The following is a blog post by Dr. Phil Willink, senior research biologist at Shedd Aquarium, about his recent research in Lake Michigan.
In 1914, a 100-foot steam ship known as the “Silver Spray” was cruising through the shallow waters of Lake Michigan a few hundred yards off the shore of Hyde Park in Chicago. Made out of wood, with the exception of its iron boiler engine, the ship soon found itself hitting rock bottom–literally. With little depth technology on steam ships in the early 1900s, the captain and crew ran the ship aground at a rocky outcrop. After two or three days, the wooden ship broke up into pieces and washed ashore, leaving behind a sunken iron boiler. 100 years later, this iron boiler still sits off the shore of Hyde Park and remains visible from nearby Lake Shore Drive when water levels are low enough. Named Morgan Shoal, the rocky outcrop is most famous for being the location of the Silver Spray shipwreck.
But the wreck isn’t the only thing that makes Morgan Shoal unique.
Most of the bottom of Lake Michigan is either sand or mud, so a rock formation like Morgan Shoal is a special point of interest. Not too far from our “office” at Shedd Aquarium, we’re really curious about what’s living out there – whether it’s the fish or amphibians or plants or invertebrates. Whenever we find something like Morgan Shoal, we are sure to survey the area to see what exactly is out there and how that compares to other areas in the Great Lakes.
Because we’re looking for a variety of species, we use a variety of methods to document what sort of life the shoal sustains. To get an accurate picture of life at the shoal, we use everything from traps, nets, and pulling seines through the water, to using divers, unmanned aerial vehicles (drones), and even remotely operated vehicles (think miniature submarines) along the bottom of the lake. These different survey methods are used to figure out what lives there, what the geology looks like, and ultimately, how to preserve the shoal in the face of potential threats like development and climate change.
From historical information and data we’ve collected through recent surveys, we’ve learned quite a bit about the geology and ecology of the shoal. The outcrop itself was formed around 425 million years ago, when Chicago was south of the equator in tropical seas. The shoal is made out a material called Silurian dolomite, which is the compressed remains of a coral reef that formed in shallow seas at the time. At Morgan Shoal, you can actually see some fossils of old sponges and corals embedded in the bedrock. The rock formations create a three-dimensional space with horizontal and vertical surfaces, smooth surfaces, and crevices. The complexity of this habitat plays host to a wide variety of plant and animal species.
Thus far, we’ve been able to identify 19 different species of fish that call Morgan Shoal home. This number includes a healthy number of native fish that are iconic to Lake Michigan, such as Yellow Perch (Perca flavescens), Largemouth (Micropterus salmoides) and Smallmouth Bass (Mictopterus dolomieu), and Lake Trout (Salvelinus namaycush). Catfish, Black Bullhead (Ameiurus melas) and Channel Catfish (Ictalurus punctatus), were found at the shoal, likely because of the abundance of food along the rock bed. Various Catostomids — White Suckers (Catostomus commersonii) and Silver Redhorse (Moxostoma anisurum) – made an appearance, and non-native species were also spotted, including Brown Trout (Salmo trutta), Rainbow Trout (Oncorhynchus mykiss), and Chinook Salmon (Oncorhynchus tshawytscha).
One of the unexpected highlights of our survey was the presence of Longnose Suckers (Catostomus catostomus). A species of Catostomid, Longnose Suckers are listed as Threatened in Illinois under the Illinois Endangered Species Act. Because of this, it is typical to find only one or two in the same area at one time. However, we found nine individuals at Morgan Shoal throughout the course of our initial survey—one of the highest recorded concentrations of the species in Illinois.
We also located a single freshwater sponge (Phylum porifera), which is not only a rare find, but also inherently fascinating: this taxon of sponge is the lone descendant of the organisms that formed Morgan Shoal 425 million years ago still found at the site.
Along with the species that have called Lake Michigan home for thousands of years, invasive species were also identified at the shoal. While “invasive” implies negative consequences for an ecosystem, their relationship with the ecosystem is sometimes more complex than that.
For example, Round Gobies (Neogobius melanostomus) are the most abundant fish at Morgan Shoal, after being introduced to the Chicago-area Great Lakes in 1993. Gobies outcompete many native fish for food and spawning sites, but they also provide an abundant source of food for larger fish such as Lake Trout, Largemouth Bass, Smallmouth Bass, and Walleye (Sander vitreus).
The Quagga Mussel (Dreissena bugensis) is another well-established species that is invasive to Lake Michigan and found at Morgan Shoal. These mussels dominate the biomass of the site, both for invertebrates and vertebrates, as sections of hard, vertical surfaces are carpeted with them. This abundance complicates the ecology of the area. All the interstitial spaces between shells trap food for amphipods, isopods, chironomids, oligocheates, and other invertebrates. These small, well-fed organisms are in turn eaten by larger organisms such as crayfish and fish. This suggests that Quagga Mussels may have actually increased the abundance of certain species at the shoal, while outcompeting and displacing others.
While we’ve established a baseline set of knowledge about Morgan Shoal, much more research is needed to fully understand it. This includes studies focusing on why sand moved by currents does not cover the shoal, what species of fishes spawn at the site, and are there other shoals in the region with similar characteristics.
As it stands, Morgan Shoal can be used as a model for various nearshore habitat restoration projects, harbor construction projects, or rehabilitation of industrial sites. While there’s much to learn about Morgan Shoal and other ecosystems like it, our existing knowledge points us to the conclusion that this ecosystem is a unique and invaluable addition to Chicago’s shoreline.
To learn more about Shedd Aquarium’s research in the Great Lakes, visit Shedd Aquarium’s website.