Placing fyke nets in the Oostvaardersplassen | Photo: Joep de Leeuw
Placing fyke nets in the Oostvaardersplassen | Photo: Joep de Leeuw
Archipelago with gradual shores
The Markermeer has some 200 kilometres of mostly hard, steep banks of basalt blocks. Development of natural shallow transitions from land to water (wetlands and soft shorelines), which are characteristic of natural lakes, requires artificial measures. In Marker Wadden, shallow wetland zones have been created that are in open connection with the Markermeer. Within this archipelago, which covers only 1 percent of the lake area, some 30 kilometres of gradual and sheltered shoreline have been added to the existing 6 kilometres [1]. Wind dynamics and fluctuating water levels there create hydrological dynamics that are particularly noticeable in the channels connecting the wetlands to the open water. These channels also appear to form the main connections for fish between the shallow wetland zones and the naturally developing shores on Marker Wadden and the open water. Especially from spring, when the water level is raised to the higher summer level, large parts are accessible to fish that use the shallows as spawning and nursery areas. By placing monthly fyke nets in pairs in four channels, we were able to monitor fish movements between the wetlands and the open water [2].
Unlike Marker Wadden, the reed wetlands of the Oostvaardersplassen are isolated and on a flat seabed with little variation in water depth and dynamics. The fish community here currently consists of a limited number of species that are best adapted to high water temperatures and low oxygen periods in summer. In the southern part of the area is a system of ditches in the grazed area of Oostvaardersplassen, where a fish passage was built some years ago, connecting to the Lage Vaart (a large canal in Flevoland). We also used fyke nets to sample fish stocks in the Oostvaardersplassen and the ditch system on a monthly basis.
Table 1. Frequency of occurrence* of young-of-the-year fish (YOY, first growing season) and older fish in gullies of the Marker Wadden islands (left), Oostvaardersplassen (centre) and Oostvaarderssloten (right), i.e. a system of small canals and ditches connected to larger channels via a fish passage.
Differences in fish fauna
Table 1 shows the results of the counts in the three areas, split into juvenile fish (year class 2025) and older fish. Figure 1 illustrates the changes during the growing season. The differences between the three areas are large. Twenty-one species were found in the channels of Marker Wadden, both older and juvenile fish, with a wide variation in habitat preferences. Three-spined stickleback, pike, roach, and later in spring, carp are typical species that use the vegetation-rich banks and wetlands as spawning and rearing areas. Other species such as perch, pikeperch and ruffe spawn at greater depths in Lake Markermeer, but many juvenile fish migrate to Marker Wadden to grow up further during the summer. In the shallow, isolated Oostvaardersplassen, 10 species were found, including carp, crucian carp, rudd and marble goby – species adapted to shallow water where water temperatures can rise and oxygen levels can be low. We also know this particular fish community from, for example, the wide floodplains of the Volga, where shallow pools can sometimes almost run dry at low water levels. Interestingly, in 2025, we found no juvenile individuals of these species at all, while in other years there may be considerably more. In the ditches in the grazed area of Oostvaardersplassen, we found 12 species and a fish fauna that was somewhere between that of Marker Wadden and the Oostvaardersplassen.
Connection to deeper water and the sea
The importance of connections between wetlands and open water is especially true for species such as eels and three-spined sticklebacks, which spend part of their lives at sea. Three-spined sticklebacks can also complete their full life cycle in isolated freshwater systems, but they remain smaller than their marine counterparts. Although three-spined sticklebacks can be common in the Oostvaardersplassen in some years, we found almost no sticklebacks there in 2025. This may be due to the exceptionally low water levels. In spring, the species was found in the fish ladder connecting the Lage Vaart to the ditches in the grazed area of Oostvaardersplassen. Given their length of 5 to 6 centimetres, these probably came from sea. On Marker Wadden, as expected, many more sticklebacks were found in early spring. The occurrence of juvenile perch, pikeperch and ruffe also illustrates the effect of connections with deeper water here.
Figure 1. Number per fyke pair juvenile (young-of-the-year) and older fish in gullies of Marker Wadden, shallow lakes of Oostvaardersplassen and canal system of Oostvaarderssloten. Blue colors: percids, green colors: cyprinids.
