Pike is an imposing carnivorous fish and a popular and tasty meal. An attractive catch – not only for fishing enthusiasts but also for the tidal marsh fishermen at the Baltic Sea. Bodden Pike stocks are not yet endangered, but scarcity is already causing conflict. An interdisciplinary research project is examining the spawning, migration routes and feeding patters of these fish in the coastal waters around Rügen. The researchers, together with local collaborators, are hoping to better understand usage conflicts and find solutions to ensure sustainable management.

With a strong, practiced grip under the gills, Félicie Dhellemmes pulls the pike out of the net and on board her dinghy. A quick look at the arm-length animal, and then it disappears into the cooler. But this fish won't be landing on a plate. Félicie Dhellemmes isn't a fisherman, rather, she is a scientist. And there's no bed of ice waiting for the fish in the cooler, but a bath of clove oil. After a few minutes the fish falls asleep and lies motionless – the clove oil has put it briefly under anaesthesia.

Meanwhile, Félicie Dhellemmes and her colleague Dominique Niessner are preparing a little operation on-board. The instruments and measuring equipment lie at the ready, as does the spray bottle to keep the gills and skin of the animal wet, and the notebook, to record the data. Both researchers are wearing thick, waterproof clothing because it is cold and windy on this March morning on the Kubitzer coastal lagoon, southwest of Rügen Island.

Niessner and Dhellemmes are part of the research team from the Leibniz-Instituts für Gewässerökologie und Binnenfischerei (IGB) (Leipniz Institute of Freshwater Ecology and Inland Fisheries) in Berlin. The aim of the five-year inter- and transdisciplinary research project „Boddenhecht“ ('Bodden Pike'), which began in 2019, is to collect and analyse data on the development of fish stocks, their spawning areas and migration routes, their fishing use and socio-economic significance. The saltmarsh lagoons offer a unique habitat: in the shallow waters, the salty brackish water of the Baltic Sea mixes with the fresh water of the Peene and the Oder and other rivers. The salt percentage is low. "We still know far too little. That leads to conflicts between user-groups, and to management recommendations which are not based on facts," says Robert Arlinghaus, Professor for Integrated Fisheries Management at the Humboldt University of Berlin and head of the 'Bodden Pike' project.

Researchers are currently collecting extensive data about the biology of pike in seven weeks of field work in the coastal shallows of the Baltic Sea off the island of Rügen. Together with participants from professional fisheries and angling clubs, management and conservation organisations and the tourism industry, collaborative concepts are being developed to ensure the sustainable management of pike populations. "We need to find out, among other things, which areas are important for their reproduction, if we want to protect the population," explains behavioural ecologist Félicie Dhellemmes.

To do this, the researchers are attaching a tracking device to the female fish they caught today whose bellies are swollen with eggs. This will enable them to track the migrations of the large predatory fish through the lagoon. "Yesterday we caught these pike a few kilometres east of here in Sehrowbach," explains project coordinator Dominique Niessner. The pike migrate there to spawn in the shallows near the bank among the aquatic plants. "We want to find out if the pike will migrate back there again, or if they might also spawn in the brackish water of the lagoon," says Félicie Dhellemmes of the aim of the investigation.

"In the coastal lagoons there are subpopulations which spawn in fresh water, similar to salmon, while others live and also lay their eggs in the brackish water. The adaptation to spawning in brackish water is highly interesting for the freshwater fish species pike, from an evolutionary point of view," explains project head Robert Arlinghaus. Indeed, spawning in saltwater conditions requires numerous physiological and genetic adaptations. The research supposes that a genetically programmed freshwater pike prefers to swim back to a river or stream to spawn, while a genetic brackish water pike can also lay its eggs in moderately salty conditions. "The first genetic analyses actually show that the freshwater and saltwater pike around Rügen differ, even though, apart from spawning, they share the same habitat in brackish water," adds Robert Arlinghaus.

The anesthetised pike female is lifted out of the clove-oil bath to be photographed, measured and weighed. At 4,500 grams and 84 centimetres she's a stately fish – but for a coastal lagoon, not unusually large. "Six months ago we had one that was 17 kilograms and 124 centimetres, it was almost as big as me," laughs Dominique Niessner. A small incision in the stomach is necessary to implant the capsule-shaped, almost 30 gram tracking device. The wound is stitched and a small piece of fin is removed for genetic analysis. The researchers need a few scales to determine the age. Lastly, a white marker is attached to the dorsal fin – in case the marked fish is caught, there is a note on it with the contact details of the research team. "The fish should be put back into the water if possible, so that further data can be collected," explains Dominique Niessner. "If that isn't possible, then the tracking device and the head should be held onto." The fish's otoliths (ears), the bones of their shoulders and their eye lenses can contain important information about each animal's life cycle, migration routes and diet and should be analysed in a lab at a later time.

