The pike stocks in the brackish lagoons around the German island of Rügen are under severe threat. | Photo: Olaf Engstedt
The population decline and its causes
Northern pike (Esox lucius) is one of the largest predatory fish in our lakes, rivers and the inner coastal waters of the Baltic Sea. In the slightly salty coastal waters around Rügen, the Bodden lagoons, the animals grow particularly quickly and to astonishing sizes of over 1.2 metres. This has made the Bodden pike famous among anglers nationwide and turned Rügen into a hotspot for fishing tourism. With a gross value added of over 10 million euros per year, the economic significance of Bodden pike fishing is around 34 times higher than that of commercial fishing for pike. Over 200 jobs in the region depend directly or indirectly on the expenditure of pike anglers.
"Various environmental changes have made life difficult for the pike in the Bodden in recent years: climate change, eutrophication and the associated loss of underwater plants as spawning grounds and habitats, the loss of spawning grounds in adjacent rivers and ditches due to migration barriers as a result of water management for agriculture and the disconnection of the pike spawning meadows flooded in spring from the Bodden. Other factors include a high mortality rate of pike eggs and larvae from sticklebacks and strong predation pressure on juvenile pike and pike prey fish due to a high cormorant population. Declining and changing prey fishes as well as fishing exploitation also play a role", explained fisheries professor Dr Robert Arlinghaus from IGB and HU, who led an extensive project on the Bodden pike from 2019 to 2023, in which the current study was also realised.
"Falling catch quotas for cod and herring – the real mainstay of commercial coastal fishing – have increased the pressure on freshwater fish such as pike from commercial fishing in the brackish lagoons in recent years. This is putting additional pressure on the already declining stocks," adds the fisheries scientist. Currently, professional fishers take around twice as many pike as recreational anglers. This recently prompted the fisheries authority in Mecklenburg-Western Pomerania to issue new protected areas during winter as an immediate measure to protect the Bodden pike spawning stock.
The role of angling
© Philipp Czapla
But what impact does recreational fishing have on pike stocks? To investigate this question, three pairs of protected areas with little or no access for anglers were compared with open areas with intensive angling in three parts of the Bodden. The researchers surveyed the pike using standardised angling methods and gillnets in order to compare the population sizes between the areas on the one hand and to investigate the catchability of the pike on the other. "The catch rates in the protected areas were on average three to four times higher than in the comparison areas. Exceptionally large pike were also more common in the protected areas. This indicates that angling also has an influence on the pike population in the Bodden waters, or did in the past", explained Phillip Roser, former Master's student and lead author of the scientific publication in the journal Fisheries Management and Ecology.
However, it is not only angling that can explain the different catch numbers in protected and unprotected areas. "It is conceivable that some of the differences in catches are due to area-specific effects of commercial fishing or different habitat qualities, even though we took care in the study to select study areas that did not differ in terms of macrophyte abundance, salinity or water depth", explained biostatistician and fish ecologist Dr Johannes Radinger, co-author of the study.
Pike are harder to catch
Another factor contributing to the reduced catch rates in the fishing grounds is the change in the pike's biting behaviour. Robert Arlinghaus: "Many Bodden pike caught by anglers are released after being caught, so they have experienced the danger posed by artificial lures or boats. In addition, pike that are particularly easy to catch are systematically removed, which can lead to an accumulation of characteristics such as low aggressiveness and lower swimming activity in the population through selection and genetic adaptation. Both effects - learning and selection - can reduce the biting motivation of newly growing or experienced pike." The research group has now been able to demonstrate the learned or genetically determined avoidance behaviour of pike that are heavily fished by anglers in the Bodden: "Of the pike that were interested in the artificial lure, a significantly higher proportion actually bit in protected areas without fishing pressure, while pike in fished areas turned away more frequently after inspecting the lure. In addition, pike from areas that had been intensively fished in the past were more likely to come off the hook after a bite, which suggests a more cautious biting behaviour", said Phillip Roser, interpreting the study results.
"Roughly a third of the differences in catches in experimental comparative angling between protected and non-protected areas can be attributed to reduced biting behaviour", added Phillip Roser, who now works at the Bavarian State Fishing Association. "Such avoidance behaviour can help to protect the pike population. Anglers, on the other hand, must adapt to the change in behaviour when choosing their fishing technique. And we as researchers realise that poorer fishing catches with rod and line only partly reflect the actual state of pike stocks", said Robert Arlinghaus.
