Abstract
Cormorants and other wildlife populations have come in real or perceived conflicts with humans over exploited fish stocks. From gut contents of cormorants, and using an extension of the Catch equation, we estimated the degree of short term competition between great cormorants and coastal fisheries in two areas along the Swedish Baltic Sea. Cormorants consumed 10 and 44%, in respective area, of the fish biomass of six fish species harvested by humans; eel, flounder, herring, perch, pike, and whitefish. On average, cormorants consumed smaller individuals than harvested in fisheries. But for perch, cod and flounder, cormorants consumed harvestable sized fish corresponding >20% of human catches. Our competition model estimated the direct decrease in fisheries catches due to cormorant predation to be <10% for all species except flounder (>30%) and perch (2–20%). When also including the indirect effects of cormorant predation on smaller fish that never reached harvestable size, the estimated decrease in fisheries catches at least doubled for perch (13–34%) and pike (8–19%). Despite large uncertainties, our model indicates that cormorants may locally have a direct impact on human catches of at least flounder, and when incorporating indirect effects also on perch and pike. The study indicates that the degree of competition between cormorants and humans varies substantially between areas. We also included economical values in the model and concluded that for the commercially most important species, eel and cod, the estimated economic impact of cormorants on fisheries was low.
Highlights
Wildlife populations can compete with humans over shared resources
Along the Swedish Baltic Sea coast great cormorants (Phalacrocorax carbo sinesis) has expanded from a few hundred birds in the 1950’s to 42 000 breeding pairs in 2009 [4]. They consume 400–600 g of fish per individual and day [5,6], so fish consumption by cormorants has multi-folded along the coasts of the Baltic Sea
When including the indirect effects of cormorant predation on fish below harvestable size the estimated impact on fisheries catches multi-folded for the species confined to an archipelago; for example up to a 34% for perch in Karlskrona archipelago
Summary
Wildlife populations can compete with humans over shared resources. In many aquatic ecosystems worldwide there is a potential competition between fish-eating birds like cormorants and humans for stocks exploited by fisheries [1,2,3]. Along the Swedish Baltic Sea coast great cormorants (Phalacrocorax carbo sinesis) has expanded from a few hundred birds in the 1950’s to 42 000 breeding pairs in 2009 [4] They consume 400–600 g of fish per individual and day [5,6], so fish consumption by cormorants has multi-folded along the coasts of the Baltic Sea. At the same time catches, and sometimes catches per unit effort, of species exploited by fisheries have declined [2,7]. At the same time catches, and sometimes catches per unit effort, of species exploited by fisheries have declined [2,7] This has raised an issue of possible competition over fish resources between cormorants and coastal fisheries in the Baltic Sea. Commercial fisheries along the Swedish coast of the Baltic Sea Proper primarily target cod (Gadus morhua), eel (Anguilla anguilla) and salmon (Salmo salar). Perch, pike, and herring are the main targeted species, but locally whitefish and sea-trout (Salmo trutta trutta) [9]
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