Bacterial abundance, population dynamics and related environmental parameters were determined in ballast water during a trans-
Pacific voyage from Japan to the west coast of Canada. Water samples were collected from four ballast tanks, two of which
underwent mid-ocean exchange (MOE) and two that remained unexchanged (control). Bacterial abundances in the unexchanged
tanks increased from ~6.4×108 cells/L on Day 0 to 1.8×109 cells/L on Day 7, whereas in MOE tanks, abundances increased from
approximately 8.6×108 cells/L (Day 0) to 2.2×109 cells/L (Day 10) before MOE (Day 14). After Day 10, bacterial abundances in
all ballast tanks declined. Despite the greater decline in MOE tanks, the final abundances in the MOE and unexchanged tanks
were not significantly different, which suggests MOE alone is not effective in reducing the risk of transferring the number of
bacteria. Bacterial abundance was assessed with respect to changes in environmental parameters within the tanks and we found a
significant increase with increased temperatures and a decrease with increased dissolved oxygen concentrations (p<0.001). If the
relationship between bacterial abundance and both temperature and dissolved oxygen concentrations is shown to be a general
characteristic for ballast water, new control protocols could be developed to minimize the number of bacteria being deballasted
into coastal harbors.