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Shark Decline Effects Marine Ecosystem - July 10, 2016

                It is the height of summer vacation, and with Discovery Channel’s famed Shark Week still fresh in everyone’s thoughts, sharks are a concern in vacationers’ minds as they make their way to the beach this summer. However, the chances of a vacationer encountering or being attacked by a shark are very slim. The United States averages about 19 shark attacks per year, with a 1 in 3,700,000 chance of being killed by a shark. The chances of being killed by a lightning strike are almost twice as high with 37 casualties a year in the coastal United States alone.

                There are over 400 species of sharks worldwide. However, only three species account for more than half of all shark attacks on humans. These species are the great white shark, tiger shark, and bull shark. Bull sharks are able to inhabit both salt and freshwater environments. This ability combined with the bull sharks’ tendency to hunt in shallow waters increases their exposure to humans. Tiger sharks are known for their tendency to eat just about anything. They also attack when they feel threatened or endangered which poses a risk to humans. Great white sharks account for the majority of shark attacks. Great whites are naturally curious creatures and most attacks on humans result in releasing the victim. This indicates that humans are not the intended source of food which typically consists of sea lions, seals, and small toothed whales. Shark species that can be found in coastal Maryland are the scalloped and smooth hammerhead sharks, smooth and spiny dogfish sharks, the sandbar shark, and the Atlantic shortfin mako shark to name a few.

                Worldwide, shark populations are declining rapidly, mainly due to commercial fishing. Sharks are sought primarily for their dorsal fins, which are used to make shark fin soup; a popular soup in Asia. After capture, the shark’s fin is cut off and often the still-living body is thrown back into the ocean. It is estimated that between 63 and 273 million sharks are killed each year, with 100 million being the medium estimate. Sharks dumped at sea are often not included in official reports, making it difficult to estimate the number killed. In the last 15 years, the populations of eight species of sharks have decreased by more than 50%, with scalloped hammerheads decreasing by 89% and thresher sharks by 80%.

                Sharks reproduce at a slow rate with a gestation period averaging between nine to twelve months but can last up to two years depending on the species. Their reproduction rate is much slower than the rate at which they are being killed, thus precipitating their decline. Sharks have two types of reproductive strategies; giving birth to live young or laying eggs. Nearly 70% of all shark species give birth to live young in what is known as viviparity. Many shark species only produce 1-2 young a year while some species such as whale sharks can produce up to 300 young. Upon birth, pups are entirely independent and receive no care from the mother.

            Sharks are vital to maintain and regulate marine ecosystems as an apex or top predator. Sharks are considered to be what is known as a “keystone” species, meaning that the function of the ecosystem largely depends on sharks and without them it would change dramatically. Sharks keep fish populations at optimal levels and often feed on old or sickly fish, keeping the population healthier.

            There are many examples of the importance of sharks as a vital component in the health of the marine ecosystem. Sea grasses are the primary food source to dugongs, cousins to manatees, and sea turtles. Dugongs prefer to feed on the best quality grass, which mostly occurs in the middle of grass beds. When tiger sharks are present in the area, the dugongs feed on the lower quality grass at the edges of the beds. Dugongs eat the entire plant when grazing, which can harm the seagrass beds if overgrazed. Their grazing habits change based on the number and location of tiger sharks in the area, which keeps grazing in check and the grass beds healthy. Tiger sharks have a similar effect on green sea turtles, which eat the top portion of seagrass blades. By altering the location of grazing, the sharks prevent areas from being overgrazed.

           Other studies have found the importance of sharks as a stabilizer in their environment.  Decreasing shark numbers can set off a series of effects called a cascade.  A decrease in shark numbers due to overfishing sets up an increase in mid-level predators such as snappers which, in turn, causes a decrease in numbers of algae eating fishes, such as parrot fish, which allows algae to overgrow coral and causing death of the coral reef. 

            As vacationers head to the beach this summer, it is important to remember the key role that sharks play in the larger marine ecosystem. The sharks that visit our Coastal Bays are vital components of our ecosystem and should be respected.

 

Phillips is the Program Manager for the Maryland Coastal Bays Program.

 



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