Recently, the increasing population size of great cormorants (Phalacrocorax carbo) in Niigata (Watanabe 2007, Fujita et al. 2011) caused serious inland fisheries damage around middle basin, but not around lower basin of Shinano river. In this study, we estimated the population growth rates of great cormorants at 2 colony sites where the fishery damages were different in degree in order to propose how to control the population size of cormorants.

We observed breeding population size, number of nests and number of fledged chicks per nest at Suidocho colony at lower basin (37°7'N, 138°9'E) and Konegishi colony at middle basin (37°2'N138°7'E) from march to may in 2011. The population size of next year in both colony calculated by using the equation (N_t+1 = N_t e^rt ; N_t+1 and N_t are population size of next and this years, r is population growth rate, t is years) was estimated as 1.23 times than this year and the population growth rate is 0.209. As the population growth rate (r) in middle basin area colony should be below 0 because of its serious fishery damage, we have to capture more than 225 chicks. On the other hand, as the possibility that population size in lower basin area must be increase by breeding, we have to capture 154 chicks in next year to maintain the present population size.