Scalloped Hammerhead Shark (Sphyrna lewini)
Nicole Balliet
"If we go on as we are, we will destroy in the next century everything that the poets have been singing about for the past two thousand years."
- Fred Bodsworth |
Taxonomy
Kingdom: Animalia Phylum: Chordata Class: Chondricthyes Order: Carcharhininformes Family: Sphyrindae Genus: Sphyrna Species: lewini |
What is it about sharks that is so fascinating to humans? Why do we have an entire week of television dedicated to them? Is it because they are vicious predators and we want to drink in every little drop of knowledge we can about how they have come to be such effective hunters? Or is it because they are one of the few types of animals that have remained relatively unchanged by time? No matter why you are interested in sharks it is easy to see that they have an appeal, but did you know that some of the greatest hunters on earth are being hunted? Many shark populations are being hunted and their populations cannot handle the stress. If something isn’t done soon “Shark Week” may be a new special on the History Channel.
The scalloped hammerhead (Sphyrna lewini), left, and the smooth hammerhead (Sphyrna zygaena), right. http://www.daff.qld.gov.au/28_12242.htm
Hammerhead Sharks belong to the order Sphyrna, which is the greek word for “hammer.” The Scalloped Hammerhead Shark is one of about 10 different species of related hammerhead sharks. Hammerhead sharks are easily identified by the hammer shape of their heads. Each species, however, has a slightly different shape to their head and other body parts. The Scalloped Hammerhead pictured above on the left is identified by the indentations along the edge of the sharks head, especially the central indent which gives it a scalloped look. Where as the Smooth Hammerhead Shark, on the right, does not have this feature.
Scalloped Hammerheads have an olive coloring on the top of their bodies and an off white or white coloring on the underside. This pattern is called countershading and acts as a type of camouflage. If another animal, such as a prey animal, looked up at the shark the light color would blend in with the light coming from the sun. If an animal were looking down on the shark the dark color would blend in with the color of the deeper, dark water. Another feature of this species, like all sharks, is that they have no calcified bones. Instead, their internal structural support system is made of cartilage. In this species females are larger than the males. The females reach maturity at a size of “8.2 feet and approximately 177 lbs.” whereas males are considered mature at “5.9 feet and a weight of approximately 64 lbs.” [1]
These sharks often feed at night, when their prey comes to the water’s surface to feed. Their prey includes several species of fish, squid, crustaceans, as well as other sharks and rays. These sharks also have another method of hunting down their prey, “a hammerhead shark uses its wide head to trap stingrays by pinning them to the seafloor.” [2] Adult Scalloped Hammerheads have no major predators, although larger sharks may prey on smaller or injured sharks.
All hammerhead sharks, including this species, are viviparous. This means that they give birth to live young. This is different from most sharks, because most sharks lay eggs that hatch outside the body. Sometimes these egg sacs are called mermaid’s purses. Hammerhead sharks have a gestation period of ~9-10 months. During this period, nourishment is provided by a yolksac placenta, or a pseudo placenta. This pseudo placenta acts very similar to a mammalian placenta by transporting oxygen and nutrients to the embryos and removing wastes [1]. Near the end of this time period the female sharks will swim into shallow water to deliver litters of shark pups. This is most likely done to avoid predation while the mother is unable to defend herself. Then she will leave her pups; they will stay in the shallow water until they are large enough to survive in deeper waters. While the pups are in the shallow water the sun will darken the skin on the top of their body, giving it the counter shaded appearance adults have.
