In addition to organisms stowing away in ballast water, ships transport aquatic and marine life on their submerged hulls. The environment created by such organisms as barnacles, sea squirts other 'fouling' organisms, serves as protective habitat for even more species.
When the vessel's hull is cleaned, the biofouling community is scraped off and may well find its way into the harbor, bringing a host of species native to elsewhere.
A SOLUTION? 'Paint' the hulls of vessels with a preselected mix of fouling organisms , of a sort that is non-invasive in nature, that is already globally ubiquitous, that nonetheless makes short shrift of any other species that tries to com aboard the hull to pull an aquatic hitchhike across the seas by fouling to the ship or to the boat.
"Go away! There's no room at the inn", the invader wannabe is told.
Thats the idea, anyway. Let's explore it further at a later time and date.