Hakai Magazine

Coastal science and societies

scientific diver beneath the hull of a ship
A diver inspects a ship’s hull for invasive species, organisms carried around the world in ballast water and often attached to the hulls of ships. Photo by Laurie Penland, Smithsonian Institution

The Migration of the Supertankers

By imagining ships as migratory animals, scientists have a new way of thinking about how invasive species get around.

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by Charles Q. Choi

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Invasive species brought to foreign shores by ships can have devastating effects on ecosystems and economies. Now, by reimagining tankers, bulk carriers, cruise ships, and other vessels as different kinds of migratory animals, Ian Davidson has offered up a way to better predict their potential to carry invasive species around the world.

The idea came to Davidson, a marine ecologist at the Smithsonian Environmental Research Center, as he analyzed ships at port for research on marine invasions. Ships “move around a lot, often on long-distance journeys like migrating animals,” he says. “But rather than being organisms, they carry organisms with them.”

“When animals—butterflies, migratory birds, and marine mammals, for example—embark on migrations, they all have a certain timing, tempo, and route, and their behavior plays an important role as to how the migrations occur,” he says. Similarly, when it comes to ships, routes vary seasonally.

Davidson realized that he could group the ships into different ship types—akin to different species—which vary in what they carry around the world, and where they go. He also clustered the ships based on physical traits, such as size and vessel complexity, as well as behavioral traits, including speed, the amount of time spent at ports, and how much ballast water they carry and release.

Davidson and his colleagues found that ships vary greatly in their ability to spread invasive species. Bulk carriers and tankers, for instance, typically discharge up to 20 times more ballast water than other ships. Bulk carriers also spend up to five times longer at port than other ships, largely because of the complexity of loading and unloading cargoes such as grains and ore. Both of these traits increase their tendency to move species around.

Jim Carlton, a marine ecologist at Williams College in Massachusetts, says the new study raises an important new layer in studying invasive species. “They’ve found that individual shipping types have remarkably different aspects to them, all of which are rarely captured in our models that try to predict the next invasion.”

These findings suggest a number of strategies to reduce invasion rates, such as focusing on ballast treatment for bulk carriers and oil tankers, or the many nooks and crannies on cruise vessels that invasive species might stick to, Davidson says. Future ballast water and hull treatment technologies could also be tailored to different ship types, recognizing that some are worse than others at spreading invasive species.

“In the future, if we can consider environmental effects as new designs emerge, we could have a higher proportion of the fleet with a smaller environmental footprint compared to today,” Davidson says.