The tagged pike is now placed into a water bath where it can wake from its anaesthesia. After 20 minutes it can be put back into the lagoon – but in the beginning only into a fishnet, as each pike is released into a different part of the lagoon. The distance from the location where it was caught in Sehrowbach is important. The captured and tagged fish are released at different distances from the location where they were caught, sometimes up to 18 kilometres away. Will the fish swim the long distances back to lay their eggs? Or will they seek a suitable place in the lagoon and tolerate the brackish water while spawning? Some animals are even released into a different stream, to see whether they will accept these waters as a spawning area. 140 underwater microphones , placed on the assumed migration routes of the fish in the lagoon and at the mouths of streams provide information about these routes. They receive the acoustic signals emitted by the tracking devices placed in the bellies of the fish. Because the devices are configured uniquely, the signals can be assigned to the individual fish.

Der besenderte Hecht kommt nun in ein Wasserbad, wo er aus der Narkose erwacht. Nach 20 Minuten darf er zurück in den Bodden– erst einmal aber nur ins Fischnetz. Denn jeder Hecht wird später an einer ganz bestimmten Stelle im Bodden ausgesetzt. Wichtig ist dabei die Distanz zur Fangstelle im Sehrowbach. Die eingefangenen und mit Sendern ausgestatteten Hechte werden unterschiedlich weit und bis zu 18 Kilometer entfernt von der Fangstelle ausgesetzt. Nehmen die Fische den weiten Weg auf sich und schwimmen wieder dorthin zurück, um abzulaichen? Oder suchen sie eine geeignete Stelle im Bodden und tolerieren somit auch Brackwasser zur Fortpflanzung? Außerdem werden einige Tiere in anderen Bächen freigelassen, um zu untersuchen, ob sie auch diese Gewässer als Laichgebiet akzeptieren. Über die Wanderrouten geben 140 Unterwassermikrofone Auskunft, die auf den angenommenen Wanderrouten der Fische im Bodden und an Bachmündungen platziert sind. Sie fangen die akustischen Signale auf, die die Sender in den Fischbäuchen abgeben. Weil die Sender unterschiedlich eingestellt sind, können die Signale jedem Fisch individuell zugeordnet werden.

While tracking devices are attached to more fish in the dinghy, PhD candidate Timo Rittweg and fishery student Phillip Roser head 25 kilometres further north-west to the shallow waters of the Breeger Lagoon to catch more pike – with fishing rods and waders. The fish caught here will also be tagged and released. This time with a transmitter attached to their backs which measures temperature, salinity and water depth. If the fish is caught again at some point and the data sent back to the researcher, Timo Rittweg uses the digital data as well as the fish's otoliths. Like fish scales, these also have growth rings which become visible when the approximately one centimetre large, bone-like pebble is cut into thin slices. "Using an isotope analysis of the rings, I attempt to reconstruct temperature and salinity levels of particular periods of the pike's life," explains the marine biologist. He compares this data with the data received from the transmitter to validate the methods.

The first fish tagged by Rittweg and Roser is, at 97 centimetres, also an impressive example. So, why do Bodden Pike get so large? A freshwater pike measures on average 65 centimetres, while in the saltmarsh lagoons it is 75 centimetres. In the regions around the lagoons the large pike are an economic factor – for professional and hobby fisherman as well as for tourism. In the shallow waters of the saltmarsh lagoons the nutritional content is high and circulates continually – "a main factor in the exceptional growth of the pike," says Timo Rittweg. "Additionally, the moderate salt content might help the fish live in a more energy efficient way. Also, the migration of herring from the ocean into the lagoon in spring gives the predatory pike an added energy boost." This is likely why the pike grow larger here than in most rivers or lakes.

The researchers are also investigating other questions about the life cycle of pike: where exactly do the pike migrate and on which environmental factors does this depend? How many fish are killed by algae, how many by commercial fishing? To what extend are the different subpopulations of pike in different lagoons connected to each other? Are there genetically distinguishable subpopulations that ought to be managed differently?

"The project is based on close cooperation with commercial and hobby fishermen. It is a project from fishery scientists for fishing practices," emphasises its initiator Robert Arlinhaus. The collaboration is based, on the one hand, on the data collection: if a tagged pike is caught at any point in a net or on a hook, the researchers require the location data. For every tagged fish reported for the first time, the finders receive a fee of 100 Euro. In total, there are currently 360 tagged animals swimming around Rügen Island. About 1,800 selected fishermen, tour guides and anglers are also helping to tag more fish. In addition, angling, professional fisheries and nature conservation associations as well as the fisheries and conservation authorities are important project partners, as they are helping to develop management options for the future of the Bodden Pike in an accompanying task force. "The interest groups are shaping this process, not us scientists – a core indicator of transdisciplinary research," explains Robert Arlinghaus.

