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Sharks and Dolphins
Love Dolphins? Love Sharks? You Can Love Both!
We here at the Dolphin Communication Project (DCP) field lots of questions about sharks. We often come across well meaning dolphin lovers who say that they hate sharks. Well, fear not dolphin lovers: you can love sharks too. Although sharks are a natural predator of dolphins, both animals are an important part of ocean ecosystems. Without one or the other, an ocean ecosystem can begin to fall apart. In Bimini, the Bahamas, one of DCP's long-term field sites, Atlantic spotted and bottlenose dolphins often bare signs of shark attacks: scars and missing parts of the dorsal and/or pectoral fins. But because both sharks and dolphins feed on many types of fish, the aquatic ecosystem around Bimini is healthy and thriving!
(Here is a photo of a Caribbean reef shark from WikiMedia)
Why are Dolphins and Sharks Important in the Ocean?
Dolphins and sharks play a crucial role in ocean ecosystems: they are both apex predators. An apex predator is a species that eats other animals, but has few natural predators of its own (1). Thus, they are at the top of the food chain. This is important because they help control the populations of all the other species in their ecosystem. More predators in an ecosystem means more diversity (2). Sharks and dolphins also feed on sick and injured fish, which helps keep the rest of the fish populations healthy (3).
Let's face it: sharks have a reputation as the "bad guys" of the ocean. Many people think sharks are "man-eaters." In fact, it is quite the opposite; humans are the greatest threat to sharks in the wild today. There are more than 350 shark species in the world, and the majority are harmless to humans. Most shark attacks on humans are from only 3 species of shark; white, tiger and bull (4). Protecting sharks from human interference is very important to maintain healthy ecosystems in the ocean.
Dolphins and sharks share not only the same home, but the same important role in their ecosystem. They keep the oceans healthy by keeping their prey populations under control, and increasing diversity in the oceans. Without top predators, the ocean ecosystems we know and love begin to degrade. Caring about the protection and safety of dolphins and sharks helps the protection and safety of the entire ocean.
3. Temple, S.A. (1987). Do predators always capture substandard individuals disproportionately from prey
populations? Ecology 68 (3): 669-674.
What Happens if Apex Predators are Removed?
When shark populations decrease to a low level, the nature of the entire ecosystem changes. Although sharks have no natural predators, humans in fact, are their greatest threat today. Sharks are in danger from two main human practices; accidental bycatch, and overfishing. Humans kill more than 100 million sharks per year (5), and sharks only kill 5-15 people accidentally in the same time period (6).
Shark finning, when sharks are caught and only their fin is taken (the rest of the shark carcass is dumped back into the water), is responsible for 26-73 million shark deaths per year (7).The fins are generally used in shark fin soup, a delicacy in some parts of Asia.
Bycatch is the other major danger to sharks. Bycatch occurs when sharks are unintentionally captured in the pursuit of another fish species. Bycatch of sharks is most common in trawl fisheries, typically in coastal areas (8).
When top predators like sharks are killed, it is easy for species further down in the food chain to become overpopulated. For example, in a reef-dominated ecosystem, when sharks and other apex predators are removed, herbivorous (plant-eating) fish become overpopulated. Because there are no predators to eat them, they over graze the coral, and the coral can die. Because so many species in a coral reef depend on the corals for survival, the whole reef system can go into decline (9).
In another example, great shark populations (sharks over 2m long) are all but extinct along the east coast of the United States. While these 11 species of sharks have decreased by over 50% in the past 15 years (10), their prey (rays, skates, and smaller sharks), have increased ten-fold! (11). In particular, the cownose ray become highly overpopulated. This ray eats scallops, clams and oysters. When the cownose rays population boomed, the scallops, clams, and oysters decreased dramatically. This completely terminated the North Carolina scallop fishery by 2004 (12). As you can see, the loss of top predators like sharks has an effect on the entire ecosystem, and sometimes, on the economic livelihood of humans.
