UW Sea Grant Research
Making sense of a tangled web within Lake Superior 11/18/2008
By Carolyn Rumery Betz
The story of food web interactions in Lake Superior—essentially who eats whom and who gets eaten by whom—is anything but straightforward. “There are a lot of things we don’t know about the world’s largest lake,” admits Jim Kitchell, director of the UW–Madison’s Center for Limnology.
In a Sea Grant-funded research project, Kitchell and his colleagues are in the process of developing a complex computer model to describe the interactions between 48 fish species and three invertebrates in the lake. The research includes examining the sea lamprey (Petromyzon marinus), a jawless, parasitic fish, and the fish it preys upon. Kitchell and his colleagues have observed that the average size of the lamprey—an invader that has made the Great Lakes its home for over 60 years—has doubled over the past two decades. That corresponds with increasing abundance of host species such as salmon, lake trout, and burbot.
As lamprey increase in size, they are more likely to kill than maim the host fish. “Little lamprey are parasites,” says Kitchell, “but large lamprey are vampires, and they can kill.”
To understand the consequences of increasing mortality rates on prey fish, Kitchell needs to understand what the lamprey are eating. Yet he can’t simply look into their stomachs. A sea lamprey uses its sucking disk and sharp teeth to attach to a host fish, drawing out its blood and other bodily fluids until satiated, without taking any visible part of the victim.
Scientists analyze tissues to determine the chemical makeup of the lamprey based upon what it has eaten in the past. The researchers then work backwards and use direct evidence (the size of the lamprey) and indirect evidence (its chemical makeup) to determine how many and what kind of fish the lamprey must have attacked.
The researchers are learning that lampreys attach to more than just trout, and what they prey upon varies in different parts of the lake. They also attack suckers, whitefish, and other species that are lower in the food chain. The deadly nature of their feeding habits may alter food web dynamics in the lake in ways that are not yet known.
Results of the project could be used by fish managers who try to eradicate sea lampreys on specific streams where they come upriver to spawn. Lampricides used to kill the larval lampreys are 99 percent effective, but some larvae escape and enter the lakes. Knowing which species are being eaten in which parts of the lake can help managers target their applications to locations that need the most protection.
Fish managers have learned that a completely successful lamprey control program is unrealistic. “It’s a continuing battle,” commented Kitchell. “Cost is a constraint, so it’s wise to focus efforts on the streams that produce lampreys most harmful to the most important fishery resource.”