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Dr. Mike Heithaus

Not many people would guess that my team and I are headed into the middle of the wilderness as we drive through the sleeping streets of Miami hours before sunrise. But just a few miles from Miami is Everglades National Park – over a million acres of wilderness. While most people think of alligators when they hear the word Everglades, we are on our way to find bull sharks. Bull sharks are one of Florida’s largest and most impressive predators. Adult bull sharks hunt coastal waters, but the juveniles are found in the rivers of the Everglades! We’ve caught them almost 30 miles from the ocean in water that is completely fresh.

My students and I are interested in why the sharks use these freshwater nurseries that are unavailable to other sharks and the role of bull sharks in the Everglades ecosystem. The first thing that we have to do is catch them. After driving an hour and a half by truck from Miami to Flamingo, FL we launch our boat and drive another hour and a half upstream. Next, we set a longline and clip 50 hooks to it baited with small chunks of mullet. After an hour we come back and check our line. Most of the sharks we catch are between 3-4’ and range from newborns to three year olds. Some of the sharks that we have tagged have been caught in the same spot more than a year later!

One possible reason the sharks are in the river is to find food. To find out if the sharks are eating in the river or on prey from marine food webs we take small samples of blood and a tiny biopsy of skin. Based on the chemical makeup of these tissues, we can tell that the sharks do eat in the rivers, but most of their food comes from the ocean – miles from where they were caught! But if the sharks prefer seafood, why would they spend so much time far upstream where they mingle with alligators? Our best guess is that by staying in freshwater they are safe from the larger sharks that patrol coastal waters and would love to eat a small bull shark.

Of course, figuring out where bull shark’s food comes from is just the first step in this study. We want to know how the bull sharks use the entire coastal Everglades estuary and how often they make trips out to the ocean. To do that, we’ll be attaching transmitters to the sharks and tracking them as they move through the rivers. But that will be another story . . .