The role of cormorants, fishing effort and temperature on the catches per unit effort of fisheries in Finnish coastal areas

Abstract

Population increase of piscivorous cormorants in Europe and in North America has created a conflict between fisheries and the species. The impact of cormorants on natural fish populations and yields of fishermen is still under debate. We investigated potential connection of the great cormorant Phalacrocorax carbo abundance, fishing effort and water temperature with the economically important perch Perca fluviatilis and pikeperch Sander lucioperca yields, measured as catches per unit of effort (CPUE) in gillnet fishing along the Finnish coastal areas (Baltic Sea) using 50km International Council for the Exploration of the Sea (ICES) grids. Since cormorants generally take smaller prey than fishermen, we expected 2–5 years time lag effect of the cormorant numbers on CPUE. Correspondingly, we expected 4–7 years lag effect of temperature on CPUE. Despite the population increase of cormorants, CPUE of perch increased in 10 out of 29 ICES grids during the study period 2005–2014. Pikeperch CPUE increased in five out of 24 grids and decreased in one. There was significant annual variation in CPUE values of perch and pikeperch, but values were not significantly associated with changes in cormorant numbers and temperature either annually or long-term. However, the CPUE values of pikeperch decreased towards the north, which is likely temperature driven as northern colder waters are less suitable for this species than southern waters. There was no clear evidence that either predation by cormorants or fishing effort are associated with long-term trends of perch and pikeperch stocks on a larger scale along the Finnish coast. The increasing CPUE values in several areas indicate that stocks are more abundant than ten years ago despite an increasing cormorant population. Our study approach can be used to monitor potential changes in stocks and impacts of cormorant in the future.