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KENNISONLINE 2021
The bream poses a dilemma for water management organisations. Is the fish a disruptive or constructive force in nature? It was considered disruptive for a long time, and therefore fished intensively. But views are changing now that bream stocks have declined by 90 per cent in the closed bay of the IJsselmeer. To learn more about the decimated species, scientists at Wageningen University & Research have implanted small transmitters in some of the bream.
“The bream, a native Dutch fish that is found in freshwater and brackish water, is both cherished and maligned,” says WUR fish ecologist Joep de Leeuw. In the past, people thought there were too many bream — they were held responsible for maintaining excess algal growth — but then the alarm was sounded three years ago. De Leeuw and his colleagues had noted before that bream stocks had fallen by 90 to 95 per cent in the IJsselmeer and the Markermeer and the bordering lakes. Every year, the Ministry of Agriculture, Nature and Food Quality asks Wageningen Marine Research to report on fish stocks in the North Sea and IJsselmeer. The ministry uses these data to decide how much scope there is for fishing.
Full nets
“In the past, you would cast a net on the open waters of the IJsselmeer to take stock of the fish there and it would be full of bream. These days, you can cast several nets and not catch a single bream,” explains De Leeuw. In 2020, the Ministry of Agriculture announced a ban on fishing for bream. What is more, De Leeuw and his colleagues are also collating more knowledge about various factors that could affect bream stocks. For example, the researchers want to figure out where exactly the fish mate and deposit their eggs in spring. “We know they prefer shallow water for this. A large proportion of the bream population always used to swim from the IJsselmeer to the lakes in Friesland to mate and then return afterwards.”
The researchers work with professional fishers who use a large net to catch the bream without harming them. Photo: Joep de Leeuw
Bream with transmitters
Last year, the researchers have implanted small transmitters in bream measuring at least 35 centimetres in length. The signal from the transmitters only reaches up to 200 metres underwater, so receiver stations have been placed on the fish's route from the IJsselmeer to the hinterland from the IJsselmeer. Examples of such locations are the sluices and pumping stations at Lemmer, Medemblik and Ketelmeer. Wageningen Marine Research is not the only organisation studying bream with the aid of transmitters: the angling association Sportvisserij Nederland, Van Hall Larenstein and the Netherlands Institute of Ecology (NIOO-KNAW) are employing this technique as well.
Algal blooms
“Twenty years ago, it was unimaginable that bream fishing would ever be a point of discussion. Back then, the general opinion was that there were too many bream,” explains De Leeuw. At that time, the water in the IJsselmeer and the bordering lakes of Flevoland contained too much phosphate from washing detergents and agriculture. That led to algal growth, including the toxic blue-green algae. The bream was held partly responsible. “The fish forages for mosquito larvae, worms and shellfish on the lake bed, which stirs up the surface and makes the water turbid. As a result, there isn’t enough light for water plants, which in turn are required for oxygen. Predatory fish like the pike, which hunts by sight, disappear too. And, like many other fish, young bream eat water fleas, which are consumers of algae. So if there are a lot of bream, there will also be more algae; it’s a whole chain of events.”
The Dutch bream has been maligned and fished out of Dutch waters for years. To learn more about the decimated species, scientists at Wageningen University & Research have implanted small transmitters in some of the bream. In this video, WUR scientist Joep de Leeuw goes to the Markermeer to measure how the bream population is developing.
Water managers no longer wanted the bream and commercial fishers saw a potential sales market. Bream are often caught live and released in fish ponds in the Netherlands and Belgium. “Anglers like it as a fish, as it can grow to a large size and weigh more than 3 kilos,” explains De Leeuw. There is a market for consumption of bream too. “While people in the Netherlands find bream slimy with too many bones, freshwater fish is actually more appreciated in Eastern Europe, where bream smoked over juniper twigs is a specialty.”
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If there are a lot of bream, there will also be more algae. It is a whole chain of events
At first, removing the bream and reducing the phosphates led to clearer waters with fewer excess nutrients in many places. Water plants now grow in large numbers in the bordering lakes, the pike has returned and other fish species finally have a chance. However, there are only a few water plants in the IJsselmeer and Markermeer, and fish stocks have declined for all species, except for pikeperch. The consequence is less fish for the fishing industry and for protected water birds such as goosanders and terns.
Spawning fish
The researchers collaborate with fishermen to find out how many bream are still there, how they use the bay and lakes, and where the young bream live. The researchers apply a small mark to the fish to track whether they are caught later and if so, where. More bream will also be implanted with transmitters in autumn 2021. De Leeuw: “We exchange knowledge. The fishermen will benefit from that, but so will the provincial authorities and the Directorate-General for Public Works and Water Management. The provincial authorities will need to decide whether to invest in fish-friendly sluices and pumping stations and whether to provide the IJsselmeer with more natural foreshores again rather than just basalt dykes.”
Inserting the transmitters is quite an operation. Joep de Leeuw: “There’s an anaesthetic fluid in the water that flows past the head and gills. That renders the fish unconscious while keeping the abdomen dry.” Photo: Joep de Leeuw
Joep de Leeuw: “We make a tiny incision and place a two-centimetre transmitter in the abdominal cavity. Then we stitch up the cut.” Photo: Joep de Leeuw
The first results from the study with transmitters are already available. Far more bream were spotted near the receiver stations during the spawning season this spring than during other periods. “A good quarter of the bream with transmitters made their way to the waterways that give access to the hinterland. Some bream are real travellers: one swam from the border lakes across the Markermeer and IJsselmeer and was then spotted by a receiver station in the IJssel.” De Leeuw is also studying the age of the fish. “They can live a long time; I know of one bream that reached the age of 39. Fish have annual rings in their otoliths in their ears. Our initial analyses show that a bream measuring 60 centimetres can range in age from 5 to 20 years.”
Engineered nature
The research by Wageningen should show what effect the fishing and the relatively low nutrient levels in the water have on bream stocks. Nature development projects such as the construction of new islands in the south part of the Markermeer lake may also have an influence. Far-reaching restrictions on fishing for bream should allow stocks to recover, but it is unclear what population level is desirable and possible for the bream. Bream, with their habit of stirring up the lake bed, are making the water richer in nutrients in parts of the IJsselmeer. Some species feel at home there, while other species prefer other spots with clear water and water plants.
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To what extent can nature be engineered? The bream confronts us with this issue
“The bream is important for species diversity, but we still have a lot of questions about its role in how an ecosystem functions. Is it a disruptive or constructive force in nature? The ecological interrelationship with chemical processes in the lake bed is complex.” De Leeuw believes that our attitude to the bream and whether we are pleased to have it around or not says something about our attitude to nature in general: “To what extent can nature be engineered to suit our wishes? The bream confronts us with this issue.”