Some 90 percent of the world’s trade spends at least part of its journey at sea. Ships carry everything from oil to cars to food to rubber duckies. They also carry huge amounts of ballast water to increase their stability. When the weight of the water isn’t needed — because the ship has taken on cargo, for instance — it gets discharged.
When ships collect the water, anything living in it is sucked up into the bowels of the ship, and when it is discharged, anything that survived the trip has a chance to take up residence in its new home. If conditions are ripe, those species can thrive and even become problems. Ballast water is known to be one of the major sources of invasive aquatic species, such as zebra mussels or Northern Pacific seastars.
Robert Cope and colleagues at the University of Adelaide in Australia sought to quantify the potential for such invasions in Australian waters, including where the invaders most likely would come from and where they would land. They created a computer simulation based on traffic data and ballast records that showed where ships took up ballast water and where they discharged it. They factored in how long the tiny creatures could survive and also considered how shipping changed over the study period, from 1999 to 2012. Their results were published April 21 in Royal Society Open Science.
During the study period, the number of bulkers visiting Australia increased. These are ships that carry commodities, mostly from mining but also include timber, grain and sugar. Unlike tanker and container ships, which are most often found in the ports near major metropolitan areas, bulkers are more likely to dock in remote ports near mines. The increase in bulkers, the researchers calculated, was largely responsible for a doubling of ballast water discharged into Australian waters between 1999 and 2012. Most of that water was coming from waters around China and Southeast Asia — and so could have been transporting Asian species to Australian waters.
There’s one big caveat to the finding, though. The researchers based their calculations on ship behavior prior to 2001. And in 2001, Australia instituted new rules for discharging ballast water. Today, if you take a ship into that country’s waters, you’re not supposed to discharge that water if it came from a non-Aussie port. But Cope and his colleagues say that they found no evidence that shipping practices actually changed after 2001 (which is very disheartening for anyone who hoped that the ballast water rules would make a difference).
What’s even more dismaying, though, is that even though the ballast water issue has been known for decades (I learned about it as an undergrad), it’s still a problem. An international convention on ballast water was adopted in 2004, but the management program it set out has not yet been fully implemented. And in the United States, where ballast water is regulated by the U.S. Coast Guard and the Environmental Protection Agency, some ships won’t have to meet the new regulations to treat ballast water until 2020.
Cope and his colleagues note that unless ballast water exchange or treatment options are 100 percent effective, “some risk of ballast-water-mediated bioinvasion will exist.” What will be the next zebra mussel or Northern Pacific seastar? It may not take long to find out.