While the idea of sharks swimming in Chicago’s waters might spark curiosity, sharks are not naturally found in Lake Michigan or the Chicago River system. The aquatic conditions of Lake Michigan are fundamentally unsuitable for the long-term survival of most shark species.
Why Chicago Waters Aren’t Shark Habitats
Sharks do not inhabit Chicago’s waters due to their distinct physiological requirements for marine environments. Lake Michigan is a vast freshwater lake with extremely low salinity, historically ranging from 0.05 to 0.60 parts per thousand (ppt). This is significantly different from the ocean’s average salinity of approximately 35 ppt.
Most sharks maintain their internal salt balance through osmoregulation. In freshwater, this system would lead to an influx of water into their bodies, causing cells to bloat and potentially rupture. This makes long-term survival impossible for typical marine sharks.
Beyond salinity, the Great Lakes ecosystem does not provide the necessary conditions to support shark populations. Lake Michigan maintains frigid temperatures too cold for most shark species, which thrive in warmer waters. The specific prey base required to sustain large predatory sharks is also absent in the Great Lakes food web.
Geographical barriers further prevent sharks from naturally reaching Lake Michigan. The St. Lawrence Seaway, connecting the Great Lakes to the Atlantic, involves physical barriers, locks, and significant salinity changes, making it highly improbable for sharks to navigate and establish populations. Similarly, rivers connecting to the Mississippi River system also face electric barriers designed to prevent invasive species from entering Lake Michigan.
Native Fish Species in Lake Michigan
While sharks are not native to Lake Michigan, the lake and the Chicago River system are home to a diverse array of freshwater fish species. Lake Michigan’s ecosystem supports native fish like Lake Trout, Whitefish, Cisco, Walleye, Yellow Perch, and Lake Sturgeon. The Lake Sturgeon, a prehistoric-looking fish, is one of the largest and longest-living native species in the lake.
The Great Lakes have also seen the introduction of several non-native species. These include popular sport fish like Chinook Salmon, Coho Salmon, Brown Trout, and Rainbow Trout (Steelhead). These introduced salmon and trout species contribute significantly to the lake’s recreational fishing industry.
The Chicago River, a part of this freshwater network, hosts its own variety of fish. Common species found in the Chicago River include Largemouth Bass, Smallmouth Bass, Channel Catfish, Common Carp, Bluegill, and Northern Pike. Walleye and Brown Trout have also been observed in the Chicago River. This rich diversity highlights the freshwater nature of these aquatic environments.
Debunking Shark Myths
Despite scientific consensus, myths about sharks in Lake Michigan persist, often fueled by misinformation. One common misconception involves the bull shark, a species known for its remarkable ability to tolerate freshwater environments. Bull sharks possess unique osmoregulation capabilities, allowing them to adjust their internal chemistry by reducing urea and increasing kidney function to produce very dilute urine when in freshwater. This adaptation enables them to travel far up major river systems, with documented sightings over 3,000 kilometers up the Amazon River and as far north as Alton, Illinois, in the Mississippi River.
However, the natural range and migratory patterns of bull sharks do not extend to the Great Lakes. Even if a bull shark were to reach Lake Michigan, it would face significant challenges to long-term survival. The frigid winter temperatures of the Great Lakes are unsuitable for bull sharks, which prefer warmer, tropical, and subtropical climates. The lack of an adequate and consistent prey base tailored to their dietary needs further reduces the likelihood of establishing a sustainable population.
Reports of sharks in the Great Lakes are consistently debunked as hoaxes, misidentifications, or isolated, non-natural occurrences. For instance, a well-known incident from 1969 involved a dead shark purportedly found in Lake Michigan near Milwaukee, which was later revealed to be a prank where a bar owner planted a shark he had kept in his freezer. Other “sightings” are often misidentifications of large native fish like lake sturgeon, muskellunge, or northern pike, whose dorsal fins can sometimes be mistaken for those of a shark.