ecology and exploitation of Pike in the Bodden

– PRELIMINARY FINDINGS

Cautionary note:
The following preliminary results correspond to the state of research in the BODDENHECHT project about one year after its start. These are preliminary results which are subject to uncertainties. It is possible that some statements will have to be modified after the availability of further data and analyses. The final results and all the methods behind the findings will appear in the peer-reviewed literature, which can take months or even years to be finalized. The contents presented here are intended as interim information to inform an ongoing discussion and should not be regarded as citable scientific source.


Fishery development

  • The number of coastal fishers in Mecklenburg-Vorpommern (M-V) who pursue herring, cod, sea trout, pike-perch, perch, pike and other species has been declining continuously since the German reunification. A similar, yet not as pronounced decline is visible for the fishers targeting the lagoon (“Bodden”) fisheries. In 2019, 161 Bodden fishers in their main occupation and 77 Bodden fishers in their sideline occupation fished in the Bodden around Rügen, in the Darß-Zingst Bodden chain and in Peenestrom and Achterwasser (Fig. 1). 

 

 

  • The importance of the individual target species for the economic success of fishers varies from one fishing enterprise to another. On average, pike contributes less than 5% to the total revenue of commercial fisheries in M-V (Fig. 2). For individual Bodden fishers, however, pike may constitute the main commercial target. Pike is also of great economic importance for the commercial angling sector (guiding).

 

 

  • The number of anglers has risen sharply in the Bodden since the German reunification (Fig. 3) and today encompass more than 50,000 people that fished at least once in the Bodden for leisure. The increase in fishing participation is mainly due to the increased attractiveness as a fishing ground for fishing tourists from other federal states than Mecklenburg-Vorpommern because the number of resident fishing licenses is rather stable. In recent years, there is a  slight decline in coastal fishing visible in the license sales.

 

 

  • The number of angling tourists from other states than M-V, who come to the Bodden to fish, is now higher than the number of local fishers. Both groups together are responsible for a fishing effort of about 400,000 fishing days per year in the Bodden (about half of the effort by tourists and the other half by resident anglers).
  • The fishing activity in the Bodden waters around Rügen (from the Darß-Zingst Bodden chain to Peenestrom and Achterwasser) generates 400 full-time jobs. Among these are 300 full-time jobs, which depend on the expenditure of angling tourists in Mecklenburg-Vorpommern. The most important sector benefiting from the expenditure of fishing tourists in the Bodden is the accommodation sector (Fig. 4). These project results were developed in cooperation with the Thünen Institute for Baltic Sea Fisheries (working group Dr. Harry Strehlow) and the Leibniz Institute of Ecological Urban and Regional Development in Dresden (Prof. Dr. Artem Korzhenevych).

 

 

  • Guiding at the Bodden is offered by about 50 businesses or individuals and has developed into an important part of the fishing sector.
  • About 50% of all angling trips to the Bodden target pike, i.e., the species is a primary target for anglers.
  • Anglers, on average, desire abundant catch rates and large “meter pike” to be satisfied.
  • Diary data show that about 2/3 of all pike are released today
  • On average 2 pike are kept per angler and year.
  • Thus, since the early 2000s angling has become the main source of fishing mortality for pike, and the angling yield is higher than the total removals of pike by commercial fisheries (Fig. 5), but for methodological reasons uncertainty in the total landings is large.
  • The combined total landings (fishers and anglers in sum) over the period from 1970 to the present day show that the Bodden generates an average yield of about 250 tonnes of pike. However, the yields fluctuate strongly from year to year (Fig. 5), which can be attributed to environmental influences or changes in fishing intensities. Currently, yields are declining. Caution: Yields are not directly proportional to stock size, but also depend on catchability and fishing effort.

Pike population status in the Bodden ecosystems

  • Non-representative survey results show: anglers, especially guides, believe that Bodden pike stocks and catches are declining (Fig. 6). Personal interviews with fishers show that a comparable decline in catches is not perceived in a comparable way by fishers. From this, it can be cautiously concluded that the reduced catches by anglers could possibly be partly explained by changing catchability patterns (e.g. hook avoidance behaviour of pike repeatedly exposed to artificial bait) or that certain commercial fishers are more effective in compensating for the reduced stock sizes.

