Barriers for eels

Leadphoto: Patrick Pleul AFP/ANP
Infographic: Steffie Padmos

In the experimental fish research facility at Wageningen University & Research, researcher Arjan Palstra takes off the lid of a water tank to reveal fat, dark-coloured eels swimming around in it. ‘Those are sexually mature females whose eggs will soon be released,’ says Palstra. Smaller, slender, silvery males swim inquisitively in and out of the PVC tubes placed in the basin.

Palstra and his colleague Leon Heinsbroek have been trying to close the life cycle of the European eel in captivity since 2016. The research is being done under the auspices of the Eel Reproduction Innovation Centre (EELRIC), which was set up in collaboration with the Dutch Sustainable Eel Sector foundation (DUPAN).

Releasing eggs

The initial aim was to get female eels that were ready to spawn to release eggs, and to fertilize the eggs with sperm from the males, in order to reach the first larva stage on the road to becoming young glass eels. ‘We succeeded in getting the eels to produce larvae straightaway back in 2016’, says Palstra. But the larvae only survived for a few weeks. In a follow-up study, the public-private collaborative LARVitAAL, the researchers are now tryingto improve the quality and health of the larvae.They are succeeding, says Palstra, thanks to innovations in the reproduction techniques. ‘Since September we have also been testing new diets to improve the growth of the larvae. We hope it will then become standard for them to reach the glass eel stage.’

If it works out, it will no longer be necessary to catch glass eels for breeding purposes. Eels are bred by 10 companies in the Netherlands. They start with wild-caught glass eels, most of them caught off the coast of France. According to figures from DUPAN, the 10 companies produced about 2000 tons of farmed eels in 2023, bred from seven tons of glass eels. For comparison: professional fisheries caught 453 tons of wild eels that year.

If a completely closed eel aquaculture is achieved, that will be good news for the wild eel too, says Palstra, as it will further reduce the pressure from fisheries.

Greatest feat

The eel population decreased dramatically in Europe between 1980 and 2010, for no clear reason. ‘The eel is a mysterious fish with a long life cycle, most of which unfolds out of sight for us,’ says Palstra. The fish spawns and produces larvae in the sea, but grows to adulthood in fresh water. Spawning takes place in the Sargasso Sea, nearly 6000 kilometres from the Netherlands. Then the larvae perform their greatest feat by hitching a ride on the gulf stream to the European coast, where they undergo a metamorphosis and change into small, transparent glass eels.

Once they are back in Europe, the eels smell fresh water, but they have great difficulty in reaching the inland waters due to hundreds of obstacles in the form of dams, locks and pumping stations. And these obstacles later form a barrier for the adult eels – known as silver eels – that head back to the sea in order to swim to the Sargasso Sea. Pumping stations and hydroelectric plants can even be deadly. In spite of improvements made to them in recent decades, these installations are still all too often a mincing machine for fish. In other countries too, such as Spain and Sweden, the numerous hydroelectric plants block the eels’ progress.

But these are not the only reasons for the decline of eel populations. Researchers point to pollution of the major rivers, and to the swim bladder parasite, which slows the eels’ growth and undermines their fitness for swimming back to the Sargasso Sea. Eel fisheries also play a role in falling eel stocks.

European research

Palstra is part of a group of about 30 Wageningen scientists who work on the eel from different angles, including ecology and the pollution of its habitat, breeding and reproduction, eel-related policy and the fisheries economy. ‘In spite of all the scientific research on the European eel, it is impossible to pinpoint a single cause for its significant decline since the 1980s,’ says Tessa van der Hammen, a researcher at Wageningen Marine Research. Van der Hammen is the project leader for the Netherlands’ statutory EU research tasks aimed at supporting the eel. In this context, Van der Hammen is doing data research on the size of currenteel stocks. ‘Let’s just say that the decline is caused by a cocktail of factors. In eel conservation, the fisheries are usually the first to come underscrutiny, but we need to address all the causes of the decline.’

Eel fisheries in the Netherlands have dwindled, although scientists don’t have a clear picture of the extent of illegal fishing. ‘There is a fishing ban from September to December to give the silver eels the chance to leave for the Sargasso Sea. And there’s a total ban on fishing for eels in the rivers because of pollution,’ explains Van der Hammen. In 2023, professional fisheries caught 453 tons of eels, 282 tons of them in the IJsselmeer and Markermeer lakes. About 50 years ago, over 1000 tons of eels were routinely caught in the IJsselmeer alone.

What makes managing eel fisheries tricky is that the fish’s habitat extends from North Africa to Iceland, says Van der Hammen. ‘That means that all the countries must join forces to protect the eel. ’For that reason, an ‘eel regulation’ has been in place in the EU since 2007, in which every member state is required to make a national and regional eel management plan. The regulation includes a fishing ban during the three months of the silver eel migration; a ban on anglers keeping the eels they catch; restocking inland waters with glass eels; and making adjustments to turbines and locks so that the fish can pass through them.

‘The long-term goal of the eel regulation is for over 40 per cent of the historical population of silver eels in every member state to be able to migrate to the sea to reproduce. Since eels are a long-lived species, recovery is expected to be slow and the EU focuses on whether the trend is in the right direction.’

