LAKE VERMILION— A team of researchers was in the area late last week hoping that sampling of lake sediments will allow them to piece together the historic trends of walleye numbers in 15 of the state’s top fisheries.
“It’s called the Walleye Lakes Project,” said Adam Heathcote, who is heading up the project. Heathcote is the Director of the Department of Water and Climate Change at the Science Museum of Minnesota.
The two-year study is funded by a $1.3 million grant from the state’s Environment and Natural Resources Trust Fund, although they had help from the Vermilion Lake Association for their local sampling. If successful, the research will demonstrate how researchers can rely on the size and variety of zooplankton remains in sediment cores to estimate fish populations in the past.
Lake sediments, much like tree rings, can offer a glimpse back in time as a core goes deeper. Heathcote said his team hopes to be able to look back about 200 years and they’ll use radioactive isotopes to age each layer. The makeup of the long-dead zooplankton in the sediments should provide an indication of the numbers of the larger predator fish, like walleye, found in those lakes over time. That’s because larger fish like walleye eat the smaller fish that tend to feed on zooplankton. Fewer walleye would presumably lead to more small fish, which would reduce both the size and abundance of zooplankton, which are small organisms that feed primarily on phytoplankton, commonly called algae. Conversely, a higher walleye population would reduce the number of small fish and allow more zooplankton to survive and grow larger.
That’s the theory anyway, and in an effort to better document the concept, one of the project’s researchers, Grace Hemmelgarn, a graduate student in conservation biology at the University of Minnesota, will be gathering current zooplankton samples from 35 different lakes and comparing the abundance and variety with the latest fish survey information from those lakes.
That information is critical to validating and calibrating the theory, according to Heathcote. Once developed, he said, the technique will “allow us to understand historical fish communities in the absence of traditional fishery survey data.” 
He noted that this “paleolimnological” approach is used routinely to assess the historical levels of nutrient pollution, water temperate, and chloride concentrations. The technique has also been used elsewhere to document historical fish populations. “So, we have reason to believe it will work here when properly tuned with Minnesota-specific data,” said Heathcote.
So why do the researchers look for zooplankton remains rather than walleye remains in the sediment cores? “It’s a numbers game,” said Heathcote. There are literally millions of zooplankton for every walleye so the odds of finding remains of zooplankton are much higher than the odds of finding any walleye parts, such as a bone or a scale in the sediments. That allows for a more useful sample size, he said.
Why do they care?
“One of the purposes of the project is to reconstruct the relative impact of the numerous drivers affecting walleye,” said Heathcote. That includes the effects of climate change, the spread of aquatic invasive species, and nutrient pollution. The study should help researchers better understand why certain lakes or more resilient against these stressors than others and which lakes might be more sensitive to certain drivers than others.