The number of Great Cormorants at a resting site within Tokyo Bay

Abstract

I conducted the number of Great Cormorants Phalacrocorax carbo resting on artificial beaches at the Kasai Rinkai Park in Tokyo Bay from March 1996 to June 2012. Annual usage patterns were observed, including a rapid annual decrease in the number of cormorants using the rest site during February to March, and an annual increase of usage from June to September. The number of resting cormorants appeared to fluctuate according to their movements between roosting and foraging locations. During the winter season, the resting site at Kasai Rinkai Park is not along the route to the foraging site, but during the summer the cormorants pass over Kasai Rinkai Park along the route to the foraging sites. Counter to this general observed pattern of annual usage, the number of resting cormorants gradually decreased from July 2004, although the population of neighborhood colonies did not decrease. This suggests that the foraging site used by this population had changed from that of other years during the survey. Daily usage pattern at this resting site was also observed by conducting surveys during three periods each day, and the maximum number of cormorants monitored daily was generally recorded at noon. The roosting colonies associated with this resting site are located in areas with little potential disturbance that could restrict the cormorants time of return to the colonies. Therefore, it is possible for the resting cormorants to continue the return trip to the colony before the evening observation period.