A Sea of Problems: Why the Scalloped Hammerhead Population is in Such Sharp Decline

Transcription

1 Neylan 1 Isabelle Neylan Dr. Durham Darwin, Evolution, and the Galapagos 15 October 2012 A Sea of Problems: Why the Scalloped Hammerhead Population is in Such Sharp Decline Despite all of the hype created by the Jaws movies and other media induced shark scares, no more than five humans are killed by a shark attack in a given year. The record for the most lethal attacks is ten. However, as many as 100 million individual sharks are killed by humans every year. The shark population worldwide is in rapid decline and the scalloped hammerhead shark is no exception. Worldwide the population has declined by 50-90% (depending on the region) and the IUCN red list has deemed it an endangered species. 1 While there is little data for the Galapagos in particular, the decline is still apparent. Divers frequent schooling areas of the hammerheads and have noted an appreciable drop in school size. 2 It has been estimated that between 1.3 and 2.7 million individuals are sold at markets each year. 3 Why is the scalloped hammerhead population declining so rapidly? I propose three possible hypotheses to explain this phenomenon. First, a leading cause of the decline is intentional human predation through commercial shark fishing and finning. The second cause is unintentional human predation as the sharks are caught as bycatch. And third, these two problems are exacerbated by the behavior and characteristics of the species itself. The scalloped hammerhead s scientific name is Sphyrna lewini and is also commonly known as the kidney, bronze, or southern hammerhead. The name Sphyrna is based off of the 1 Sphyrna Lewini, Scalloped Hammerhead, iucn.org. 2 Carlos, one of the guides, personally confirmed this observation. 3 Sphyrna Lewini, Scalloped Hammerhead, iucn.org.

2 Neylan 2 Greek word for hammer and belongs to the entire hammerhead family including the smooth hammerhead, the great hammerhead, and the bonnet head. The scalloped hammerhead gets the named scalloped from the ridges found along the front of its hammer. It can grow up to 14 feet long and be up to 330 pounds. They are apex predators which places them at the top of the food chain. They are an integral part of the marine ecosystem because they control prey populations all the way down the chain. They feed mostly on bony fish and squid. The species is found in semi-coastal regions all over the world. In the Galapagos, they are the most abundant species of shark and are most commonly found around Wolf and Darwin Islands in large schools. A large debate among scientists and marine biologists has surrounded the odd shape of all hammerhead heads. On the one hand it would seem that having such a large space between the two eyes would give the animals a giant blind spot. On the other hand, from an evolutionary standpoint it would make no sense to evolve such an odd shaped head without gaining any benefit. Finally this debate has been put to rest. Recent studies have shown that the sharks not only have completely binocular vision, (no blind spot,) but that their vision is actually greatly improved. Their eyes are tilted forward allowing them to have a thirty-two degree overlap in the front and when the head is rotated they have 360 degree vision and can see above and below them at the same time. Despite being beautifully adapted, the question remains, why is such a unique creature in this perilous situation? The first hypothesis is that a major cause of population decline in Sphyrna lewini is intentional human predation through commercial shark fishing. Sharks are hunted mainly for their fins which are used in shark fin soup. The soup is considered a delicacy and a social status symbol in many Asian cultures. It is served on special occasions ranging from weddings to business and government banquets. The market for shark fins is mainly in Asia,

3 Neylan 3 centered in Hong Kong, but there are markets throughout the world. California is the second largest market and British Columbia in Canada is another large one. With the burgeoning economy in China and other parts of Asia, the wealthy, upper class is growing exponentially and with it the demand for shark fins. S. lewini forms a large part of this market. A study by Clarke et al. in 2006 took DNA samples from hundreds of shark fins in the Hong Kong market to test which shark species were being represented. 4 They found that about 4.4% of the fins tested were from Sphyrna lewini or Sphyrna zygaena, the smooth hammerhead which is a close relation to the scalloped hammerhead (see figure 1). These two species formed a sizeable part of the test subjects and were second only to P. glauca or the blue shark, the most abundant shark species in the world, at 17.3%. It is estimated that about one to three million scalloped hammerheads are killed for fins each year. 5 The second hypothesis is that the other leading cause of the lewini population decline is inadvertent human predation where sharks are caught as bycatch. Bycatch means that the fishermen intend to catch one particular marine species, in the Pacific usually tuna, and unintentionally catch another one. Dolphins, sea turtles, and sharks are common bycatch. Longlines and gill nets are particularly problematic as they immobilize the shark that then drowns. In 2004, Hinke et al. performed a study looking at the top-down effects of fisheries in the Pacific. 6 Their primary concern was the effects on the tuna population, another predatory fish, but they also collected data on sharks. Because the scalloped hammerheads in the Galapagos are adults, the data gathered on large shark species was particularly telling (see figure 4 Clarke et al., Identification of Shark Species Composition and Proportion in the Hong Kong Shark Fin Market Based on Molecular Genetics and Trade Records, Conservation Biology 20.1 (2006). 5 Ambercrombie et al., Global Scale Genetic Identification of Hammerhead Sharks: Application to Assessment of the International Fin Trade and Law Enforcement, Conservation Genetics 6.5 (2005). 6 Hinke et al., Visualizing the Food-Web Effects of Fishing for Tunas in the Pacific Ocean, Ecology and Society 9.1 (2004).