 

 

  • Due to lack of monitoring, the ability to objectively look back in time and say exactly how the development of the pike stock was is impossible; the monitoring data to reconstruct stock indices simply do not exist at a reliable level.
  • That said – a trend analysis of stock development of the past 20 years based on an abundance index of commercial pike landings per boat (normalized catch per unit effort) suggest that a decline in pike stocks has happened in Greifswalder Bodden. For the other Bodden there are no comparable declining trends. Instead, there are relevant fluctuations between individual years and the population index also varies in absolute terms between individual Bodden (Fig. 7), suggesting that the pike abundance is systematically higher in certain Bodden compared to others where pike-perch dominate.

 

 

  • Further stock assessment analyses show that the Bodden pike stock is fully or even slightly overfished, and the current biomass trend is negative. The reference point of maximum sustainable yield (MSY) is used as a basis to judge the overfishing status (Fig. 8). The MSY describes the maximum possible biomass extraction that can be realised permanently by the self-reproducing stock. If the pike are fished at a rate higher than the fishing pressure that produces the MSY, overfishing sets in, i.e. the pike are caught too young and have not yet reached their maximum growth potential. This point is a popular overfishing indicator in fisheries science. According to available information, the productive capacity of the pike stock is either fully realized (i.e. fishing pressure is equal to that which reaches MSY) or the stock may even be slightly overfished (i.e. fishing pressure is higher than that which reaches MSY = overfishing has begun, Fig. 8). 
  • As the current fishing mortality on pike is high, it is very likely that the pike size are no longer maximal. This means that the combined fishing mortality by anglers and fishers is currently so high that the fish no longer reach their maximum length because they cannot grow old enough. The reduced average sizes are particularly problematic for anglers who are often after trophy pike.

 


Environmental impacts on pike

  • Main determinant of successful recruitment of pike is the availability of vegetated habitat serving as spawning and refuge habitat (Fig. 9): the more weed there is, the more pike are found in the water. Pike spawn on underwater plants and the young pike need these vegetated habitats as refuge. For this reason pike are abundant in clear, macrophyte rich sytsems, while pike-perch can also cope very well with very turbid and water plant-poor situations: the more turbid and rarer the aquatic plants in a body of water, the fewer pike and the more pike-perch can be found. These patterns are also visible in the Bodden.

 

 

  • In addition, for successful reproduction of the freshwater fish species pike, the salt content must be permanently below 10 PSU or 10 per mille.  Basically, all Bodden offer these conditions all year round on average (Fig. 10). However, especially in the saltier West Rügen Bodden (Fig. 11), it cannot be ruled out that saltwater inflow events during the spawning season in certain years may have a negative impact on pike numbers and also influence pike distribution. The latter is currently being investigated by pike telemetry in the area.

 

 

  • The different Bodden differ in degree of nutrients (eutrophication) and in turn in visibility (Fig. 12). The availability of macrophytes decreases with the turbidity. The more turbid a Bodden, the fewer aquatic plants persist and pike abundance declines, while pike-perch rise (Fig. 7). Typical pike-perch-dominated Bodden are Saaler and Barther Bodden, Peenestrom or Kleiner Jasmunder Bodden, while the clearer, macrophyte-richer Westrügen Bodden or the Greifswald Bodden generally offer higher pike abundance. 

 

 

