Researchers have suspected that the invasive round goby spread throughout the Great Lakes by hitching a ride in ballast water tanks of cargo ships. There was only one problem: The round goby lives on the bottom--below the intakes of ships--so it was hard to fathom how the fish could have hopscotched from one port to another. Now scientists have discovered that at night the fish's larvae rise to the surface, where they could be pumped into ballast tanks.
About 18 centimeters long, the round goby (Neogobius melanostomus) is an aggressive predator that eats the eggs of native fishes and chases them out of their habitat. The species was first spotted in the St. Clair River in 1990, and it presumably came to the United States in ballast water that cargo ships sucked up in the Black and Caspian seas.
But how did gobies get into the ballast tanks in the first place, and how were they able to spread so rapidly through the Great Lakes? Although the fish are bottom-dwellers, limnologist David Jude of the University of Michigan, Ann Arbor, who first noticed the goby in the United States, and his graduate student Stephen Hensler, observed that goby larvae in lab aquaria would occasionally swim up from the glass floor. That contradicted all the literature on round gobies, however, so Jude doubted it happened in the wild.
Then in 2002, Hensler found a few larval gobies, each less than a centimeter long, in the surface water of Lake Michigan. "It was a giant surprise," he recalls. In the following years, Hensler and Jude netted gobies in the surface water of Lake Erie as well. Although scarce in Lake Michigan, there were up to 80 larvae per 1000 cubic meters of water in Lake Erie, the pair reports in the current issue of the Journal of Great Lakes Research. The larvae only came up to the surface during the night, probably following the zooplankton that they eat, Hensler says.
Jude says that freighters could avoid spreading gobies by pumping water into their tanks only during the day. "A lot of the damage has been done already, but [round gobies] haven't colonized all the harbors yet," Jude says. "It might slow down their transmittal." It's not a perfect solution, because other alien species might make it into the ballast tanks during the day.
Fisheries biologist Philip Moy of the University of Wisconsin Sea Grant Institute in Manitowoc thinks a requirement for daytime-only ballasting is "not a likely prospect," because the carriers would probably consider it an economic hardship. Hensler notes other good news in the fight against Great Lakes invaders: Michigan recently enacted regulations that require oceangoing ships, the major source of new species introductions, to treat their ballast water to kill aquatic hitchhikers.