So why are our fish getting smaller? A recent study by biologists from the University of Minnesota and Minnesota Department of Natural Resources found that freshwater fish may be significantly smaller after their first year as a result of the presence of exotic Zebra Mussels (Dreissena polymorpha) and Spiny Water Fleas (Bythotrephes cederstroemi)

The study, which took place over a 35-year timespan, compared the growth of Walleye (Sander vitreus) within nine lakes in Minnesota. Walleye are a popular game fish and are a top predator in many of North America’s lakes and rivers. Unfortunately, the study found that Walleye in lakes with nonnative Zebra Mussels and Spiny Water Fleas are up to 14% smaller than their counterparts found in lakes without the invaders.

pictured above is a Walleye chasing after a bate fish. Walleye are a top freshwater predator in North America and can grow up to 3ft in length and weigh as much as 20lb.

The Zebra Mussel and Spiny Water flea are originally from northern Europe and Asia, and were unintentionally introduced into the United State where they rapidly colonized North America’s rivers, lakes, and waterways. Both species are currently listed as an aquatic invasive species (AIS) in the United States due to their ability to cause significant ecological and economic damage.

As its name suggests the Spiny Water flea has spikes that make it inedible to most fish species. The Water Flea is a species of predatory Zooplankton that in North America feeds exclusively on native zooplankton species. (picture credit of Minnesota Department of Natural Resources)

The study suggests that Walleye growth is most stunted between the ages of zero and one. The researchers believe young fish are particularly vulnerable due to competition for prey species from the Zebra Mussel and Water Flea. Newly hatched Walleye are small, and rely on tiny organisms called zooplankton for their first year. Unfortunately, Zebra Mussels and Water Fleas feed almost exclusively on these tiny organisms, which are causing critical zooplankton populations to crash.

Larger Walleye as well as more opportunistic fish species such as Yellow Perch (Perca flavescens), have been able to make an adjustment and feed on alternative food sources. Younger Walleye however, feed almost exclusively on zooplankton for their first year and aren’t big enough to find alternative prey. The result, according to the study is smaller Walleye.

According to Gretchen Hansen, an assistant professor from the Department of Fisheries at the University of Minnesota “growth of fishes is critical to survival to later life stages”. Dr. Hansen goes on to state “Larger fishes are less likely to succumb to size-related predation and have access to a more diverse prey base”. Essentially, what Dr. Hansen is saying is the bigger you are the more prey you can swallow and the less predators that can swallow you.

Overall, smaller Walleye also mean less Walleye, and the implications are far broader than disappointing wranglers in search of a fish worthy of a trophy. According to the researchers, the decline of a top predator such as the Walleye, could set into motion a “trophic cascade” wherein the consequences reverberate throughout an ecosystem. The researchers warn that if populations of Zebra Mussels and Spiny Water Fleas aren’t controlled, trophic cascades could unfold throughout North America’s aquatic environments and many other species could face declines.

Zebra Mussels are notorious for forming dense colonies where they feed on zooplankton within the water column. Above a colony of Zebra Mussels were observed covering the seabed on Lake Michigan. (Photo credit of Mel Clark photography)

Going forward, the researchers plan to conduct research examining the effects of AIS on early growth stages in various fish species across a broader coalition of lakes. The researchers hope their findings can be used to help elucidate the mechanisms of AIS and their broader impacts on native fish species.






Hansen, J. A. G., Ahrenstorff, D. T., Bethke, J. B., Dumke, D. J., Hirsch, J., Kovalenko, E. K., LeDuc, F. J., Maki, P. R., Rantala, M. H., and Tyler, W. 2020. Walleye growth declines following zebra mussel and Bythotrephes invasion. Bio Invasions: 22, pp. 1481-1495.