Adaptation of fisheries and ecosystem management is required
© Philipp Czapla
The results of this and previous IGB studies on pike in the Bodden emphasise the need for improved management of pike in Bodden waters. "It would be advisable to give the pike in the Bodden the opportunity to reproduce and grow to a large adult size. This is the only way to ensure that professional fishers and recreational anglers can continue to benefit from good catches and high productivity in the Bodden in the future", emphasised Robert Arlinghaus.
As part of the Bodden pike project, recommendations for future management were developed and recommended to the Ministry for Climate Protection, Agriculture, Rural Areas and the Environment of Mecklenburg-Western Pomerania for implementation. This was based on an intensive participation process lasting more than three years with interest groups from commercial fishing, angling, tourism and nature conservation. The first measures have already been implemented: since 1 November 2024, the Udarser Wiek, Koselower See and Neuendorfer Wiek have been considered winter conservation areas for spawning stocks of pike with severely restricted removal and fishing opportunities for professional and angling fishers. For example, gillnet fishing for pike is no longer permitted during this period.
Other recommendations agreed by consensus by all parties involved, but not yet implemented, include the introduction of a size-based harvest window that protects both small and large pike, the definition of maximum mesh sizes for gillnet fishing and the reduction of the daily bag limit for anglers from the current three to one pike per fishing day.
"From a scientific perspective, however, these measures do not go far enough. What would be important is a network of protected areas without any fishing, which is currently only the case for around one per cent of the lagoon area. These areas could be periodically opened up to commercial fishing and angling, so that the fishery benefits from the protective effects in terms of higher stock sizes and increased catchability. It would also be conceivable to introduce fisher-level pike quotas for commercial fishing and a maximum annual removal quota for pike for each person involved in recreational angling. This is because partial or year-round protected areas alone will not fully solve the problem of overfishing, as professional fishers and anglers often shift their fishing effort to open areas following the introduction of spatial protection zones, which can increase fishing pressure in unprotected areas. At present, there is no upper limit on the total removals of pike from the Bodden, which is problematic for the current stock, especially if, as is currently the case, commercial fishing opportunities for other coastal fish species decline, thus increasing the incentive to switch to non-quota species such as the Bodden pike", explained Robert Arlinghaus.
Beyond fishing reglementation: improving the Bodden environment, making rivers passable, creating pike spawning meadows
The research team makes it clear that recreational and commercial fishing is not solely responsible for the decline of the coastal pike. The coastal pike is also affected by major changes in the environment of the Baltic Sea. These have reduced the current growth rate and stock productivity of the pike.
"In a stressed ecological environment, the pike is no longer able to compensate for the current fishery removals”, explained Johannes Radinger. Longer-term measures aside from the short-term regulation of angling and commercial fishing are therefore necessary. "The various pike spawning grounds in the Bodden shall be renaturalised and shallowly flooded pike spawning meadows shall be created in the Bodden fringes in spring so that sufficient juvenile pike can grow back. To this end, the connectivity of the watercourses and ditches flowing into the lagoons must be improved so that pike can reach their spawning sites, and the water management adapted.
Sustainable pike stock management requires thinking at the level of the "Bodden" or "Baltic Sea" ecosystem, which also includes managing the effects of agriculture via nutrient inputs, modified water management and modified management of the pike's predators", explained Robert Arlinghaus.
Working together with nature conservation, agriculture and water management for a healthy Baltic Sea
These and other recommendations have been available to the state ministry responsible for the environment and fisheries in Mecklenburg-Western Pomerania since June 2023. The amendment of the Coastal Fishing Ordinance is planned for 2026, and further fishing regulations could be adapted here. "Irrespective of this, it would be desirable to initiate and swiftly push ahead with restoration projects. After all, many organisms benefit from these measures, not just the pike. The pike is only representative of deeper problems in the Baltic Sea that extend into the inner coastal waters. Cod and herring are also in decline. Unfortunately, the Bodden pike is not alone", concluded Robert Arlinghaus. He points out that the co-operation of stakeholders far beyond fisheries is required, including nature conservation, agriculture, water management and tourism.
