Niche overlap among anglers, fishers and cormorants and their removals of fish biomass: A case from brackish lagoon ecosystems in the southern Baltic Sea
Introduction
Conflicts in fisheries are well documented (Bavnick, 2005; Bishop and Samples, 1980; Kearney, 2001, 2002; Spijkers et al., 2018). From a social science perspective, conflict among stakeholders is caused by goal interference (Jacob and Schreyer, 1980). Accordingly, stakeholders may perceive reduced satisfaction attributed to somebody else‘s behavior, which produces outcomes that conflict with their own expectations (Arlinghaus, 2005). For example, when two types of fishers, such as commercial or recreational fishers, target the same fish species (i.e., have the same goal), elevated landings of a particular species by one fisher type might be perceived by the other group as preventing reaching their own goals (e.g., expected catch) in the future. Conflicts do not necessarily involve direct interactions and are often value-based (Arlinghaus, 2005; Boucquey, 2017; Loomis and Ditton, 1993). Moreover, conflicts in fisheries are often asymmetric, i.e., they may only be felt by one party (Manning, 1979, 1999; Martinson and Shelby, 1992). Conflicts may be founded on the behavior or even the mere existence of other people, groups, or animals. Well-documented examples in fisheries are the conflict around fish-eating wildlife (Brehens et al., 2008; Cowx, 2003; Steffens, 2010) or conflicts among resident and non-resident anglers that differ in values and behaviours (Aas and Skurdal, 1996). Solving conflicts typically requires some form of managerial intervention, including facilitation, communication and conflict resolution (Redpath et al., 2013).
Within the sector of fisheries, conflicts between commercial and recreational fisheries are common in situations of co-exploitation of fish stocks (Boucquey, 2017; Kearney, 2001, 2002; Loomis and Ditton, 1993). Often, both groups blame each other for reduced access to fishing grounds, interference during fishing, overfishing, damaging or stealing of gears or illegal harvest (Dorow and Arlinghaus, 2012; Quinn, 1988; Sullivan, 2003). Beyond competition for fish, fisher-angler conflicts often have a root cause in different interpretation of value, e.g., is a fish generating more value when captured by a recreational angler or a commercial fisher, and general discussions about the moral appropriateness of the use of fisheries resources by either fishers or anglers (Boucquey, 2017).
Another common conflict in fisheries is, broadly speaking, intersectoral when conservation and fisheries sectors collide (Arlinghaus, 2006; Brehens et al., 2008; Hansson et al., 2018; Marzano et al., 2013; Steffens, 2010). For example, as a result of European bird conservation regulations, cormorants (Phalacrocorax carbo sinensis) have strongly increased in abundance in many areas of Europe, in some cases causing depletion of local fish populations (Dieperink, 1995; Jepsen et al., 2019; Östman et al., 2013; Rudstam et al., 2004; Skov et al., 2014; Veneranta et al., 2020). Protection of the cormorants in Europe has been elevated to a highly symbolic conflict between bird conservationists and fishers, creating a polarized management dilemma due to the multitude of actors involved, poor scientific basis and the pan-European nature of cormorant population dynamics and associated conservation regulations (Brehens et al., 2008; Marzano et al., 2013). Current scientific knowledge indicates that ecological predation effects of cormorants depend on local context and are sometimes non-detectable (Lyach, 2020), but can also be substantial and affect fish abundance and size-structure of fish populations (Klimaszyk and Rzymski, 2016; Östman et al., 2013; Veneranta et al., 2020).
Certain conflicts in fisheries, e.g., those among commercial and recreational fishers targeting the same resource (Bishop and Samples, 1980; Kadagi et al., 2021; Kearney, 2001, 2002; Ngoc and Flaaten, 2010; Samples, 1989), may be framed from a human ecological perspective drawing on the ecological theory of competition for resources among foragers (Berkes, 1984; Monk et al., 2018). Competition occurs when at least two species or stages within species desire to acquire the same scarce resource (Begon et al., 1996). One way to understand the degree of possible competition among foragers is to measure and compare the trophic niches of the foragers (Berkes, 1984) and to additionally assess and evaluate total resource extraction by each forager (Hansson et al., 2018). In humans, competition between stakeholder groups may arise when their trophic niches overlap substantially in terms of the composition of fish species that are targeted. To properly understand the conflict potential between different forager groups that involve humans it is important to measure both trophic overlap among foragers and their total resource extraction in the same environment, and simultaneously to use stakeholder surveys to understand whether specific groups indeed are perceiving conflict and desire to regulate or constrain the party with whom they are in conflict.
Mixed commercial and recreational fisheries are present in many coastal waters (Ihde et al., 2011; Hyder et al., 2018). While commercial fishers (Pauly et al., 1998) and recreational fishers (Donaldson et al., 2011) may both desire the catch and harvest of adult upper trophic level predators, cormorants have been described as opportunistic foragers that mainly target smaller-bodied, highly abundant species of lower trophic levels (e.g., Hansson et al., 2018; Keller, 1995; Suter, 1997). However, in the coastal environment of the Baltic Sea there are also low-trophic level fish that are intensively targeted by commercial fisheries, e.g., herring (Clupea harengus) or cyprinids such as roach (Rutilus rutilus), where impacts of cormorants may be economically relevant. Work by Hansson et al. (2018) and Östman et al. (2013) has suggested that cormorants in the Baltic Sea exert their greatest impact on the more accessible coastal fish species rather than on those regularly using offshore habitats.
