Colony-forming eutrophic marine microorganisms in ballast water were counted in samples taken on board in 2002 and 2003. In
the ballast water in Japan, viable cell numbers were highly variable but not by more than 105.1 colony-forming units (CFU)ml1
regardless of season. Even when ballast water was discharged offshore, values varied but not by more than 105.0 CFUml1
. The
effectiveness of the ballast water exchange was unconfirmed, except for the February 2003 voyage. No microbial colonies were
counted in the reloaded ballast water in the high seas on that voyage, which contributed to the reduction of the total number of
viable cells sampled in the discharged ballast water at the Ras Laffan port in Qatar. In sediment samples, the values of 105.2–
106.0 CFUml1 were estimated for all seasons in which voyages took place. The maximum of the marine Vibrio species,
110 CFUml1
, was observed in the ballast water sample taken in July 2003. The estimated total viable cell numbers in sediments
were higher than those counted in the ballast water throughout the experiments, indicating the importance of sediment management
as well as ballast water management on vessels traveling from Japan.