Young walleye growth rates decline by 12 to 14% in Minnesota lakes invaded by zebra mussels and spiny water fleas, researchers concluded in a study published recently.

Led by University of Minnesota Assistant Prof. Gretchen Hansen, it becomes one of the few studies to show impacts of aquatic invasive species on high-level fish such as walleye. A summary of the study in the journal Biological Invasions said slower growth makes it more difficult for baby walleyes to survive.

Fourteen percent might not seem like a big number, Hansen said, but a fish smaller than normal at the end of the first growing season can struggle to survive over winter.

The study out of the Minnesota Aquatic Invasive Species Research Center looked at a 35-year data set of first-year walleye growth in nine of Minnesota's best walleye lakes. The numbers were routinely recorded over the years by the Department of Natural Resources. In the early years, none of the lakes was invaded. But seven of the lakes became invaded by spiny water fleas or zebra mussels (Mille Lacs was invaded by both).

Hansen said researchers went to great lengths to make comparisons fair by correcting for many variables, including temperature.

Data were collected on hundreds of thousands of fish sampled since 1983 on Cass, Red, Kabetogama, Rainy, Vermilion, Lake of the Woods, Leech, Winnibigoshish and Mille Lacs. In their first year of life, the walleyes were 12% smaller in the presence of spiny water fleas and 14% smaller in the presence of zebra mussels as compared to walleye in uninvaded lakes.

Hansen said walleye growth was particularly stunted in baby fish studied at Mille Lacs. "In Mille Lacs they were quite a bit smaller,'' she said.

Both invaders reduce lake levels of zooplankton, an important food source for young walleye. Zebra mussels act as filters, reducing the amount of algae in the water, which is a food source for zooplankton. Spiny water flea eat zooplankton directly.

Hansen's research team looked at the growth of walleye and yellow perch. Perch growth was less affected by invasion, the study said.