4 Neylan 4 2). It is clear that from 1950 to 2000 the amount of sharks caught by all fisheries is in sharp decline. The methods of fishing have changed very little during that time span so this trend shows that shark populations are indeed declining. However, the amount of sharks caught by longlines has stayed fairly constant without an appreciable decline showing that this method of fishing is still a threat to large shark species in this part of the Pacific. For scalloped hammerhead sharks, being killed as bycatch is still a problem and is contributing to their reduced numbers. These two problems, however, are faced by shark species worldwide. The third hypothesis addresses why the scalloped hammerhead specifically is being so heavily affected. Overall, the behavior and physiology of the Sphyrna lewini species itself leaves it more vulnerable to population decline due to both intentional and unintentional human predation. First, scalloped hammerheads are known to travel great distances. Hearn et al. in 2010 looked at the scalloped hammerhead population specifically in the Galapagos. 7 By tracking specific individuals around Wolf and Darwin islands, they found that the sharks gathered there in greater numbers at certain times of the year. This suggested that the sharks migrate to another location for the other part of the year. This behavior poses a problem for conservation because it means that these animals regularly leave the protected marine preserve and are more vulnerable to fishermen. The team also installed tracking devices directly onto the fins of the sharks and discovered what they called the Golden Triangle. 8 (See figure 3). The scalloped hammerheads were splitting their time between the Galapagos, Cocos, and Malpelo islands. This discovery also reveals complications for conservation because while the Galapagos belong to Ecuador, Cocos belongs to Costa Rica and Malpelo belongs to Colombia. To truly and effectively protect these animals it would take international cooperation among these three countries. The Hearn et al. study also 7 Hearn et al., Hotspots within Hotspots? Hammerhead Shark Movements Around Wolf Isalnd, Galapagos Marine Preserve, Marine Biology (2010). 8 Hearn, Shark Migrations: Discovering the Golden Triangle, Galapagos: Preserving Darwin s Legacy (2009).

5 Neylan 5 found another behavioral problem. Scalloped hammerheads tend to group during the day in schools that can number in the hundreds. In addition, they also tend to school in predictable places. These facts have helped fuel the many dive companies in the Galapagos, but also makes it easy for fishermen to find the animals. Hearn et al. discovered that it is the numerous seamounts surrounding the Galapagos Islands that attract these animals. It is unclear why exactly, but the seamounts serve as a diurnal refuge for the animals where they can participate in social behaviors. It is thought that they pick out seamounts because they are easier to locate and provide some protection from currents. At night, the animals disperse to hunt in the open ocean usually beyond the bounds of the marine preserve. On a more global scale, these animals are classified as semi-oceanic, meaning they are found near coasts all over the world. This behavior also makes it easier for fishermen to catch them because they do not have to travel as far to fish. The physiology of the scalloped hammerhead itself poses many problems. This species has particularly large rays and fins which make them more valued in the markets. They are also ram ventilators, which means that they must move to breathe. Their gills work on a system of counter-current exchange where water flows through their mouth and through the gills over filaments full of capillaries. The blood flows in the opposite direction of the water flow to optimize the amount of oxygen absorption. To make the water flow over the filaments, however, the sharks must be swimming at all times. The reason they are killed so often as bycatch is because if they are caught on a line or in a net and can t swim they will drown. In general they also have a low resilience to exploitation. 9 While they can have between twelve to thirty-eight pups at once, their generation time is long at around fifteen years. This generation time is due to the fact that these sharks are actually viviparous, or give live birth. Their gestation takes around 9 Sphyrna Lewini, Scalloped Hammerhead, IUCN.org.

6 Neylan 6 twelve months per litter. Because it takes so long for them to reproduce, it is very difficult for their numbers to rebound. The two leading causes of the scalloped hammerhead population decline are intentional human predation in the form of commercial fishing and unintentional human predation where the sharks are caught as bycatch. These problems are exacerbated by the characteristics of the species itself. There are, however, ways to try and alleviate the problem. The biggest and potentially most beneficial thing to do would be to eliminate the demand for shark fins. Without the market and the demand there would be no shark fishing at all. One option is to have governments ban shark fin soup and the selling of shark fins. This has been implemented in California and British Columbia, Canada. In Asia, however, the governments seem less receptive. The closest was a movement in China that banned the serving of shark fin soup at state functions in the next three to five years. While a slight nod in the right direction, this ban clearly does not fix the problem. Government officials in Hong Kong, the world s leader in shark fins, will not even acknowledge a problem. A second option is to work from the ground up rather than the top down by forming a grassroots movement in Asia. A huge problem is simple ignorance. Most people in China are not aware that shark fin soup even has shark in it, (in Chinese the name more closely translates to fish fin soup,) let alone that the fins come from endangered species. This movement must come from the Chinese themselves. People are much less receptive to outsiders telling them they are doing something wrong. In 2006, the Chinese basketball player Yao Ming began an anti-shark fin soup campaign in China and had moderate success. More attempts like this would be the most effective way of reaching people. Some other possible solutions would be to cultivate international cooperation in maintaining marine reserves and to continue to police the poaching of these animals. The

