They come out at night: Goby fish mystery solved
Written by Greg Houtteman   
Thursday, 13 September 2007 15:32

Oceangoing freighters were long suspected of being the vehicle that carried nonnative round gobies into the Great Lakes, and now a University of Michigan researcher has documented goby behavior that may explain how it could have happened.

U-M fishery biologist David Jude discovered the arrival of round gobies in 1990 in the St. Clair River northeast of Detroit, and they eventually spread throughout the Great Lakes. Because they are bottom-dwelling fish, the mystery has always been how significant numbers could get inside a ship that takes on ballast water close to the surface.

Now Jude and U-M graduate student Stephen Hensler have documented for the first time that during the summer breeding season, countless newly hatched round gobies leave their lake-bottom homes and swim to the surface. The nocturnal migration boosts the chances that large numbers of hatchlings will get sucked into the ballast tanks.

Newborn fish of many other species - as well as tiny aquatic animals called zooplankton - are known to rise to the surface at night and descend to the depths after sunrise, following food and evading predators, Hensler says.

"If you had some sort of policy whereby ships would only take on ballast water at the surface and only during the day, it would reduce the likelihood of introducing new species and spreading existing ones around,'' said Hensler, a doctoral student at the U-M School of Natural Resources and Environment.

The voracious and aggressive round goby is native to the Caspian and Black seas in eastern Europe. In the Great Lakes, an adult round goby is typically 3 to 5 inches long. Anglers despise them because they steal bait from hooks.

Scientists say round gobies have disrupted the Great Lakes ecosystem and are responsible for local extirpations of at least three species of small, native, bottom-dwelling fish: the mottled sculpin, the greenside darter and the logperch.

Jude and Hensler report their findings in the current issue of the Journal of Great Lakes Research.

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