Pike is currently heavily fished in the Bodden lagoons around Rügen. In technical terms, this means that the pike population is 'fully used'. "Stocks that are used to their full capacity such as this consist mainly of young and small fish that don't live to their maximum age due to the high mortality rate," explains Timo Rittweg. "Moreover, the pike population is only about half as large as when it was not fished, and the Bodden Pike population has been gradually declining for ten years," states Robert Arlinghaus. The anglers, especially, have an interest in the particularly large and therefore coveted animals. Anglers from all over Germany vacation in the Bodden region hoping to catch one of the big fish. What is important is finding the right balance so that, on the one hand, fish stocks are fished sustainably and not endangered, and on the other hand, the needs of all users are met. This includes the needs of both professional and hobby fishermen.

The researcherDieter Kömle is investigating how professional fishing, angling, tourism and nature conservation can best be reconciled. "Two groups with significant interest in the use of the pike are the free time fishermen – and by association the guides who offer guided fishing tours – as well as the professional fishermen," explains the agricultural economist. Around 50,000 fishermen are in the region yearly, about 15,000 of these are local. Professional fisheries currently catch between 50 and 100 tonnes of pike in the lagoons. The anglers, who, contrary to the commercial fisheries, are not required to report their catches, are estimated to harvest a significantly larger amount of pike per annum, according to the Thünen Institute for Baltic Sea Fisheries. In general, the fish stocks are not yet grossly overfished, however the mortality rate due to fishing should not increase. Even now, the conflicts for resources are palpable.

In order to find out why exactly the pike is so attractive, Dieter Kömle is conducting detailed interviews, evaluating historical catch statistics and examining the catch diaries of anglers. "With angling, the experience is often the primary incentive," knows Kömle. "That can be either the experience of catching a huge pike and taking a photo of it, or it can be the experience of nature which is connected to fishing. Using the fish takes second place for some anglers, approximately two thirds of the pike are thrown back after having been caught." It is different with commercial fishing – here the purpose of catching pike is to market it as a food product. However, in the coastal fisheries of Mecklenburg-Western Pomerania, pike makes up only about one percent of their total intake. Next to traditional bread-fish such as cod and herring, freshwater fish such as pikeperch and perch play a much bigger economic role than pike.

For Dieter Kömle, 'Bodden Pike' is more than simply a research project. It is also a platform where all stakeholders, including local authorities and nature conservation organisations, can come together at one table. Together they can reach an agreement about which targets goals to be achieved, and how to manage pike stocks in order to do this. Scientists advise on the matter.

There are already standards regulating the fishing of pike: during the spawning season from the 1st of March until the 30th of April, no pike are allowed to be caught. Anglers are allowed to take a maximum of three pike a day and these must be a minimum of 50 centimetres long – this also applies to commercial fisheries, though they are not subject to a pike quota. Extended closed seasons, catch-quotas, changing mesh sizes for fishnets or setting a larger minimum mesh size could all have a significant influence on the development of pike stocks. There are also discussions about introducing a cap on maximum size – meaning a regulation requiring pike over a certain size to be thrown back. Kömle is investigating which of these measures meet with agreement, and which with resistance. "As scientists, we aren't here to set the regulations," he stresses. "But we can provide the data to show what kind of effect different measures would have on the fish stock and which measures would be supported by a majority. In this way we can support the different interest groups in their search for solutions. This is exactly what we do with the task force."

The field research at the Bodden lagoons, together with the socio-economic findings, are an important foundation for this. The data on the biology of the pike are entered into computer models that simulate the developments of the fish stocks and show the impact of various fishery management measures. The days in the field are hard: they start at 7.30 am and often end after dusk, when the last fish have been caught and tagged, the instruments stowed and the boats pulled ashore. "The cold is tough," says hydro-geographer Dominique Niessner. Even when they are damp, after hours of working in and on the water, their fingers have to take exact measurements and stitch up the operation wound. Sometimes the sharp teeth of a pike will catch a finger and cause a cut that bleeds badly. Nevertheless, the team is happy, and glad to be able to connect the useful with the beautiful: all of them are more than simply scientists, they also love the Bodden lagoon landscape and working with the fish. "If our project contributes to a better understanding of the pike and at the same time helps to maintain and improve their populations and fishery use, we will have reached our goal," states Robert Arlinghaus.

• Information flyer for commercial and hobby fishermen: what to do if you catch a tagged pike?
• Website for online registration of tagged pike
• Information sheet on the monitoring stations for locating the pike