Effect of mid-oceanic ballast water exchange on virus-like particle abundance during two trans-Pacific voyages.

Ballast water is a potential source of invasive species, including viruses that target a variety of hosts. We sampled ballast during two trans-Pacific voyages and analyzed the efficacy of mid-oceanic exchange in reducing virus-like particle (VLP) abundance. Exchange did not significantly reduce virus abundance during the first voyage (P=0.874), whereas it reduced viral abundance 3.9-fold from 1.8 × 10(7) to 0.47 × 10(7) VLP mL(-1) during the second voyage (P<0.0001). Despite the impact of exchange during the second voyage, virus abundances were not significantly different between exchanged and unexchanged tanks upon arrival in Canada (P=0.363) and Canadian port water samples (P=0.502). Regressions between environmental parameters and VLP abundance uncovered negative correlations between salinity and viral abundance during one, and dissolved oxygen and viral abundance during the second voyage. In summary, ballast tanks are highly variable with respect to total virus abundance, and the efficacy of exchange requires investigation into the dynamics of specific viruses.