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Why do sharks swim at surface with their dorsal fin exposed?

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By Blaise Jones

Few images are as iconic and terrifying as that of a shark’s dorsal fin knifing across the surface of the water. Hollywood has taken advantage of this, both for its threatening imagery and for simple practicality; it’s much easier to build a convincing looking shark fin than it is to build a whole shark. But why do sharks do this? Do they know it scares the dickens out of us surface dwellers? Or is there a deeper behavioral instinct that drives them to the surface?

Con-fin-mation Bias

Sadly, Hollywood has yet again proven to be an unreliable source of information on sharks. Sharks do not typically swim at the surface of the water with their fins exposed. The reason this image is so prevalent is because, for the most part, the only time people ever see sharks are when they are at the surface. Sharks can’t bend or retract their dorsal fins, so when they come near the surface they can’t help but stick their fins out. They are not doing it on purpose.

Getting to the Bottom—er, Surface of Things

However, what brings sharks to the surface in the first place? While most sharks tend to hunt near the bottom or the middle of the water column, filter feeding species like the basking shark and whale shark feed near the surface, where the sunlight creates plankton blooms that lure in their preferred prey items.

More than just plankton swim near the surface. Schooling fish, when chased by a predator, may rush upwards to the surface in an attempt to escape. The predator, in this case a shark, will chase them upward and during the hunt its dorsal fin will break the surface.

Many times that sharks arrive at the surface, people are involved. Maybe the water is being chummed by fishermen, or maybe the shark is curious about all the noise and splashing going at the surface. This just adds to the myth that sharks always swim at the surface .

Multipurpose Fin

However, one species of shark might actually use this iconic behavior for more than just scaring swimmers. The oceanic whitetip shark has been observed repeatedly breaking the surface with its dorsal fin and dipping back down below the surface. This is because oceanic whitetips are one of the few species of shark that can detect scents outside of the water.

Scent travels better through air than it does water, and the oceanic whitetip takes advantage of this. Since they live out in the open ocean, they use scents traveling over the wind to find their prey. By sticking their dorsal fins out of the water and then pulling them below the surface theses sharks can drag down air bubbles into the water that contain scent particles, which lets the shark know which direction to head when looking for prey.

Just goes to show that not every species of shark use their dorsal fins just to scare swimmers.

Sources

  1. “Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess
  2. http://www.elasmo-research.org/education/topics/b_dorsal_out.htm
  3. “The Encyclopedia of Sharks” by Steve Parker
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