Wind dynamics and water level
Wind-driven differences in water levels (higher with westerly winds, lower with easterly winds) create inflows and outflows of nutrients and sediment through the channels at Marker Wadden. This contributes to the dynamic formation of habitats that are favoured by stream-loving species such as ide, asp and bleak. In the Oostvaardersplassen, such dynamics are much more limited and these species are not found except for an occasional ide. Differences are also seen for non-native goby species, which have expanded in the Netherlands over the past two decades: on Marker Wadden, species that like open and/or slightly flowing water (such as round goby and monkey goby) are common, while in the Oostvaardersplassen, a species that likes stagnant water and is well-adapted to muddy beds (western tubenose goby) is numerous.
Fish and birds
Fish-eating waterbirds such as spoonbills, herons, grebes and little grebes play an important role in the management of Marker Wadden and the Oostvaardersplassen. For these birds, small fish such as three-spined sticklebacks, gobies and juvenile fish of other species are the main source of food. In the shallow waters of Marker Wadden, the presence of these small fish annually follows a predictable seasonal pattern. In the Oostvaardersplassen, where reproduction and a greater supply of juvenile fish are not guaranteed, it varies from year to year. However, the large areas of shallow water do mean that small fish, when sufficiently present, are widely exposed to predators. In the ditches in the grazed area of Oostvaardersplassen, the constructed fish ladders seem to be able to provide a more predictable source of small fish, with similar seasonal dynamics as observed at Marker Wadden. Accordingly, we see that herons and spoonbills use both wetlands, but the extent to which this happens varies greatly during the growing season and between different years. Conversely, the fish population of the Oostvaardersplassen is characterised by a predictable, high abundance of large fish, especially carp. These, especially when the water dries up, are an attractive food source for white-tailed eagles, for example.
Habitat variation
The comparison we have made here between the Oostvaardersplassen and Marker Wadden provides some guidance that may be of interest for wetland design. Connecting wetlands with open, deeper water significantly broadens the habitat choices of fish during the growing season and their life cycle.
The unnatural water level in Lake Markermeer does not seem directly unfavourable in this regard. During the spawning period, the water is high enough for adult fish to reach shallow areas on Marker Wadden, and these areas are also accessible to juvenile fish in summer. The open connections then allow fish to migrate to deeper water for the winter. An important condition is that water level management should not be at the expense of wetlands development and a suitable fish habitat.
Even in the shallow Oostvaardersplassen, it is important to maintain water levels such that wetlands vegetation remains accessible to juvenile fish and that there is sufficient water for small fish to survive. Besides fine-tuning the water level, connections to deeper parts in surrounding waters, such as the Lage Vaart or Lake Markermeer, can also be important for the survival of juvenile fish and species more sensitive to oxygen depletion. While fish can migrate to the ditch system via fish ladders, it is still unknown whether this is a significant route for out-migration and whether it can also benefit the Oostvaardersplassen.
We are conducting desk studies to investigate the extent to which fish passes can help enable fish connections between the Oostvaardersplassen and Lake Markermeer. While such artificial connections can certainly contribute to a richer fish fauna, connecting wetlands to open waters via channels has the added value of increasing environmental dynamics and habitat variation and thus ecological diversity on a larger scale. Apart from that, the isolated Oostvaardersplassen, with a fish community adapted to it, has an intrinsic value as a habitat that is scarce in the Netherlands. It can be concluded that the interplay of water level management, wetlands development and connectivity determines the fish fauna, requiring well-considered management choices to be made.
Marker Wadden's open connections to Lake Markermeer create a dynamic system and enable a rich and robust fish population. The Oostvaardersplassen are home to a more limited number of species that are best adapted to warm and oxygen-poor water. Fish passages increase species diversity here. While such artificial connections can contribute to a richer and more robust fish fauna, connecting wetlands to open water via channels has the added value of increasing habitat variety and ecological diversity on a larger scale. The interplay of water level management, wetlands development and connectivity ultimately determines the fish fauna, requiring well-considered management choices to be made.