Interactions between Sharks and Seals off the coast of Cape Cod
One of the big draws for tourists to the Cape over the past few years has been the ever growing Grey Seal population. This has also attracted another kind of visitor to the waters off Cape Cod: the Great White Shark! Great White populations off the coast of the Cape have increased significantly over the past decade, and especially during the past 3 years with sightings going from 80 individuals in 2014 to 147 in 2016. There have already confirmed at least 9 individuals so far this season. These apex predators are here following their favorite prey: seals.
The Great White Shark (Carcharodon carcharias) lives mostly in temperate, coastal waters around the world, but has been known to make appearances in colder northern waters. They can grow to be about 20 ft long, and up to 5,000 lbs. They can swim at speeds of up to 35 miles per hour, and they have 300 teeth in their mouths at a time in 7 rows. As they lose teeth, a tooth from the next row pushes forward to replace the lost tooth. Great White’s are mostly solitary creatures, but they will occasionally hunt in small groups. They also congregate during mating season, but scientists are still trying to pinpoint where white shark mating actually occurs. These sharks are considered “vulnerable” on the endangered scale, so conservation efforts are very important right now to keep these beautiful creatures from becoming endangered and/or extinct in the future.
Juvenile Great White Sharks will eat fish and small skates and rays, but as they grow larger they transition to a diet based almost solely on marine mammals, particularly seals. They are ambush hunters, and will often target seals from below. The seals won’t see the sharks thanks to an adaptation the sharks have known as countershading. Countershading refers to the coloring of a Great White Shark’s skin, white on the bottom and gray on top. This allows the animal to blend in with the dark depths of the oceans if viewed from above, and with the bright, dappling sunlight if viewed from below. This gives the shark a huge advantage, in addition to their size, speed, strength, and teeth, over the seals.
With the number of White Sharks off the coast increasing drastically there is a growing concern for swimmers and those engaging in water sports at beaches all across the Cape. There’s no need to be overly concerned, but it is important to read posted signs at the beaches about recent shark activity, and listen to the proper authorities if you’re asked to refrain from swimming. Being vigilant is key. If there are seals around there’s a good chance there are sharks as well, and make sure not to go swimming at dawn or dusk, the shark’s favorite feeding times.
Posted by Emily B.
Emily is a summer, 2017 intern at the National Marine Life Center.