There is a general consensus among marine scientists that sharks have an exceptional sense of smell. This is definitely very true and absolutely necessary for their survival. The idea though that sharks have super sniffers that they use to sniff out the minutest amounts of blood in vast bodies of water is, for the most part true, but greatly embellished.
Let’s first look at how sharks use their noses to sniff out prey, blood and other interesting titbits in the ocean. Smell is incredibly important to sharks and how they use this sense is quite fascinating. Just under the snout of a shark are two small openings, or nares (nasal cavities). Each nare consists of two more openings. The shark sucks or pulls the water into the nares. The water goes into nasal sacs and over a series of skin folds known as olfactory lamellae. The nasal cavities are big spaces, which gives the shark more time to register the smells. In the shark’s brain, the olfactory lobes kick in and analyse the scents, which can be those of a potential mate or prey. With smell being so very important to sharks, it is no surprise that about two-thirds of a shark’s brain is made up of olfactory lobes. From here, it is up to the shark to decide if it wants to pursue the scent or not.
So, how do sharks “pursue the scent”? A study has found that not only are a shark’s nares super sensitive to smell, but they also work in stereo. Sharks can detect the smallest delays in the time that it takes for a smell to reach one nare versus the other. The shark will turn towards the nare that first registered the smell. The time delay between the smell registering in the first nare and then registering in the second can be as little as half a second. If a shark doesn’t experience such a delay, it will go with the flow and turn whichever way it chooses.
The belief that sharks can smell a drop of blood in the ocean from miles away, is vastly exaggerated though. Sharks can detect one part of blood per million parts of water which is hundreds of times better than a human’s sense of smell. And consider the length of time that it would take for the scent of the blood to travel from its origins to where a shark is, and then the shark still has to retrace the path of the blood, back to its origin. This could take anywhere from minutes to hours. Not to mention the interference of ocean currents, other animals, other scents and human interference such as boat activity, etc.
And just because they can smell it doesn’t mean they’ll follow it. Sharks really don’t like human meat and generally aren’t interested in the bodily fluids people excrete. “You can smell a landfill, but it won’t make you want to eat it,” noted Dr Steve Kajiura of Florida Atlantic University’s Shark Lab.