Monitoring migratory fish with sound signals

In Stellendam harbour, Melanie Meijer zu Schlochtern demonstrates how to fit a transmitter to a migratory fish. Outside the little lab is a tank into which fresh seawater is constantly being pumped. The fish are swimming in the tank. The scientist scoops out one of the fish and transfers it to a smaller tub of water. ‘An anaesthetic has been dissolved in this water,’ explains Meijer zu Schlochtern.

Once the fish is fully anaesthetised, she lifts it onto the operating table. The fish is assigned a number, measured and weighed, and Meijer zu Schlochtern takes a DNA sample. She then makes a small incision in the abdominal cavity while a hose squirts water over the fish’s gills. ‘There is a lot of room in the cavity,’ she says. She inserts a transmitter the size of a large pill in the opening. After two stitches, the incision is closed again. ‘That’s that one done,’ she says cheerfully and takes the fish outside again. She holds it in the tank with flowing seawater until it comes round.

Meijer zu Schlochtern is a PhD candidate in the Aquaculture Biology and Fisheries Ecology group. With the aid of research firm ATKB, she is fitting a total of 350 migratory fish — salmon, sea trout, sea lamprey, North Sea houting and shad — with transmitters. The fish are then released into the North Sea. Meijer zu Schlochtern will monitor them and be able to see exactly what happens to the fish that try to reach their spawning grounds via Haringvliet.

The rivers of the Netherlands used to be teeming with migratory fish. Large numbers of Atlantic salmon, for example, would swim up the Rhine and Maas from the North Sea to get to waters in Belgium, Germany and France where they reproduced, but this silvery green salmonid has now disappeared almost entirely from the Netherlands’ inland waterways. There have been efforts since the 1980s to reintroduce the salmon and other species, but without much success to date.

Scientists point to a wide range of causes: climate change, pollution, invasive species, the loss of spawning habitats and overfishing. The construction of barriers such as dams and sluices has been another factor in the decline of migratory fish populations and may be preventing them from returning to the rivers. Meijer zu Schlochtern’s research looks at one of the largest man-made migration barriers in the Netherlands: the kilometre-long, 56-metre-wide Haringvliet sluices in Zuid-Holland, sometimes known as the sixth Delta Work.

The sluice complex, which was built in 1966, separates the Haringvliet inlet (which as a result is now freshwater) from the North Sea. The Directorate-General for Public Works and Water Management is also aware that the sluices could be getting in the way of migratory fish. That is why, as of 2019, gaps are opened up in the configuration for a short period every day. Depending on the water level and the weather, among other factors, one or more sluices open about one metre for roughly 15 minutes during high tide. The hope is that the fish will find their way to the rivers through these gaps.

‘Managing the sluices is about responding to circumstances,’ explains Aniel Balla, a senior advisor on water quality at the Directorate-General for Public Works. ‘That is why we opted for “implementing by learning” when we introduced the gap procedure. As we experiment with opening the sluices, we look at what happens to the salt water that flows in while Meijer zu Schlochtern is investigating which migratory fish swim through the passage and into the rivers, and how. We will ultimately be able to use that knowledge to optimize the gap procedure.’

Migratory fish were already able to reach the rivers even before the gap system was introduced. ‘We know that from a long-term study in which the fish were monitored using radio transmitters,’ explains Meijer zu Schlochtern. ‘Twenty-nine per cent of the fish released into the North Sea managed to get into the Haringvliet inlet.’ However, that study was unable to show how the fish were able to get past the sluices. A fish fitted with a radio transmitter will be recorded when it swims past a receiver station, but that gives you no more than a couple of data items per station.

This time, Meijer zu Schlochtern is using acoustic telemetry. These new transmitters send out a high-frequency sound signal rather than a radio signal. ‘Sound waves travel well through water. They can be emitted continuously and received in a targeted way.’ The receivers look like a cross between a thermos flask and a boom microphone. About 80 of them have been suspended from buoys underwater, with equal numbers in the North Sea and the Rhine-Maas delta.

‘Acoustic telemetry gives us a far more detailed picture of the route taken by the fish because the receiver records each fish for as long as it is in the vicinity,’ explains Meijer zu Schlochtern. ‘That means we can now see not only whether a fish passes through the sluices but also how long it takes to do so and how many attempts it makes.’ Meijer zu Schlochtern has fitted some of her test subjects with even more sophisticated transmitters that provide additional information, such as the depth at which the fish swims or the speed.

A special predation transmitter can even tell you whether the fish fitted with it has been eaten. ‘Of course, that’s another reason why fish are unable to reach the spawning grounds,’ says Meijer zu Schlochtern. ‘That transmitter reacts to gastric acid and then communicates the temperature at the time the predation took place.’ The researcher can use this information to get a rough idea of the kind of predator the transmitter has ended up in. ‘Fish stomachs are colder than the stomach of a mammal such as a seal.’

Meijer zu Schlochtern and two colleagues tie their little boat to one of the buoys. They haul up the steel cable to which the receiver is attached. ‘That doesn’t always go smoothly, especially if a rope or cable gets entangled with the underside of the buoy.’ The object the researcher retrieves from the sea no longer looks like a high-tech device. ‘Over time, the receiver gets fouled with marine life, such as algae and mussels,’ she explains. ‘That’s also why it’s wrapped in a pair of tights.’ The scientist deftly cuts the device free of the tights, after which she sets up a wireless connection between the cylindrical receiver and the laptop on board the boat.

This is how the researcher collects all the new data every six months — along with the angling association Sportvisserij Nederland, which manages half the receivers. Meijer zu Schlochtern expects the total number of detections to be up to 20 million. She hopes this massive amount of information will let her draw precise conclusions about the journeys made by the migratory fish. ‘How long do fish wait at the sluices? When do they pass through and what route do they take? Do they continue on their journey immediately? And what do the fish do that don’t get to the rivers?’

Some of the fish were only fitted with their transmitters recently, but Meijer zu Schlochtern already has interesting results from the fish that got their transmitters in 2023 or 2024. ‘Salmon, sea trout, sea lamprey and North Sea houting often get past the sluices when the gaps are open,’ she says. ‘Salmon and sea trout pass mainly during daytime. We know from the literature that these fish prefer to navigate complex situations by sight.’ Meijer zu Schlochtern cannot yet say for certain whether the system of gaps results in a higher percentage of fish being able to swim into Haringvliet.

Even if the system doesn’t increase the numbers of fish reaching the rivers, the gaps could still offer benefits. ‘Before the controlled temporary gaps came into operation, fish could only enter the Haringvliet inlet swimming against a strong current when the sluices discharged excess water. That takes a lot of energy. Fish that enter through a gap may have more energy left to cope with other obstacles they encounter during their migration. In that sense, the gap system could still result in more fish managing to reach the spawning grounds.’