Eel stocks

The Wageningen researchers provide the EU with an estimate of the eel stocks every three years, to show how far the Netherlands is from the agreed goal. To this end, sampling is done in inland waters such as the IJsselmeer and the major rivers. ‘Particularly in the IJsselmeer, there is a clear increase in the number of large eels,’ says Van der Hammen. Such a rise has not been observed in all the countries, however. ‘The methods of establishing eel stocks vary a lot per member state, making it hard to draw comparisons,’ she says.

Three photos show glass eels are marked with a specific colour indicating the location and time, which allows scientists to work out where they face obstacles.

As a consequence, the International Council for Exploration of the Sea (ICES) focuses mainly on the numbers of glass eels returning to European shores from the spawning grounds. ‘By comparing those numbers with previous years, you can find out whether there’s a rising or falling trend in overall European stocks.’

The estimates are surrounded by many uncertainties, but according to Van der Hammen, the decline in eel stocks in Europe has been halted over the past decade. ‘But the supply of glass eels from the spawning grounds is still very low all over Europe, compared to what it was.’

Restocking

To boost eel stocks in the Netherlands, and with funding from the ministry of Agriculture, Fisheries, Food Security and Nature, DUPAN released glass eels purchased in France in parts of the Netherlands including Friesland and the string of lakes along the border of Flevoland province, explains Van der Hammen. It remains to be seen whether these eels originating from France will manage to reach the Sargasso Sea from the Netherlands, says Ben Griffioen, a PhD candidate in the Aquaculture & Fisheries chair group.

Griffioen does research on the migration of fish including eels at Wageningen Marine Research in Ijmuiden. He also helps monitor the glass eels at so-called junctions, where the little fish reach Dutch inland waters in early March after a long journey. ‘The glass eel is often smarter than we think,’ says Griffioen. ‘If it can’t access the inland waters at Katwijk, the young fish will just try again at IJmuiden or the Afsluitdijk.’ He has also repeatedly seen that glass eels quite easily slip through with ships going to Amsterdam via the locks at IJmuiden. ‘Especially if the locks are opened in the evening, the eels can easily get in,’ he says.

For his research commissioned by the ministry of Agriculture, and the national and regional water boards, Griffioen catches glass eels and colour-codes them (under anaesthetic) with different colours per time and place. The transparent little fish are about two years old at that point, and about seven centimetres long. ‘They are still too small to have a transmitter attached to them, which is why we colour-code them in order to track them along their route towards fresh waters.’ In the last couple of years, more than 150,000 glass eels have been colour-coded. ‘We are trying to find out how many glass eels there are, how long they get are held back for by obstacles like locks, and whether they can get through in the end.’

To find out how the fish continue to migrate through an area, the researchers released thousands of colour-coded glass eels in several batches at locations including IJmuiden harbour. ‘We saw in the North Sea Canal, for example, that glass eels get in all right, but once they are in the canal there are bottlenecks at the various locks into the hinterland. Some of the fish passages work well, such as the one at Halfweg pumping station, where 40 to 80 per cent of the glass eels manage to get through. But sadly, at most of the fish passages, we have seen only a low success rate, sometimes of only a few per cent.’

Solving bottlenecks

Recent findings by various researchers will be published at the end of 2024 in a national glass eel barrier list providing information about over 100 locations, says Griffioen. ‘This will provide water management organizations with more insight into the state of affairs and where much can be gained by taking measures.’ The key thing now, the researcher explains, is to solve the bottlenecks for incoming glass eels, while not neglecting eels at other stages in the life cycle, like the resident yellow eels and the departing silver eels, says the researcher.

So it is bad news that the deadliest pumping station in the Netherlands is located precisely at the big lock in IJmuiden, says Griffioen. ‘Silver eels often choose the route with the most water discharge. At IJmuiden, that often means swimming out to sea through the pumping station. We have calculated that 10 to 15 per cent of all the departing silver eels die, which is a terrible shame,’ he says. You can take measures such as adapting the way the pump is managed or scaring the eels o. with noise or strobe lights, but these are just sticking plasters, says the researcher. ‘The solution really lies in installing fish-friendly pumps which a silver eel can and does swim through without coming to any harm. But it will take a while before every pumping station in the country has a pump like that. Pumps last a long time, and do not usually get replaced before their time is up.’

Layer of slime

Meanwhile, Palstra and Heinsbroek are working not just on breeding the larvae, but also on nurturing the eels from the glass eel stage to adulthood. ‘From the glass eels we breed beautiful silver eels with a good layer of slime, no sign of stress or aggression, by simulating the trip to the Sargasso Sea in artificial channels,’ says Palstra. The eels swim against the current. ‘A propellor creates a current moving at 0.57 metres per second, with which we get them to swim 3000 kilometres – on the spot, of course.

That suffices to turn them into maturing silver eels.’ After a year and a half, the researchers can already use these eels as parent fish, and stimulate further maturation using hormones. ‘So we don’t have to wait between seven and over 50 years, the time it takes for wild eels to mature.’

It would be nice if the fish could decide for themselves when to ovulate and spawn, says Palstra. He is working with Finnish researchers and the Maretarium, a saltwater fish aquarium in Kotka, Finland, where a 43-year-old female eel spontaneously matured to the spawning stage. ‘She was one of a group of eels of over a metre long, which are monitored continuously. The knowledge gained like this is of great importance because we don’t have any information about spawning eels in the Sargasso Sea. Naturally matured European eels have never been caught, neither in the Sargasso Sea nor elsewhere.’