Abstract
To evaluate the effect of mass stocking of fish on the distribution of communal roosts or breeding colonies of Great Cormorants Phalacrocorax carbo, we examined the relationship between the size of cormorant roosts or colonies and the amount of Ayu Plecoglossus altivelis released within 20 km of each cormorant aggregation site in the Kanto region of Japan for three years (2006-2008). Approximately 14,000 Great Cormorant occur in the Kanto region and about 120,000 kg of Ayu are released in the region each year. This quantity of fish represents a substantial potential food resource for cormorants. Assuming that a cormorant consumes 500 g of fish per day, the Ayu stock is equivalent to the food requirements of all the Cormorants in the Kanto region for 17 days. Two hypotheses were examined by using generalized linear mixed models: first we tested whether the size of cormorant roosts or colonies in March was affected by the amount of Ayu released within their foraging range (20 km); and second we tested whether changes in the size of cormorant roosts or colonies from March to July were affected by the amount of Ayu released per cormorant. No relationship was observed between the size of cormorant roosts or colonies and the amount of Ayu released, suggesting that cormorants do not select the location of their roosts or colonies according to the amount of Ayu released. The rate of increase in cormorant roost or colony size from March to July was significantly greater in areas with larger quantities of Ayu per cormorant, suggesting that cormorants may increase their roost or colony sizes during the breeding season either through enhanced reproductive success or immigration.
