Niche overlap among anglers, fishers and cormorants and their removals of fish biomass: A case from brackish lagoon ecosystems in the southern Baltic Sea

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We used time series, diet studies and angler surveys to examine the potential for conflict in brackish lagoon fisheries of the southern Baltic Sea in Germany, specifically focusing on interactions among commercial and recreational fisheries as well as fisheries and cormorants (Phalacrocorax carbo sinensis). For the time period between 2011 and 2015, commercial fisheries were responsible for the largest total fish biomass extraction (5,300 t per year), followed by cormorants (2,394 t per year) and recreational fishers (966 t per year). Commercial fishing dominated the removals of most marine and diadromous fish, specifically herring (Clupea harengus), while cormorants dominated the biomass extraction of smaller-bodied coastal freshwater fish, specifically perch (Perca fluviatilis) and roach (Rutilus rutilus). Pike (Esox lucius) as large-bodied freshwater fish was the only species where recreational fisheries were responsible for the major fraction of the annual biomass extraction. A strong trophic overlap and hence a similar foraging niche was documented among commercial fishers and recreational anglers and among non-resident and resident anglers, indicating that the aversion expressed by anglers against commercial fisheries in a survey had an objective underpinning related to resource competition. By contrast, the foraging niches of cormorants and of both fishers and anglers differed strongly as evidenced by largely non-overlapping sets of species that were caught and removed by cormorants and by commercial as well as recreational fishers. However, for individual species of commercial and recreational interest, specifically perch, cormorants were responsible for a major fraction of total biomass extraction, suggesting that at the individual fish species level competition with fishers and anglers may still occur. In an angler survey, respondents expressed a preference for cormorant control, indicating the existence of conflict between fisheries and cormorants. We recommend that conflicts in the lagoon fisheries be proactively managed, e.g., through improved communication, zoning, predator control and outreach. Further research should clarify the population-level impacts of cormorants on target species of commercial and recreational fisheries as well as the relative impact of commercial and recreational fisheries on selected species of joint interest.