Effects of Egg Replacement on the Reproductive Success and Population Abundance of a Great Cormorant Colony

27 Mar Effects of Egg Replacement on the Reproductive Success and Population Abundance of a Great Cormorant Colony

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Abstract

魚類への食害など人間との軋轢が顕在化しているカワウPhalacrocorax carboの被害防除を目的とし，3年間にわたり山梨県下曽根コロニーにおいて擬卵置き換えによる繁殖抑制を行った．擬卵には鶏卵および石膏製のものを用いたが，置き換えを行ったすべての巣で親鳥が擬卵を抱卵した．置き換えを行った巣での一巣あたりの巣立ち雛数は，置き換えを行っていない巣よりもはるかに少なく，繁殖抑制効果が認められた．特に2006年はコロニー内のほとんどすべての巣で産み足し卵についても置き換えを行った結果，194巣からわずか12羽の雛しか巣立たなかった．営巣数が年々増加したものの，コロニー全体の巣立ち雛数は2004年をピークに減少したため，擬卵置き換えは雛数抑制に有効であることが示された．しかし，雛の加入が制限されているにもかかわらず，個体数の減少傾向はみられなかった．そのため，擬卵置き換えは，個体数抑制ではなく，雛数抑制による食害防除を短期的な目標として実施されるべきである．

English 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 ma gement, but at reduction of damage to fisheries by reducing the food intake of chicks during the breeding season.