This is how Wageningen monitors the eel population

Eels are important for the fishing industry as well as an alarm bell for the state of Dutch waters. The water system needs to be clean and accessible for eels to thrive and be able to migrate to the sea after 5 to 20 years. They then spawn in the Sargasso Sea off North America. The offspring return to Europe as glass eels. There, they first grow up into 'red eels’, then turn a yellow-green colour and eventually become adult silver eels that can reproduce. At least that is the intention, but it happens much less than it used to. Researchers from Wageningen Marine Research are monitoring the eel population on behalf of the Ministry of Agriculture, Fisheries, Food Security and Nature. The main question is: are enough eels migrating out of the Netherlands again? 

Talking to the eel team, it becomes clear that mapping the life cycle and distribution of eels is no easy task. It requires monitoring in many different places. From the long Dutch coastline to the national waters and smaller lakes, streams and polder ditches, the eel team collects data everywhere. They combine this data in population models in a European effort to protect the eel.

Researcher Ben Griffioen explains how they try to track the arrival of glass eels and the departure of silver eels. The first monitoring of elvers began in the Netherlands as early as 1938 after the construction of the Afsluitdijk, a major barrier to swimming up the river from the sea. The eel monitoring is still carried out annually in spring. The elvers are caught with cross nets that professional fishermen or volunteers lower into the sea with a rope. The catch is then carefully counted. These days, some glass eels are injected with a colour tag too. After one or more barriers along the coast, researchers check how many coloured eels they catch there. This makes it possible to estimate how many glass eels pass the obstacles. 

Since 2019, a device has been added called the glass eel detector, developed by Waardenburg Ecology en Visserij Service Nederland. This device attracts glass eels into a collection tank via a freshwater stream. Every few days, the elvers are counted in the detector, checked for color markings, and released.

Until a few years ago, researchers relied on catches from 40 ditches a year in the polder ditches, where the eels grow up and spend a large part of their lives. Monitoring there now has also been renewed. “Along the bank, we fish for 250 metres with an electrofishing net,” explains Joey Volwater. “We then measure the length of the anaesthetised fish, enabling us to calculate the ‘probability’ that it is a red eel or silver eel and whether it is a male or female. Because it is not always easy to see by looking. A good method, but unfortunately there were times when we often caught nothing.”

Because the many barriers along the way mean that few ditches contain eels, such a sample therefore provides less and less data on the composition of the eel population. In collaboration with Waardenburg Ecology, the researchers therefore first take water samples from 100 ditches to see whether they contain eel DNA. They then select 40 spots to go electrofishing. Volwater: “We used to catch maybe one, now we catch 10 in those 40 spots combined.” He is happy with that, because a larger catch thus provides more data on the population. He may not know exactly how many eels there are in ditches, but he does know whether any eels mature. Later, the researchers also hope to use the eDNA samples to determine the exact age of the eels at that site, without having to fish.

For the silver eel migration to the sea, besides traditional trap monitoring, the researchers tested underwater cameras and AI for image analysis. Ben Griffioen: “Cameras are much more fish-friendly than nets and are not a snapshot.” However, camera tests led to all kinds of practical challenges, such as in the Nieuwe Waterweg near Rotterdam. “Power supply, severe weather conditions or busy shipping make it difficult to record good images in such places. Moreover, even AI can still hardly distinguish between a red eel and a silver eel,” admits Jorn School, who is responsible for monitoring in large inland waters. However, the researchers are not giving up and hope to use the technology in the future. 

The researchers use monitoring data to calculate population trends. This shows that the entry of glass eels alone fluctuates enormously and a clear trend is far from obvious. Giffioen: “One year 2 million eels come in, the next year 11 million. That's already a big difference.” Researchers cannot yet explain the fluctuations.

For catches in the IJsselmeer and large rivers, the proportion that reach maturity within a year is estimated based on length and sex. From this, the researchers deduce the number of migrating eels. Also at specific locations, such as the North Sea Canal near IJmuiden, the researchers are trying to say something about the number of eels passing there on their way to the sea. “At IJmuiden, an average of 100,000 migrate in one year. We calculated this based on an experiment in which we recaptured some eels, which we had previously given a small mark, when they migrated,” Griffioen says. 

Despite all the counting methods and calculations, the picture is far from complete. Because the eel's habitat spreads from the big rivers into the countless small capillaries of our water system. Bottlenecks such as locks, weirs and pumping stations block the free passage of eels. Ever since the 1980s, eels have been struggling as a result. The numbers were rapidly decreasing, but they are now fairly stable at a level many times lower than before. The current fluctuations therefore no longer mean a sharp decline, but also no recovery.

The situation at some of these bottlenecks has been improving in recent years, through the construction of fish ladders, for example, but this does not seem to be sufficiently helping the population as a whole. Griffioen: “Ultimately, the solution is simple: to restore the eel population, we need to restore the migration routes. Eels need space for free migration as small glass eels, to grow up as red eels and safely migrate again as silver eels. That whole cycle must be in order.” “In addition, water quality in the major rivers must also improve,” says Project Manager Tessa van der Hammen. “Because that is still not good enough, according to the European Commission, too.”

For eels to migrate freely and easily again, more than just Dutch efforts are needed. Apart from Dutch ditches, juvenile eels also seek out English, Irish, French and Spanish waters, for example. Van der Hammen: “Measures in one country affect return and distribution in other countries. All eels from all over Europe and North Africa converge in the Sargasso Sea. The more eels return, the more eels can reproduce.” Since 2007, the European Union has required all member states to monitor eels and take measures to restore the eel population. “Each country still takes a different approach to monitoring and modelling, but we are trying to make a single model of the population. We want to have a good picture of the entire eel population of Europe.”

Researchers are cautiously optimistic now that the population is no longer rapidly shrinking. New methods of monitoring help to constantly zoom in on bottlenecks, with the aim of solving them. Except we still know very little about the journey from Europe and North Africa to the Sargasso Sea and back. Van der Hammen: “The ocean phase and the challenges along the way are still a big question mark.” “Glass eels are very dependent on wave currents in the ocean,” Griffioen adds. “If global warming changes anything relating to currents, we don't know how that will affect glass eel migration. So this may also change the distribution of glass eels along the European coast.”