The Baltic Sea is an example where commercial fishing for herring, cod (Gadus morhua) and other marine species is prevalent, but where the total removals of marine fish by recreational fishing are non-negligible (Hyder et al., 2018; Radford et al., 2018). In the Baltic Sea cormorants have increased strongly in abundance and may thus lead to competition for fish, particularly in inshore coastal habitats (Östman et al., 2013; Hansson et al., 2018; Mustamäki et al., 2014; Veneranta et al., 2020). We studied the lagoon fisheries around the Island of Rügen in the southern Baltic Sea in Germany as a model coastal European ecosystem where commercial and recreational fisheries as well as cormorants are all abundant and jointly exploit a shared multi-species resource composed of both freshwater and marine species. Pike (Esox lucius), pikeperch (Sander lucioperca) and perch (Perca fluviatilis) are examples of stenohaline freshwater fish that are common in the lagoon ecosystems around Rügen, and in many areas of the Baltic Sea under oligo- to mesohaline conditions. These species are targeted by commercial fisheries with varying intensity (Östman et al., 2013; Winkler, 1990), and they are also prime targets of recreational fisheries (Basan, 1988; Larsson et al., 2015; Östman et al., 2013; Weltersbach et al., 2021). Concerns that fishing and other forms of predation might impact stock dynamics of freshwater predators in the Baltic Sea have emerged (Olsen, 2019; Östman et al., 2013). Specifically, cormorant predation may negatively affect the abundance of certain coastal species in the Baltic Sea, such as pikeperch or perch (Hansson et al., 2018; Östman et al., 2013; Mustamäki et al., 2014; Winkler et al., 2014). Yet, important knowledge gaps remain. For example, Hansson et al. (2018) presented zero pike removals for the entire ICES subdivision 24 by cormorants. This does not agree with local studies in this area, which reported substantial removals of fish by cormorants (Winkler et al., 2014).
The present study had four objectives. The first objective was to describe the development and current size of the three forager populations (commercial fishers, recreational anglers, and cormorants), with a special focus on recreational fisheries, to deal with the widespread lack of local data on recreational angling harvest in the Baltic Sea (Östman et al., 2013; Hyder et al., 2018). The second objective was to analyze the niche overlap of the three forager populations from the brackish coastal zones of the German Baltic Sea following the methods of Berkes (1984) and thereby examining the potential for between-group conflicts. The third objective was to present novel estimates on the total fish removals for the three forager types following the example of Hansson et al. (2018) but tailored to local scales. The fourth and final objective was to examine whether the perceptions of the human stakeholder groups about each other and preferred management actions would match objective data about niche overlap and resource extraction. We hypothesized a strong foraging niche overlap among commercial and recreational fisheries as well as among resident and non-resident recreational anglers due to similar target species, and little overlap among cormorants and either fishers or anglers. Following earlier research on eel (Anguilla anguilla) fisheries in the same study area (Dorow and Arlinghaus, 2012), we also hypothesized that anglers would have a negative attitude towards commercial fisheries and cormorants, specifically assuming both groups to have a stronger impact on target stocks than recreational fishing.
Section snippets
Study area
The study area (Fig. 1) encompasses classical and choked lagoon ecosystems and estuaries around the island of Rügen, Germany, covering a total area of 1.584 km² (Biester, 1991; Schiewer, 2008). The lagoons are eutrophic to polytrophic and offer a mix of freshwater, marine and diadromous fish species (Winkler and Schroeder, 2003). Typical upper trophic level fish are pike, pikeperch and perch. Other commonly targeted marine and diadromous fish species are the western Baltic spring spawning
Development of recreational fisheries
The total number of general fishing permits (which are issued only for residents of M–V and allow to use either inland or coastal waters depending on whether the permit holder buys a separate water-specific angling permit, Fig. 2) in the state of M–V rose from about 40,000 in the mid-1990s to close to 100,000 today, but has recently shown a saturating pattern (Fig. 2). By contrast, the total number of coastal fishing permits required additionally to actually be able to fish at the coast of M–V
Discussion
Both study hypotheses were supported. We detected a high potential for conflicts among commercial and recreational fisheries due to strong trophic overlap. By contrast, limited trophic overlap was found for both fishery forms and cormorants when the entire species mix harvested by the foragers was analyzed. Yet, for individual species, individual foragers, either fishers, anglers, or cormorants, were dominating the total biomass removals, suggesting at the species-specific level the dominant
Conclusions and implications
Our work has shown that there is a solid basis for conflict in terms of competition for the same resources among commercial and recreational fisheries in the Rügen area and among resident and non-resident anglers. Therefore, reported conflicts among these stakeholders (Vogt, 2020) are backed up by objective data. The potential for conflict is aggravated by the fact that there is often one fishery form that strongly dominates the total removals for a given species or species group. With the
Author contribution
RA, CK, HVS conceptualization of study, RA, MSW, DK, HMW, HVS data generation, RA, JL, MSW, DK, HMW, CR data analysis, RA, JL, MSW, DK, HMW, CR, CK, HVS interpretation of results, RA writing of paper, with all others involved in revisions.
Declaration of Competing Interest
The authors report no conflict of interest.
Acknowledgements
We thank the European Union through the European Maritime and Fisheries Fund, and the State of Mecklenburg-Vorpommern (Germany) (Grant MV-I.18-LM-004, B 730117000069) for funding of the Boddenhecht project. The 2014/2015 telephone-diary survey was conducted in the framework of the German marine angling program of the Thünen Institute of Baltic Sea Fisheries and co-funded by the European Commission’s Data Collection Framework (DCF) and the State Office for Agriculture, Food Safety and Fishery M–V
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