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Mapping the “Secret” Spawning Grounds of Lake Trout

Little is known about what the bottom of Lake Superior looks like near the shore. This is a vital area because many fish spawn at depths of less than 30 meters (100 feet), including lean lake trout (Salvelinus namaycush). Lake trout are the top predator in the Lake Superior fish community and they’re what most anglers try to catch. To ensure a healthy population, resource managers need to know where their spawning areas are.

Lake Superior scuba diver

A diver swims over prime lake trout spawning habitat off the North Shore of Lake Superior.

In fact, the idea for the project to map the bottom of the lake near the shore came from Minnesota Department of Natural Resources (DNR) personnel. The DNR teamed up with me and researchers from the U.S. Geological Survey (USGS) in Ashland, WI, the Natural Resources Research Institute (NRRI), and the University of Minnesota Duluth. We used a unique combination of technologies to discover habitats where lake trout are likely to spawn: a global positioning system (GPS), a high-tech sonar device similar to a fish finder, and a computer mapping program called a Geographic Information System (GIS).

This combination of technologies has not been used in the Great Lakes before. Fifteen years ago, this would have been cost-prohibitive. Now it is much more feasible to use it to answer the common fishery question: ‘how much fish habitat is out there?’

Previous maps of the area came mainly from shipping charts that consist of only a few depth measurements per square kilometer and contain no information about the type of bottom.

Lake trout are well adapted to the cold, clear, infertile waters of Lake Superior. They generally require boulder and cobble -- rocks that vary in size from softballs to Volkswagens -- for spawning and survival of eggs and fry (young fish). Resource managers believe that young lake trout stocked in suitable habitat will return to these areas to spawn. Knowing where spawning takes place helps both wild populations and stocked fish.

Our team used two different boats furnished by the USGS to gather data: a 25-footer, the Daphnia, and the 57-foot Siscowet (ironically named for another species of lake trout). We collected data in 1995, 1996, and 1998.

With a global positioning system that recorded the exact location of the boat, we slowly cruised the near-shore areas and bounced sonar waves from the boat to the lake’s bottom. Using this method, we had the ability to collect data at a rate of two samples per second. The sonar and position information were recorded on a computer. Using a special computer technique, we translated the sound waves into bottom types and water depth. Using various colors to represent different bottom types, we created GIS maps that look like underwater topographical maps.

We used an underwater video camera to double-check the accuracy of the computer data. The camera helped us identify eight categories of substrate: sand, sand and cobble, sand over bedrock, smooth bedrock, rough bedrock, conglomerate, cobble, and boulders.

These are the first maps created of the near-shore lake bottom. The area we mapped is approximately 80 miles long, between Duluth to Grand Marais, and out to a depth of 100 feet.

“This was our first attempt to put together a map of what goes on below the surface of the water,” said Don Schreiner, Lake Superior fisheries supervisor for the Minnesota DNR. “We’re hoping that others will build on it.”

The process was an eye-opener for me. I learned that there’s much more diversity in substrate than was originally thought. While this project is important for lake trout, it also tells us more about the entire aquatic ecosystem that exists in Lake Superior.

Funding for the project was the result of an environmental incident in 1993 where tons of taconite ash slumped, some spilling into Taconite Bay during a heavy rainfall. LTV Steel reached an agreement with the Minnesota Pollution Control Agency to fund an environmental project for $240,000. The incident was the impetus for the mapping study and involved the cooperation of several state and federal agencies. Minnesota Sea Grant provided additional funding.

“The Minnesota Pollution Control Agency had the foresight to put the money back into the area where the incident occurred,” said Schreiner. “We now have a baseline map that we can add details to for 50 years.”

The information is being shared among state and federal regulatory agencies that make decisions regarding land and water use such as building and development permits, fish stocking, road construction, safe harbor development, and shoreline protection. Information about this project is available on the Web.

Marie Zhuikov and Sharon Moen with Minnesota Sea Grant, and Brenda Maas and Anna Nelson with NRRI contributed to this article

By Carl Richards
September 2000

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