Researchers use sound waves to map potential rock reef restoration sites in Saginaw Bay

Sonar helps identify conditions at the bottom of the lake.

This 3D image is created using a side scan sonar. It shows the relative hardness of the lake bottom. The yellow coloring delineates a harder substrate while the purple coloring shows areas of comparatively soft substrate. Photo: Michigan DNR

This 3D image is created using a side scan sonar. It shows the relative hardness of the lake bottom. The yellow coloring delineates a harder substrate while the purple coloring shows areas of comparatively soft substrate. Photo: Michigan DNR

Across the Great Lakes several efforts have been undertaken to restore historic rocks reefs providing improved spawning grounds for a variety of fish species. Projects in the St. Clair -Detroit River System, Grand Traverse Bay, and Thunder Bay have restored reef spawning habitat for lake trout, walleye, whitefish and lake sturgeon. Michigan Sea Grant and Michigan State University Extension played a role in a number of these reef restoration projects.

New research is currently paving the way for a similar reef restoration effort in Saginaw Bay. Part of this research requires understanding the current state of historic reef sites in the inner Saginaw Bay including assessing conditions on the lake bottom. In order to lay the groundwork for future effective reef restoration efforts, researchers need to understand how much hard or rocky substrate currently exists at potential and historic reef sites. To do this they use a special technology called side scan sonar.

Side scan sonar uses sound waves to find and identify objects in the water. It can also give an indication of the relative “hardness” of the lake bottom. Special equipment carried on a boat sends out sound energy and analyzes the return signal (the sound that bounces back). Hard surfaces like rocks reflect more sounds producing a stronger return signal compared to soft surfaces like sand. Researchers continuously monitor the strength of the return signal building a picture of the lake bottom. Objects that rise above the lake bottom such as large boulders can be identified by the “shadow” they cast in a sonar image, the area where no sound hits.

Scientists from the Michigan Department of Natural Resources Alpena Fisheries Research Station used side scan sonar to develop images of lake bottom conditions at two potential reef restoration sites in Saginaw Bay as well as two sites where the remnants of historic rock reefs can be found. They also recorded video images of the lake bottom at these locations. Their research produced detailed maps of substrate conditions that will provide valuable information for future reef restoration efforts.

Other research team members including scientists from Purdue University are studying the biological conditions of the reef site. By collecting egg samples they are able to determine what fish species may already be using the site as a spawning ground and how often. Visit the Michigan Sea Grant website for more information on the pre-restoration assessment of fish spawning reefs in Saginaw Bay and updates on project status.

Michigan Sea Grant helps to foster economic growth and protect Michigan’s coastal, Great Lakes resources through education, research and outreach. A collaborative effort of the University of Michigan and Michigan State University, Michigan Sea Grant is part of the NOAA-National Sea Grant network of 33 university-based programs.

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