In Germany, fishing is a popular sport – or should we say an outdoor adventure? Indeed, the German Fishing Association names nature and waterway conservation, spending time in nature and stress relief as number one in their '10 Good Reasons to Go Fishing'. But are fishing and conservation compatible? And could anglers maybe even encourage biodiversity? The project 'Quarry Lake' hopes to answer these questions.

Three to four million anglers head out to the water at least once per year. Some are seeking time spent outdoors in the natural environment, some are hoping for an impressive catch, while yet others mainly enjoy the social aspect of time spent with friends. While anglers are pursuing their hobby, the danger arises that they might adversely affect the natural environment – for instance, by scaring nesting birds as they walk along the path to their chosen fishing spot, by flattening plants on the banks or by introducing foreign fish species thereby affecting the fish populations and the lake's ecosystem. And, of course, broken fishing lines and discarded hooks can trap and injure birds.

So, do anglers adversely affect biodiversity, or could they somehow contribute to the conservation of nature? The question of if and how fishing and biodiversity are compatible is being considered by Robert Arlinghaus, Professor of Integrated Fisheries Management at the Humboldt University, and his team, using quarry lakes in Lower Saxony as an example. These lakes are created when earth is excavated to obtain sand, gravel and other materials for road and housing construction. In Lower Saxony, unlike in other water-rich areas such as Brandenburg, more than three-quarters of the lakes are of artificial origin.

"Inland bodies of water are often ignored when it comes to biodiversity," believes Arlinghaus. "And especially artificially created bodies of water. We want to show that there's an underestimated biodiversity in these places and a user group, namely anglers, who care for this diversity." Biologists, fisheries scientists and environmental ecologists from the Leibniz Institute of Freshwater Ecology and Inland Fisheries (Leibniz-Institut für Gewässerökologie und Binnenfischerei) , along with the Technische Universität Berlin and Lower Saxon Fishing Association (Anglerverband Niedersachsen) have been working together on the project Baggersee ('Quarry Lake') since 2016. Funded by the Federal Ministry of Education and Research (BMBF) and the Federal Agency for Nature Conservation (BfN), they are looking at more than 40 quarry lakes between the Elbe, the Weser and the Ems rivers. Most of these gravel lakes are owned by fishing clubs, though some lakes, investigated for comparison, are not. One thing to note, however: owning or leasing fishing club lakes or other waterways comes with rights and obligations – on the one hand the right to fish, and on the other hand the duty to care for the body of water.

In order to be able to compare the two different types of lake, researchers have measured random samples of life in and around the water for two years: they counted trees, bushes, grasses, ferns and flowering plants, even the aquatic plants by diving. From the boat they searched for frogs and frog eggs in the water, and along the bank they searched for dragonflies. Fish were caught from the bank using nets, insect larvae were counted in sediment samples and the presence of water birds and song birds was assessed using sound recordings. The researchers also determined the number of anglers, hikers or bathers, the amount of rubbish they left behind as well as the size of paths and fishing spots at the lake.

In total, researchers identified more than 290 different animal species and 274 different plant species. This included 33 different dragonfly species, 34 water bird and 36 song bird species, as well as 22 kinds of tree. That artificially created quarry lakes would be such a biodiverse environment, approaching the biodiversity of natural lakes, was a surprise for the scientists. "The fish populations in the quarry lakes do not differ from natural waterways in terms of their biodiversity and composition," says Robert Arlinghaus.

So how do the flora and fauna at the lakes managed by fishing clubs compare to those at quarry lakes where there is no intervention? The nature there was at least as diverse as at the fishing club lakes. Only in the case of the fish did the statistical analysis reveal a noticeable difference: while five to twelve species of fish were found in the lakes managed by fishing clubs, in the unmanaged lakes it was a maximum of eight species, though usually just three to five. And this, even though the lakes owned by fishing clubs are frequented much more often by people for fishing and recreational purposes. The species diversity has clearly not been harmed by it. This also applies to the conservation value of the extant animal and plant species. The biologists found species classified as threatened or endangered on Germany's 'Red List' with the same frequency in both lake types. This includes the critically endangered Bitterling fish, a member of the Carp family, the Osprey, the Beaver and the Lilypad Whiteface, an endangered species of dragonfly. In one of the fishing club lakes, the team even found a kind of algae long considered extinct in Lower Saxony.

Especially important for biodiversity are the shorelines around the lake. These are among the most dynamic habitats of a lake's ecosystem. "Here can be found the most structures," explains environmental scientist and project spokesperson Eva-Maria Cyrus. "You can imagine the zone along the shore, with leaves and branches protruding from the shallow water, as a well furnished apartment. Fish and other animals can hide away there and, unlike in the deeper water, there is food, warmth and light." For this reason, researchers are also investigating whether biodiversity can be fostered by specifically creating such areas of shallow water. This is where the fishing clubs come in. In workshops, the anglers discussed and planned how to improve the habitats with the scientists. Club members took the implementation into their own hands. In 2017 and 2018, they established shallow water zones in four lakes, where there were previously steep banks. They are hoping that aquatic plants will grow better here in the future and will offer fish and other animals some protection. 12,000 cubic metres of earth were moved using excavators to make this possible. Additionally, club members placed bundles of wood under the water along the banks. In total, 800 of these arrangements of sticks and twigs, each measuring three metres long and weighing several hundred kilos, were placed into the four lakes.

"One year after the habitat improvement, we could already observe that aquatic plants had colonised the newly created areas of shallow water. And we saw that the bundles of brush-wood were frequented by fish and other small creatures such as water fleas, as well as fly and dragon fly larvae," reports Eva-Maria Cyrus. However, what is not yet clear is whether this has, in fact, increased the number of animal species, or if they have simply rearranged themselves. As chairperson of the fishing association Angelvereins ASV Neustadt am Rübenberge, Holger Machullahas helped to improve two of the association's lakes and knows that the natural environment needs time to grow and flourish. "We are hopeful that there will be a follow-up project that we can get involved in. Perhaps then we can find out if there are more fish and more water birds. I would find it fascinating to follow the developments over a longer period of time."

For Robert Arlinghaus, the project has already shown that artificially created lakes can be very similar to natural ones. "Nature conquers such bodies of water, so to speak, and fills them with natural species populations." After 20 or 30 years, a quarry lake will have blended into the landscape so much that it will be barely distinguishable from a naturally formed lake. Quarry lakes are important for the preservation of biodiversity, but also as habitats for endangered species – and anglers can significantly contribute to this too.

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• Flyer zum Projekt Baggersee
• Studie zum Einfluss anglerischer Bewirtschaftung auf die Biodiversität von Baggerseen (Fachartikel, Deutsch)
• Baggerseen sind Refugien für die Artenvielfalt (Fachartikel, Deutsch)
• Studie zur Biodiversität in künstlichen Gewässern (Fachartikel, Englisch)