Salmon and eels are struggling

Salmon are one of the many migratory fish that use the Netherlands as a corridor. "They spawn in the tributaries of the Rhine and Meuse in Germany, Belgium and France," explains researcher Jacco van Rijssel from Wageningen Marine Research: "After spawning, the small salmon, called smolts, migrate to the Atlantic Ocean, returning later to spawn."

Once, the Rhine and Meuse rivers were full of salmon. Not any more: by canalising rivers and building dams, humans have made life very difficult for salmon. For the past 20 years or so, this fish species has been a symbol of European recovery programmes aimed at returning migratory fish to rivers. A great deal of time and energy is devoted to that goal. For example, fish passages have been created, and millions of salmon eggs and smolts are released every year in Germany to help boost the population."

Commissioned by the Ministry of Agriculture, Fisheries, Food Security and Nature (LVVN), Wageningen Marine Research has mapped the salmon population in the Netherlands. Van Rijssel: "The Ministry wanted to know the impact of the fishing industry on the population. Based on interviews with fishermen and by travelling on fishing vessels as researchers, we got an idea of how much salmon is caught as by-catch. To measure the impact on the total population, however, you need to be able to reliably estimate the size of that population. Using acoustic transmitters, we have been following the migration of salmon for years. We therefore know how long it takes salmon to migrate from spawning grounds to the Atlantic Ocean, and vice versa. We also know how long fish linger at barriers, such as locks."

Van Rijssel compiled all the available data. Conclusion: despite the many millions of eggs, larvae and smolts released in Germany, remarkably few salmon return to the Netherlands. "At the beginning of the century, things were going quite well, but around 2010 we see the numbers declining sharply. And they have not recovered since. There are probably cumulative effects: low river discharges in dry years, heavy shipping and many predatory fish, such as catfish. Together, this significantly reduces the salmon’s chances of survival."

The species is also sensitive to hotter summers: “These days when they go up the Rhine in spring, they have to endure a hot summer. The number of days when the water temperature is above 18 degrees has increased considerably in recent decades. Salmon then suffer heat stress, making them susceptible to all kinds of diseases. Many salmon die from these and fail to reach the spawning grounds."

Other fish actually like the warmer water, says Van Rijssel: “Catfish, for example, love the heat, allowing this species to reproduce better and faster and therefore thrive. Many other invasive species also thrive in warmer temperatures. These in turn serve as food for the catfish."

Like salmon, eels also suffer from barriers. In the Netherlands, we are familiar with the Anguilla anguilla, the European eel, from trade and in the kitchen. It is considered a true European fish that also does well in the Netherlands: from IJsselmeer to ditches and canals. From there, they swim thousands of kilometres to the Sargasso Sea to spawn as a single population. The glass eels born there move towards northwestern Europe with the warm Gulf Stream.

"The eel likes warm water," says Van Rijssel. "We do a lot of monitoring in spring and autumn and we see that more eels are caught when the water is warmer than normal. This is probably because it is easier to catch the eels then. It doesn’t mean that the species is doing well, quite the contrary. Our analyses show that the population collapsed some time in the 1980s. Since then, it has not recovered. Over the last 10 to 15 years, the eel population has been stabilising at low levels. If things get worse, there won't be many eels left."

As in the case of salmon, a combination of factors appears to impact eels. “Changes in sea currents and conditions at sea may play a role," says Van Rijssel.

The state of salmon is an indicator of the health of ecosystems: if they are doing well, then the system is functioning. Their complex life cycle makes them dependent on many links. "If something goes wrong anywhere, you notice it immediately," says Van Rijssel. However, the picture is not all bleak: "With the shad, another migratory fish species, we suddenly saw an upturn in 2023, whereas for years we hardly saw them. This offers hope that unexpected turning points may also occur in salmon. If we were to have another few years of cooler temperatures, more salmon might survive and natural spawning would also increase. And if the resulting salmon are slightly better able to withstand higher temperatures, this might boost the population. Nevertheless, I expect salmon will have a tough time."

North Sea houting shows that adaptation can be a good survival strategy: “This species was released in Germany and has adapted by spawning on the IJssel and Vecht rivers and less frequently, as it used to do, in the North Sea. The more flexibly a fish species adapts to changing conditions, the greater its chances of survival."

There is a partial fishing ban on eels in the Netherlands. This is one of the ways that we can improve the status of the species. "But in the Netherlands, eel is part of our food culture, just like in Denmark, for example. That plays a role in the considerations."

Research into salmon and eel fits seamlessly with the studies conducted by Wageningen Marine Research, in which the effects of climate change on biodiversity is one of the priority areas. And thanks to the international network, Van Rijssel and his colleagues can gather and pool a huge amount of knowledge. "Ultimately, it is up to politicians and policy makers to make good trade-offs," he concludes. “And it is up to us to provide politicians and policy makers with reliable scientific knowledge and sound advice.”

• Ministry of Agriculture, Fisheries, Food Security and Nature
• ICES