7 Neylan 7 golden triangle would be a great place to start. Another solution would be to implement more ocean-friendly forms of fishing that avoid bycatch. There is a hook specially designed to slip out of a shark s mouth that could be used. Just a small step like that could really help the problem. And generally, more research into the life history of these animals is necessary. The majority of the studies cited in this paper are in their infancy. Very little is known about these unique animals and new found information should be shared with the public. The issue of shark finning should be brought to everyone s attention. Most people are completely unaware that there is a problem. Awareness is the first step towards action. These solutions, however, need to be implemented soon if there will be any hope of saving the scalloped hammerhead in the Galapagos and worldwide before it is too late.

8 Neylan 8 Works Cited Abercrombie, Debra L., Shelley C. Clarke, and Mahmood S. Shivji. "Global-scale Genetic Identification of Hammerhead Sharks: Application to Assessment of the International fin Trade and Law Enforcement." Conservation Genetics 6.5 (2005): Print. Branstetter, Steven. "Age, Growth and Reproductive Biology of the Silky Shark, Carcharhinus Falciformis, and the Scalloped Hammerhead, Sphyrna Lewini, from the Northwestern Gulf of Mexico." Environmental Biology of Fishes 19.3 (1987): Print. Camhi, Merry. "Industrial Fisheries Threaten Ecological Integrity of the Galapagos Islands Merry Camhi." Conservation Biology 9.4 (1995): Print. Clarke, Shelley C., Jennifer E. Magnussen, Debra L. Abercrombie, Murdoch K. Mcallister, and Mahmood S. Shivji. "Identification of Shark Species Composition and Proportion in the Hong Kong Shark Fin Market Based on Molecular Genetics and Trade Records." Conservation Biology 20.1 (2006): Print. Hearn, Alex, James Ketchum, A. Peter Klimley, Eduardo Espinoza, and Cesar Peñaherrera. "Hotspots within Hotspots? Hammerhead Shark Movements around Wolf Island, Galapagos Marine Reserve." Marine Biology (2010): Print. Hearn, Alex. "Shark Migrations: Discovering the Golden Triangle." Galápagos: Preserving Darwin's Legacy. By Roy Tui. De. Richmond Hill, Ont.: Firefly, Print. Hinke, Jefferson T., Isaac C. Kapan, Kerim Aydin, George M. Watters, Robert J. Olson, and James F. Kitchell. "Visualizing the Food-Web Effects of Fishing for Tunas in the Pacific Ocean." Ecology and Society 9.1 (2004): n. pag. Print.

9 Neylan 9 Kinney, Michael John, and Colin Ashley Simpfendorfer. "Reassessing the Value of Nursery Areas to Shark Conservation and Management." Conservation Letters 2.2 (2009): Print. Klimley, A. P. "Highly Directional Swimming by Scalloped Hammerhead Sharks, Sphyrna Lewini, and Subsurface Irradiance, Temperature, Bathymetry, and Geomagnetic Field." Marine Biology (1993): Print. Orchant, Rebecca. "Threatened Species Used In Shark Fin Soup Across U.S." The Huffington Post. TheHuffingtonPost.com, 10 Aug Web. 08 Sept < "Scalloped Hammerhead." (Sphyrna Lewini) FAQ. N.p., n.d. Web. 05 Sept < "Scalloped Hammerhead." Wikipedia. Wikimedia Foundation, 09 May Web. 05 Sept < Sonnino Sorisio, L., A. De Maddalena, and I. N. Visser. "Interaction between Killer Whales (Orcinus Orca) and Hammerhead Sharks (Sphyrna Sp.) in Galápagos Waters." Latin American Journal of Aquatic Mammals 5.1 (2006): n. pag. Print. "Sphyrna Lewini." (Scalloped Hammerhead). N.p., n.d. Web. 05 Sept < Stevens, J. "The Effects of Fishing on Sharks, Rays, and Chimaeras (chondrichthyans), and the Implications for Marine Ecosystems." ICES Journal of Marine Science 57.3 (2000): Print.

10 Figure 1 Neylan 10 Figure 2 2 Large sharks Purse seine Longline Rel ati ve Bio ma All fisheries Year

11 Figure 3 Neylan 11

12 White tip reef shark attacking a sea lion. Neylan 12

13 Another species of shark, the white tip reef shark, named for the white tip of its dorsal fin. Neylan 13

14 Potential scalloped hammerhead food Neylan 14