Abstract
In Japan, the abundance of Great Cormorants Phalacrocorax carbo disturbs sports fishing communities. Replacement of cormorant eggs with artificial eggs has been used as a method of decreasing the reproductive success of Great Cormorants. In this study, we evaluated the effects of egg replacement on the reproductive success and population abundance of a cormorant colony over three years. Both chicken eggs and artificial (gypsum) eggs were successfully accepted by cormorants. Although some pairs relaid after their eggs were replaced, the number of fledged chicks in nests with replacement eggs was lower than in nests without replacement. In 2006, relaid eggs were also replaced in 191 of 194 nests, resulting in only 12 chicks fledging. Therefore, replacement with artificial eggs was proven to be a very effective means of reducing the reproductive success of cormorants. Despite the recruitment of young birds into this population having been reduced experimentally, the colony has increased in size, perhaps because of the immigration of birds from other large colonies. Our results suggest that egg replacement can only reduce the numbers of new chicks, and the abundance of Great Cormorants within a colony may be influenced by the population size over a larger area. In the future, methods of egg replacement of Great Cormorants should aim not at population management, but at reduction of damage to fisheries by reducing the food intake of chicks during the breeding season.
