Fishing clubs often invest money in juvenile fish for their lakes to increase stocks. But a new study shows that better habitats, such as shallow water zones, help fish and other species in small lakes significantly more. "The introduction of fish, a so-called fish stock, only makes sense in very specific cases," said Robert Arlinghaus from the Berlin Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) and the Humboldt University in Berlin. The money is often better invested if it goes into renaturation.
Arlinghaus is the head of the study presented in the journal Science. His team studied 20 quarry ponds in Lower Saxony for six years. "We created shallow water zones in some lakes, introduced deadwood into others and released fish into yet other lakes. Unmanipulated lakes served as controls," explained the first author of the study, Johannes Radinger from IGB.
A targeted stocking with five different species did not bring anything. The introduction of deadwood into the bank area only had an effect in isolated cases. The most successful was the establishment of shallow water zones. These ecologically significant habitats are usually absent in quarry ponds. The water there warms up quite quickly in spring, young fish can grow up well and are better protected from predators. "It's actually something quite obvious: creating natural habitats is more effective than tinkering with individual species," Arlinghaus said.
"Then the competition will only increase"
In the case of a freshly excavated quarry pond without any fish worth mentioning, stocking with juvenile fish is usually sensible and successful, said Arlinghaus. This usually works well with certain species of fish such as carp - but not with others such as zander, especially in small, nutrient-poor quarry ponds, since the fish need large, nutrient-rich waters.
The study also failed to include species that already existed in the lakes and naturally reproduced there, such as pike, roach or tench. "If more individuals are added to an existing fish stock, only the competition increases. The fish stock regulates itself back to a quantity that is adapted to the body of water," says the researcher.
"We do not only focus on individual species in fisheries management, but it is widespread in nature conservation in general," explained co-author Christian Wolter from IGB. "With our study, we show that it helps more to look at the entire habitat," adds Radinger. That helps the whole community. For example, aquatic plants and dragonflies would have benefited from the more naturally designed quarry ponds with shallow water zones.
River expansion would be wrong
"The principle also applies to other lakes and rivers. Whenever you can improve the critical habitats, the biological communities should respond better than if you use alternatives that only benefit individual species," said Arlinghaus.
The planned expansion of the Oder for shipping goes in the opposite direction. Instead of renaturation, from an ecological point of view after the fish die-off in 2022, it means a further deterioration of the already difficult constellation, said Wolter.