Estimating Competition between Wildlife and Humans–A Case of Cormorants and Coastal Fisheries in the Baltic Sea

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.

Perch (Perca fluviatilis) and pikeperch (Sander lucioperca) in the diet of the great cormorant (Phalacrocorax carbo) and effects on catches in the Archipelago Sea, Southwest coast of Finland

“…These results are in line with several recently published results indicating that percids are vulnerable to cormorant predation (Rudstam et al, 2004;Fielder, 2010;Vetemaa et al, 2010;Skov et al, 2013;Östman et al, 2013). Especially in situations similar to the Archipelago Sea, where cormorant predation has occurred together with an intensive fishing, effects on the catch level have been found (Mustamäki et al, 2013;Östman et al, 2013).…”

Temporal development and spatial scale of coastal fish indicators in reference ecosystems: hydroclimate and anthropogenic drivers

“…The WFD initiated a development of fish indicators for assessment of broader ecological status in freshwaters and transitional waters (estuaries, lagoons;European Commission 2000). Although fish indicators within the WFD are associated with anthropogenic pressures, they are typically national or locally specific, likely a consequence of small systems with well-defined boundaries (P erez-Dom ınguez et al 2012). In contrast, marine and coastal waters are open large-scale ecosystem and MSFD indicators need more generic indicators reflecting key properties of fish communities over larger areas (HELCOM 2012(HELCOM , 2013.…”

Black spots for aquatic and terrestrial ecosystems: impact of a perennial cormorant colony on the environment

“…Considered by humankind as the competitors for fish resources, they were largely exterminated and their number remained low over the decades (Ostman et al, 2013). However, over the past 40 years, a great rise in the population of some species such as the North American double-crested cormorant (Phalacrocorax auritus Less.)…”