  • An anadromous migration behaviour of pike comparable to that of salmon has been described in various areas of the Baltic Sea. Accordingly, pike spawn in freshwater inlets and then migrate to the brackish environment to feed. It is also known for the region around Rügen that pike sometimes migrate into tributaries, ditches and streams. However, recent studies by the University of Rostock show that a large proportion of the pike from Kubitzer Bodden and other Bodden around Rügen have recruited from brackish water (Möller et al. 2020). Accordingly, the pike around Rügen have adapted over time to spawning in brackish water conditions.
  • Since reunification, the main environmental factors (see Figs. 9 and 10), on average across the year, have not shown any directed changes in areas that could become a problem for pike. For example, there is no evidence of directional trends towards saltier conditions (Fig. 9). Also the nutrient load has decreased in many Bodden. And even if turbidity has not decreased in all Bodden (Fig. 11), there is no evidence of massive changes in aquatic plant coverage over the last 5-10 years. Although there are currently significantly fewer aquatic plants detectable in the Greifswald Bodden than in the 1940s, the current underwater vegetation cover are comparable with the conditions at the end of the 1980s. Even the melioration in the 1970s in the GDR, which led to the loss of formerly important pike spawning grounds in wetlands, is so long ago that it cannot have caused current declining pike stocks. There is therefore no obvious evidence that environmental factors have developed in a dramatic negative way for pike over the last 10 - 20 years.
  • The current knowledge on how the environment drives the pike population in the Bodden can be summarized as follows:
    • The pike stock is heavily exploited and perhaps not optimally managed, currently declining, at the brink of overfishing or even overfished, but certainly not collapsing.

    • Key contemporary impacts on Bodden pike involve mainly fishing (recreational angling and commercial). Stickleback (through predation of eggs and larvae), cormorant and seals might additionally impact the pike stock perhaps locally, e.g. in Greifswalder Bodden, but data gaps are large and statements are highly uncertain. Studies by Winkler et al. (2014) on cormorant diets on the coast M-V show that cormorants predominantly eat fish other than pike. One bachelor thesis on seal nutrition at the University of Greifswald also showed that not a single pike was eaten, but the sample size was small.

    • Historically, the separation of pike spawning grounds by the GDR's melioration in the 1970s certainly destroyed spawning grounds in wetlands. However, this restriction did not lead to the collapse of the pike stock in the 1980s, and perhaps merely increased the adaptation to brackish water spawning. Currently there is no evidence of declining submerged macrophytes that have a negative impact on pike stocks, but especially in the Greifswald Bodden, the plant cover in the 1940s (90%) was significantly higher than today (6%) (Kainstinger et al. 2018). Any increase in the weed biovolume will have a positive effect on pike.

    • Further possible impacts stem from pollution and reduced herring abundance in recent years in selected Bodden areas, which might affect pike growth (herring) or affect health status (pollution). The extent of the impact cannot be quantified precisely. Once again the Greifswald Bodden stands out, which historically was one of the most important herring spawning grounds and currently offers a reduced abundance of herring as documented by the Thünen Institute of Baltic Sea Fisheries.

    • Limited obvious influence on the pike population in the Bodden emerges from the long-term trends in average salinity and temperature. The salinity values are on average relatively stable on a monthly average and although climate change has led to a verifiable warming of the Bodden, the degree of warming is currently unlikely to threaten the stock. These statements are however uncertain.


Pike management

  • Management of pike is possible through three approaches: stocking, habitat enhancement and regulation of fishing mortality.
  • Based on many years of research in freshwater (Arlinghaus et al. 2015) and experiences from the Baltic Sea in Denmark (Larsen et al. 2005), stocking can be discarded as useful in self-recruiting pike. Whenever pike reproduce naturally, as in the Bodden, stocking is ineffective.
  • Improvement of habitats (e.g., through reconnected wetlands) is done in Sweden (pike factories), but the success in terms of elevated the stock size of the entire population is currently unclear.
  • Regulation of fishing mortality is possible through a range of tools and approaches, many of  which are already in use. First models developed in BODDENHECHT suggest that harvest slots may reach better compromises among multiple fisheries objectives compared to minimum-length limits (Ahrens et al. 2020). Other harvest regulations such as protected areas, bag limits, minimum-length limits, spawning seasons and gear restrictions as well as regulations of gear types and forms of fishing (e.g., ban of commercial trawling) are already in place and can complement and perhaps even substitute size-based harvest regulations.
  • Making concrete recommendation is the task of the ongoing working group and depends on stakeholder wishes and management objectives. Defining these is not the role of us scientists and shall be done through the stakeholders in the working group (Fig. 13). Due to the COVID 19 pandemic, the work of the working group is currently suspended and will be resumed as soon as possible.

 

 


Robert Arlinghaus, Dominique Niessner, Henry H. Hansen, Rob van Gemert und Dieter Kömle.