The loss of dolphins in an ecosystem can have similar detrimental effects. If the number of dolphins decrease, their prey increases, which can change the dynamics of the entire food chain. In the 1970's, 80's, and 90's, high numbers of dolphins were being caught as bycatch in tuna fisheries. The world was outraged at the loss of such a dynamic aquatic mammal, and many new laws regarding the use of tuna nets were established with the Agreement on the International Dolphin Conservation Program in 1999. Dolphin bycatch levels in tuna fisheries dropped from 500,000 in in 1970 to 1,000 in 2006 (13).
International cooperation and regulation can have a big effect in reducing the loss of apex predators like dolphins and sharks to bycatch.
Both species can be protected by working to decrease bycatch through improved management and education, ending shark finning, and reducing the demand for shark fin soup and other apex predator products (14).
5. WildAid and Oceana. (2007). The end of the line? Global threats to sharks.
6. International Shark Attack File. /www.flmnh.ufl.edu/fish/Sharks/ISAF/ISAF.htm>
8. Bonfil, R. (2000). The problem of incidental catches of sharks and rays, its likely consequences and some
possible solutions. Sharks 2000 Conference, Hawaii, 21-24 February.
9. Stevenson, C., Katz, L.S., Micheli, F., Block, B., Heiman, K.W., Perle, C., Weng, K., Dunbar, R., and
Witting, J. (2007). High apex predator biomass on remote Pacific islands. Coral Reefs 26: 47-51.
10. Baum, J.K., Myers, R.A., Kehler, D.G., Worm, B., Harley, S.J., Doherty, P.A. (2003). Collapse and
conservation of shark populations in the Northwest Atlantic. Science 299: 389- 392.
11. Baum, J.K., Myers, R.A., Kehler, D.G., Worm, B., Harley, S.J., Doherty, P.A. (2003). Collapse and
conservation of shark populations in the Northwest Atlantic. Science 299: 389- 392.
12. Myers, R.A., Baum, J.K., Shepherd, T.D., Powers, S.P., and Peterson, C.H. (2007). Cascading effects of the
loss of apex predatory sharks from a coastal ocean. Science 315: 1846-1850.
Dolphins and Sharks in Bimini, the Bahamas
Although sharks are often depicted as monsters, they are clearly crucial members of every healthy ocean system in the world. Dolphins and sharks both serve an important function as top predators. Even if sometimes sharks take a bite out of a dolphin! In Bimini, DCP researchers have observed evidence of shark attacks on the Atlantic spotted and bottlenose dolphins. Despite scarring, the surviving dolphins heal well and appear to function normally. They can swim, they can eat, and they can play!
In Bimini, there are 2 species of dolphins: Atlantic spotted dolphins (Stenella frontalis) and bottlenose dolphins (Tursiops truncatus). The researchers from the DCP have been studying these dolphins since 2001.
Several species of sharks are also found around Bimini, including tiger sharks (Galeocerdo cuvier), bull sharks (Carcharhinus leucas), and nurse sharks (Ginglymostoma cirratum). The good folks at the South Bimini Biological Field Station have dedicated themselves to learning more about Bimini's shark populations. Tiger and bull sharks are probably responsible for the attacks on the dolphins, but we do not know for sure (hammerheads and makos may also attack Bimini dolphins). Despite the overlap between sharks and dolphins, they are all able to live and survive in the waters of tropical Bimini. Again, their role as apex predators keeps the ecosystems in this area in check.
Tilly is a juvenile female Atlantic Spotted dolphin, and is still developing her spots. In the past DCP had a difficult time identifying her, but in May 2010, she showed up with a severe shark-related injury to her dorsal fin. Although the bite took out most of her dorsal fin and left scarring around it, she is doing great and her injuries continue to heal nicely.
Learn more about Tilly at www.adoptawilddolphin.com
Billy is a juvenile male Atlantic Spotted dolphin and his full name is Prince William Joseph of Wyckoff. He was first seen in 2002 as a calf with a "C" shaped scar on his right side. The injury healed well and we enjoyed observing Billy as he began developing spots. In 2005, we observed Billy with evidence of yet another shark attack; this time the injury was to his peduncle (the area behind the dorsal fin and before the tail fluke). Despite these two shark related injuries there has been no observable change to his swimming or interactions with other dolphins. You can learn more about Tilly and Billy at www.adoptawilddolphin.com!