All oceangoing ships carry seawater ballast tanks whose weight improves stability  To maintain the ship' s proper stability, water must be pumped out of these tanks when cargo is loaded and into them when cargo is unloaded  As a result, sea creatures often get into the tanks and are then inadvertently deposited into new habitats, where they can wreak ecological havoc
One viable way of addressing this problem would be to empty and then immediately refill the tanks in midocean, since midocean creatures and coastal sea creatures usually cannot survive in one another' s habitats
An oceangoing ship's ballast tanks could be emptied and refilled in midocean only in conditions of calm air and flat seas.