A bull shark off the coast of the Bahamas. Image: Wikimedia Commons
By Brooke Kansier
Let’s be realistic, here. The only sharks in the Great Lakes region can be found behind glass in an aquarium. Right?
But between Megalodon specials on the Discovery Channel’s Shark Week earlier this month, one title caught my eye: Monsterquest: Jaws in Illinois. Now, Chicago is home to a lovely aquarium, but the Shedd was decidedly not the topic of the re-run — rather, it told a tale sharks swimming up the Mississippi River as far as Illinois.
That may sound like a low-budget horror you’d catch on Syfy, but the phenomenon of sharks swimming up rivers — and even into lakes — is real.
But in the Great Lakes?
“There may be one kind of shark that could survive — some of the time — in the Great Lakes,” said Amber Peters, an assistant professor specializing in Marine Ecology in Michigan State University’s Department of Fisheries and Wildlife. “It couldn’t really deal with the winter temperatures, or the lack of food, but it’s possible that if one swam in it could live for a while.”
That one kind, the bull shark, isn’t as well known as the Jaws-featured great white, but it has a unique trick — its kidneys recycle salt vital to the animal’s cells. Normally, a freshwater dip would dilute the salt in a shark’s body, causing its cells to rupture and kill it, according to National Geographic. Recycling the salt its body already contains allows bull sharks to adapt to strange environments, like freshwater.
Bull sharks are one of a very few of the 375 species of sharks with this physiology-altering ability, and they do it better than any other.
Bull sharks have been found in freshwater around the world: thousands of miles up South America’s Amazon, in Central American lakes and as far up the Mississippi as Illinois, according to National Geographic.
They have been reportedly seen in Lake Michigan, although some instances, like this dead bull shark found on the lake’s shore, are a bit uncertain.
No shark reports have been scientifically documented in the lake.
The Illinois River has seen at least one documented case. Dams now keep any wandering sharks from entering the river.
Like similar cases with whales, many of these Great Lakes shark sightings are merely hoaxes, Peters said, spurred by all of the attention the species gets.
“People always want something exciting to talk about,” she said. “There’s not a lot of animals that have a whole week to themselves, that people look forward to.”
Very little is understood about the bull shark’s physiological abilities, and any competitive advantages it brings are as yet unknown.
The disadvantages to sharks of a freshwater environment, especially one like Lake Michigan, are clearer, according to Peters. There aren’t as much food and friends.
“The fish that are in the Great Lakes are overall much smaller than the prey items (sharks) can get in the ocean,” she said. “It takes them out of their normal environment. They have to deal with the stress from being in fresh instead of salt water. They don’t have normal prey and they definitely aren’t going to find mates.”
A bull shark’s typical diet consists of large, bony fish and smaller sharks. They occasionally go after much larger prey — all creatures that are scarce in most freshwater environments.
Of course, the lack of salt and companionship and appropriate grub are hardly the only obstacles keeping a shark from reaching the upper Great Lakes. Even a bull shark venturing up the Mississippi River basin would have to get through the electric barrier at Chicago that’s designed to keep invasive species out of Lake Michigan. And one venturing from the Atlantic coast into the St. Lawrence River would have to negotiate that seaway’s lock system — or swim up Niagara Falls.
Due to these barriers — and the sheer unlikelihood of the phenomenon — you shouldn’t expect to see any fins popping out of the surf at any regional beaches this summer.
For those interested in seeing some sharks, the region is home to the Shedd and some other great aquariums, including Ohio’s Greater Cleveland Aquarium, which even offers diving with sharks. The Sea Life Aquarium in Michigan added sharks to its roster earlier this year.
And there is always Shark Week highlights on the Discovery Channel’s website.
Or, you know, grab some popcorn and a VHS of Jaws.