Net-entrapment of great cormorants Phalacrocorax carbo sinensis in relation to individual age and population size

Abstract

Population control of great cormorants Phalacrocorax carbo sinensis has been suggested as a tool to mitigate fish losses by cormorants foraging in pound nets, gill nets and fykes. The objective of our study was to quantify the difference between first-year birds and older birds in the risk of drowning in fishing gear, and to explore the influence of population size on the proportion of individuals drowning. An index of the proportion of first-year birds and older birds that drowned was obtained for cormorants ringed in the Danish Vorsø colony using resightings of colour-ringed individuals and ring recoveries of individuals found dead. First-year birds were approximately 10 times more likely to drown than older birds. We used ring recoveries of cormorants ringed in Denmark and found dead in Denmark, North Germany and South Sweden (i.e. in the main post-breeding area) to reveal changes over a 25-year period in the proportion drowned among those recovered. Among first-year birds the proportion drowned declined from 66% in 1978–1984 to 24% in 2000–2002, and among older birds the proportion declined from 46% in 1978–1984 to 26% in 1999–2002. During 1978–2000, breeding numbers in Denmark increased from 1,400 to 42,500 nests, and the proportion of cormorants drowned among those recovered was significantly negatively correlated with population size. This suggests that the proportion of the population foraging in nets declined as the population increased. The damage caused by cormorants foraging in pound nets is, therefore, unlikely to decrease in proportion to reductions in population size reached through population control.