kilometres.

A trap is placed in the North Sea Canal by an employee of Visserij Service Nederland. The catch is counted and the lengths of the eels measured. Photo: Wageningen Marine Research

Eel populations have fallen substantially in the Netherlands and Europe more generally, explains biologist Tessa van der Hammen of Wageningen University & Research (WUR). “The numbers of glass eels reaching the North Sea coast are less than two per cent of what they were in the 1960s and 70s. The figure for other parts of the European coast is nine per cent.” The decline is due in part to a reduction in the available habitat for eels because of increasing obstacles in the form of dykes, dams and hydroelectric power stations — the Netherlands has around 15,000 obstacles that make it harder for eels to migrate. Other factors cited as reasons for the population decline are the fisheries, parasites, surface water pollution and predators.

European regulation

The European eel is a widespread species that is found from Norway to North Africa. Van der Hammen: “That’s why it’s important to have as many countries as possible working together to help eel populations recover.” To facilitate that recovery, in 2007 the Council of the European Union instructed EU member states to take measures and incorporate them in eel management plans. The Dutch eel management plan includes protective measures such as adapting or removing physical barriers, a requirement for amateur fishermen to return eels they have caught and a closed season (September to November) for eel fishing. The regulation aims to bring numbers of silver eels returning to the Sargasso Sea to spawn back up to 40 per cent of the original levels. Now all EU member states have to measure how many silver eels are able to escape back out to sea.

Researchers from Wageningen look at the numbers and behaviour of the endangered eel species in Dutch lakes, rivers and ditches. But how does it work when you take samples like that? WUR biologist Ben Griffioen takes us to the Dutch coastline to see how many glass eels pass by a glass eel detector, placed in cooperation with Bureau Waardenburg and Visserij Service Nederland. Those results can be used to approximate the relative numbers of glass eels that swim past the barriers along the coast.

In the Netherlands, the measurements are being carried out by researchers at Wageningen Marine Research, part of WUR. They look at the presence, numbers and behaviour of eels in Dutch lakes, rivers and canals. Using a variety of samples, they calculate relative indices that let them draw conclusions about eel populations in the Netherlands.

Glass eels

The researchers use glass eel detectors when sampling glass eels. These detectors have been installed at five locations along the Dutch coast where incoming eels enter the freshwater. “Fresh or brackish water is pumped over the dyke through a hose into a container in the sea,” explains WUR biologist Ben Griffioen. “When the container is full, the freshwater flows over a coconut mat into the sea. The glass eels smell the freshwater and crawl up the coconut mat, ending up in the container where we can count them.” Griffioen emphasizes that this method is geared to obtaining relative numbers: it provides information about differences between years. It is much more difficult to say anything about the absolute numbers of glass eels arriving at the coast, though.

Researchers mark the glass eels with a fluorescent marker as this will let them estimate how many eels there are in total later on. Photos: Ben Griffioen

To get some indication of those numbers, the researchers are imprinting the eels with a fluorescent marker that remains visible throughout the migration period, says Erwin Winter, an aquatic ecologist at WUR. “The idea is that the marked eels will mix with unmarked eels. If you take another sample some time later and you find one marked eel in a hundred, you can calculate how many eels there would have been in total.”

Silver eels

Wageningen Marine Research is also monitoring silver eels. Griffioen: “We are counting how many silver eels leave the Netherlands at Kornwerderzand, Den Oever, Nieuwe Waterweg, the North Sea Canal and Haringvliet. We are also counting how many silver eels are coming into the Netherlands at the borders with Germany and Belgium.”

The European eel is a widespread species that is found from Norway to North Africa. Photo: Shutterstock

Some silver eels are being fitted with transmitters so that their behaviour can be studied properly. This will let the researchers advise the Ministry of Agriculture, Nature and Food Quality and water management organizations, such as the Directorate-General for Public Works and Water Management and the water boards, about measures to encourage the migration of silver eels and prevent their deaths. “Our monitoring has shown that silver eels pass the pumping stations mainly in the first four hours of the night,” says Winter. “If you were to stop the pumping station or hydroelectric power station during that period, it would prevent a lot of dead silver eels.”

Van der Hammen says it is important for the monitoring to provide information on how numbers of young and adult eels increase and decrease over the years. With this information, the government can take targeted measures. The researchers are moderately optimistic about the eel population. Measurements over the past few years do not show a further decrease in eel numbers, however it is unclear if this trend will be permanent.”

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