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.
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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.
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,
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).
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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 157.9 (2010). 8 Hearn, “Shark Migrations: Discovering the Golden Triangle,” Galapagos: Preserving Darwin’s Legacy (2009).
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