Mellanskarven (Phalacrocorax carbo sinensis) i Krankesjön - Analys av långtidserier av skarvförekomster och fisksamhällets utveckling i kombination med bytesval

Sammanfattning: This study analyzed highly detailed long-term information about occurrence of cormorants (Phalacrocorax carbo sinensis) and the fish community composition in lake Krankesjön, southern Sweden. Moreover, cormorant predation with regards to species and size preference were estimated using data from recovered PIT (Passive Integrated Transponders) tags. The results shows that the cormorant population have increased over time whereas the distribution of temporal occurrence frequency within years have decreased. Thus, in recent years, cormorants have shifted towards primarily arrive in flocks after breeding season. The fish populations in Krankesjön is shown to be stable during the last fifteen years, dominated by roach (Rutilus rutilus) and perch (Perca fluviatilis). However the pike population (Esox lucius) had a tendency of decreasing although they do not get representatively caught in the fishing nets. Neither the fish biomass and biodiversity nor the species evenness had decreased or could be explained by cormorant abundance. However body condition of roach was lowest when cormorants peaked in number. Cormorant diet during autumn/winter miss-matched the overall consumption for some of the nine predated species, indicating season differences in diet composition. Additionally my results show that cormorants do not catch species according to their abundance, probably due to fish behavior and morphology. The cormorant showed a preference for larger individuals of perch which could affect lower trophic levels and thus water turbidity. Finally there was a significant higher predation of cyprinids than commercially important species. The overall results suggest that cormorant predation have no substantial effect on the fish community in Krankesjön nor is there any support for a conflict with recreational fishery. Nevertheless, effects on fish populations cannot be excluded if predation pressure exceeds 3-4 %. My results are of great importance to mitigate the ongoing conflict between cormorant predation and fisheries and for local management.