News - May 17, 2018

Disco fish swim into the polder

Research into the migration of glass eels in Dutch waters should shed light on the likely causes of the decline in eel numbers. A report.

The marked fish light up under the UV lamp.
Tessa Louwerens photo Ben Griffioen

Ben Griffioen waits patiently while the Emma bobs gently up and down in the sluice at IJmuiden. Fortunately, it is a lovely sunny day. The signal is given and the sluice gates to the sea swing open. From here, it is less than a minute by boat to the glass eel detector.

Every spring, millions of glass eels, young eels, migrate from the sea to the freshwater waterways of the Netherlands, where they will grow up. Along the way, on the seaward side of the sluices at IJmuiden, they encounter the glass eel detector: a climbing wall of sorts covered with a coconut mat that leads to a basin. ‘We pump brackish water from the North Sea Canal into the basin that stands on the pontoon,’ explains Griffioen, a researcher at Wageningen Marine Research. ‘That water then flows over the coconut mat and attracts glass eels that slither up it during the night and fall into the basin.’

Griffioen and his colleagues from the Netherlands Fisheries Service (VSN) and elsewhere are studying the migration of glass eels. Their work has been commissioned by various parties, including water authorities. As the Netherlands has a network of some fifteen thousand waterworks such as weirs, sluices, dikes, dams and pumping stations, the glass eels have to overcome a fair number of obstacles to reach the inland waterways. These waterworks ensure that our feet stay dry, but also make it difficult for eels to enter and leave the fresh water. This, says Griffioen, can have disastrous consequences. Eel are doing poorly worldwide. Griffioen, ‘In the Netherlands we have been monitoring glass eels since as early as 1938 and their numbers have declined drastically. The catch in the period since 1980 is down by 95 percent compared to the preceding period.’ Exactly why that is, the researchers do not know. As well as the obstacles that hinder the arrival and departure of the eels, it may also be due to factors like water pollution, changing ocean currents and the fishing industry.

IJmuiden is an important hub for the glass eels, because it offers access to the 26-kilometre long North Sea Canal. From there, the glass eels swim into the Dutch polders. The researchers are keen to find out how many glass eels enter the North Sea Canal, how they spread, how long they take to do this and whether they encounter any obstacles. To study this, this spring more than six thousand glass eels in total are being caught, marked and released.

The researchers fit the glass eels with a VIE tag, as the fluorescent mark is called. It is inserted under the skin using a very fine needle while the fish are anaesthetized. Marking these small, slippery creatures is quite a fiddly job and they are indeed as slippery as an eel. ‘Once you've acquired the knack, it is a pretty quick process,’ says Griffioen. ‘With a team of three, we do three hundred an hour and we have pricked our fingers only twice.’ The glass eels suffer no adverse effects, he believes, and once they are adult, there is no trace left of the tag.

In the meantime, several dozen glass eels – the night's catch – are wriggling madly in a bucket on board the Emma, the researchers' sloop. Some glass eels have retained their natural transparency, others are an even brown. These ones number among the 7,600 glass eels marked with the aid of a colour bath. ‘That is a lot quicker and it was useful because we had to mark so many of them,’ says Griffioen. ‘The disadvantage is that for this group we can't say exactly when they were marked.’ A single glass eel has a striking orange stripe. ‘That orange fellow is one we marked and released five days ago, together with some nine hundred others, near the fortress island on the seaward side of the sluice.’

UV lamp
The boat is now back at the landing stage. Here, the glass eels are placed in an aquarium and in a darkened room held under a UV lamp. The marked ‘disco fish’, as the researchers call them, light up. The colours reveal when and where the fish were released. Some of the glass eels were released on the seaward side of the IJmuiden sluices, others on the landward side. ‘This enables us to establish whether and how fast the glass eels pass through the sluices,’ explains Griffioen. The ratio of marked to unmarked glass eels also gives the researchers an idea of the total number of glass eels trying to swim inland and whether their efforts to do so are slowed down in any way.

Based on these data, targeted measures can be taken to improve the flow of glass eels. In the North Sea Canal area, regional water authorities are already taking steps to improve their migration conditions. At various other places in the Netherlands fish passages have been installed, enabling glass eels to bypass a pumping station, for example.

According to Griffioen, it will take a while before the effects of the measures are evident. ‘It will take ten to fifteen years before we see the effects of all we are currently doing for the glass eel, when their offspring arrive at IJmuiden.’ Moreover, there is little point in taking action limited to the Netherlands if no European measures are taken. After all, there is no such thing as a ‘Dutch eel’ whose offspring return dutifully to the Netherlands.  

On the landing stage, meanwhile, all the data have been gathered and the bucket is being emptied over the side of the boat. The glass eels weave their way rapidly to the riverbed. If everything goes well, some of them will return to this spot in more than ten years' time as adult silver eels. To embark on the long journey back to the place of their birth to reproduce and then die.


6000 kilometres for sex

Life cycle of the European eel: Infographic ©Wageningen Marine Research

As young adults, eels live in fresh water. When they are ready to mate and lay their eggs, European eels (Anguilla anguilla) make the journey to the Sargasso Sea. When the larvae hatch from the eggs they are roughly a half centimetre long and, for the first year of life, shaped like a willow leaf. The larvae are transparent, which is necessary because they live at depths of up to 200 metres where there are few places to hide. After this stage, the larvae swim and drift along on the Gulf Stream for a couple of years towards Europe, a journey of some 6,000 kilometres, becoming glass eels in the process. In European fresh waters, they become adolescent elvers, then adults. Some ten to fifteen years later, they embark on the long journey back to their spawning grounds. This is also their last journey, for after mating the eels die.