Matt Herbert

November 2022

Under the Surface: Restoring Reefs in the Great Lakes

You might think we know everything there is to know about our Great Lakes—a focus of science and management for many years—but we still have a lot to learn. Especially underneath the surface. This is the case for Great Lakes reefs. 

When most people picture reefs, they picture coral reefs in tropical areas. But a reef is just a feature of rock, coral or sand that rises from the sea/lake bottom toward the water’s surface. In the Great Lakes, reefs are usually large piles of rock or gravel (or a mixture of those and other materials) deposited by glaciers—the same glacial deposit found in landforms that occur across our local landscapes, like moraines or eskers. 

Most reefs are relatively small, but they have a disproportionate influence on the Great Lakes. Highly productive reefs can produce tens of millions of fish that can make up major portions of a population within large areas of a given Great Lake. Many of the most important Great Lakes fishes, including lake whitefish, walleye, perch and lake trout, use reefs for spawning.

Why Are Reefs Important for the Great Lakes?

Many reefs have been degraded by excessive sediment from rivers or coastal erosion, dredging or navigation, excessive growth of filamentous algae due to high nutrients, or a wide variety of impacts from invasive species, including habitat degradation due to quagga mussel infestation and egg predation by dense populations of invasive round goby or rusty crayfish.

Past research on reefs has helped us to understand how important reefs are for fish production and some of the reef conditions that help drive that production. The Nature Conservancy (TNC) has participated in some of this work in our Lake Michigan project partnership with Michigan DNR and Central Michigan University. As part of that effort, we rehabilitated a reef in Grand Traverse Bay and have worked to develop and test techniques to control invasive egg predators on reefs.  

How Much Do We Know About Great Lakes Reefs?

While this work has helped us to understand the importance of reefs and healthy reef conditions, most reef research has been conducted on a small number of reefs scattered around the Great Lakes. We know a lot about a handful of reefs but surprisingly little about most. In fact, most reefs have never been explored by natural resource researchers or managers. So, we don’t know the basic characteristics of these reefs, what condition they are in or what species are using them. 

For a long time, many Great Lakes habitats were generally “out of sight, out of mind.” Many reefs can be hard to get to, and underwater habitats are difficult to assess without divers or a lot of technology. Improvements in technology and diving capacity are increasing among Great Lakes agency scientists. TNC now has a Great Lakes dive team focused on reef assessment.

Because of the reef research done around the Great Lakes in recent years, there is growing recognition of the importance of healthy reef habitats and increasing interest in reef restoration.

Anglers love the restored reef in Elk Rapids, and many want similar projects elsewhere. However, because we don’t know the status and condition of most reefs and which ones are already working well for fish production, we don’t know where the restoration needs and opportunities are.

More information is needed before even considering reef rehabilitation work—we have to ensure we are providing benefits and, most importantly, that we don’t harm the functioning habitat.

How Can We Better Understand the Status and Condition of Great Lakes Reefs?  

We are working with project partners from the Michigan DNR, Grand Traverse Band of Ottawa and Chippewa Indians, U.S. Geological Survey (USGS) and Central Michigan University to assess reefs in Lake Michigan. Starting with the existing Great Lakes Aquatic Habitat Framework (GLAHF) reef layer, we use navigation charts and other available information to identify additional reefs and to create maps of their likely footprint.

Our project partners at USGS conduct multi-beam sonar at each reef, which results in a detailed map of elevations across each site. We use underwater drop cameras to take hundreds of photos along a grid across each reef. From those images, we can identify the bottom material (e.g., cobble, boulder, gravel, sand, etc.) across the reef, how deep the rock layer is (important for understanding spaces for fish eggs to hide during incubation) and the area covered by filamentous algae and quagga mussels.

Divers on our team then collect additional detailed information on the composition of the bottom, conduct counts of native and invasive crayfish hiding within the rocks, measure quagga mussel densities and the height of filamentous algae and collect invertebrate samples to be processed in the lab.

The work described above occurs during summer and early fall. We return to these reefs in November and December to determine whether reefs are being used for spawning by lake trout, lake whitefish and cisco. We return again in the spring to sample for young larval fish of these same species that have just emerged from the reef from hatched eggs.

This need for reef assessment isn’t novel to Lake Michigan. TNC is involved in very similar projects in Green Bay and Lake Erie. We have been hosting coordination calls for all the partners associated with each project to promote information sharing and to ensure that the results from these different reef assessments are comparable.

Assessments aren’t occurring everywhere in the Great Lakes. Each one requires a time commitment with multiple visits at different times of the year. Even for the geographies we are working on, we won’t be able to map and characterize every reef with this initial effort. However, we will have information on enough reefs to draw better generalizations about likely reef characteristics and conditions based on factors such as geology, depth and exposure to wave action. We are confident these assessments will be valuable enough to inform restoration and protection needs as well as encourage future efforts to complete the assessment of reefs across the Great Lakes.

People who live in the region love the Great Lakes. As awareness and interest continue to grow, reefs are becoming less "out of mind," which is important for the future of the Great Lakes and its fisheries.

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