Yes, that’s a bonnethead shark (Sphyrna tiburo) … and yes, that’s the Detroit River.
Reaching a maximum length of 150 cm (Compagno, 1984), bonnetheads are relatively small and inoffensive congeners of the better-known hammerheads. S. tiburo are common inhabitants of both estuaries and shallow coastal waters in the tropical and subtropical Western Atlantic and Eastern Pacific (ibid.), but monitoring of specimens along Florida’s Gulf of Mexico coast suggests that they’re unable to tolerate salinities lower than 15 ppt (Ubeda et al., 2009). No bull shark, the little fellow clearly never had a chance.
So how this? Richard elaborates:
It was clearly a pet release in part and a prank second. The fish was found off of the public beach on Belle Isle in Detroit Michigan on the Detroit river. I have no idea how one managed to keep a 3 foot bonnethead alive to do this but it certainly was never swimming around for long. Stomach content check showed it empty.
I got a call early in the day that the staff there found a shark on the shoreline and it was very much alive. Personally I suspected just another Sturgeon but the lady on the phone was telling me NO it was an actual shark. I told them to corral it and I’d be by. Sure enough when I arrived it was an honest to goodness bonnethead shark. It was pretty much dead but still slightly breathing.
Fish is currently in my freezer… I plan to have it mounted and displayed at the Belle Isle Nature zoo as an example of the stupid things people try to release.
Deliberate release — as anyone familiar with irresponsible aquarists, game-fish stockers, and fang sheng practitioners can attest — is a major vector for aquatic invasive species. Even the dumping of marine critters into decidedly non-briny waterways, bizarre though it might sound, is not without precedent; look to this report (with photo), of a 6-foot octopus pulled up from the Ohio River by an Indiana fisherman.
Some more photos of the expired elasmobranch:
Compagno, L.J.V. (1984). FAO Species catalogue. Vol. 4. Sharks of the world: an annotated and illustrated catalogue of shark species known to date. Part 2. Carcharhiniformes. FAO Fish Synop., 125, 251–655.
Ubeda, A., Simpfendorfer, C., & Heupel, M. (2009). Movements of bonnetheads, Sphyrna tiburo, as a response to salinity change in a Florida estuary. Environmental Biology of Fishes, 84(3), 293-303.