Scalloped Hammerheads have an olive coloring on the top of their bodies and an off white or white coloring on the underside. This pattern is called countershading and acts as a type of camouflage. If another animal, such as a prey animal, looked up at the shark the light color would blend in with the light coming from the sun. If an animal were looking down on the shark the dark color would blend in with the color of the deeper, dark water. Another feature of this species, like all sharks, is that they have no calcified bones. Instead, their internal structural support system is made of cartilage. In this species females are larger than the males. The females reach maturity at a size of “8.2 feet and approximately 177 lbs.” whereas males are considered mature at “5.9 feet and a weight of approximately 64 lbs.” [1]
These sharks often feed at night, when their prey comes to the water’s surface to feed. Their prey includes several species of fish, squid, crustaceans, as well as other sharks and rays. These sharks also have another method of hunting down their prey, “a hammerhead shark uses its wide head to trap stingrays by pinning them to the seafloor.” [2] Adult Scalloped Hammerheads have no major predators, although larger sharks may prey on smaller or injured sharks.
All hammerhead sharks, including this species, are viviparous. This means that they give birth to live young. This is different from most sharks, because most sharks lay eggs that hatch outside the body. Sometimes these egg sacs are called mermaid’s purses. Hammerhead sharks have a gestation period of ~9-10 months. During this period, nourishment is provided by a yolksac placenta, or a pseudo placenta. This pseudo placenta acts very similar to a mammalian placenta by transporting oxygen and nutrients to the embryos and removing wastes [1]. Near the end of this time period the female sharks will swim into shallow water to deliver litters of shark pups. This is most likely done to avoid predation while the mother is unable to defend herself. Then she will leave her pups; they will stay in the shallow water until they are large enough to survive in deeper waters. While the pups are in the shallow water the sun will darken the skin on the top of their body, giving it the counter shaded appearance adults have.
ScallopedHammerhead Sharks have a long history with humans. This species of shark is considered to be “dangerous to humans.” This species is one of only 3 hammerhead species that have attacked humans in the past. However, no deaths have occurred from these attacks. These attacks occurred in shallow water by female sharks having pups, who presumably felt threatened or by juvenile sharks that were still living in the shallow water. Whether you feel these sharks are dangerous or not, there is another issue that must be looked at. These sharks are being fished at an alarming rate.
Fisherman are collecting as many sharks as they can at one time, and several behaviors of this shark are being taken advantage of in order to do this, including the fact that these sharks are often found in large schools and that they all come into shallow water at nearly the same time to give birth to their pups.
Why are these sharks being fished so intensely? Most commonly for soup! Shark fin soup, can sell for as much as $300 a bowl. The fishermen will often cut the fins off of the sharks and toss it overboard, leaving it to die. They will do this because the rest of the shark’s meat is not valuable so tossing it overboard will save space for the rest of their catches.
Fisherman are collecting as many sharks as they can at one time, and several behaviors of this shark are being taken advantage of in order to do this, including the fact that these sharks are often found in large schools and that they all come into shallow water at nearly the same time to give birth to their pups.
Why are these sharks being fished so intensely? Most commonly for soup! Shark fin soup, can sell for as much as $300 a bowl. The fishermen will often cut the fins off of the sharks and toss it overboard, leaving it to die. They will do this because the rest of the shark’s meat is not valuable so tossing it overboard will save space for the rest of their catches.
Sharks are highly threatened worldwide due to overfishing, a trend that is largely driven to fuel the shark fin trade as well as from accidental bycatch from commercial fishing operations. Even more disturbing than the fact that these sharks are being overfished is that an “assessment of the population rebound potential of 26 shark species in the Pacific Ocean ranked Sphyrna lewini as one of the species with the poorest ability to recover from increased mortality”[3]. This means that if we do not stop the overfishing of this species soon, they will not be able to recover and this beautiful, unique species will be lost from this world forever. Studies in several different oceans and different countries all show that this population is rapidly declining, sometimes by as much as 98%. “Estimates of trends in abundance are available from two long-term research surveys conducted on the U.S. east coast, both of which indicate this species has undergone substantial declines in this region (98% between 1972 and 2003)” [4].
“The USA data for the scalloped hammerhead is often combined with two other species, the great and smooth hammerheads. The scalloped hammerhead is by far the most abundant of the three species, so the data probably reflects best the condition of scalloped hammerhead stocks. Cramer reported that the CPUE (catch per unit effort) index for hammerheads in the US Atlantic fisheries fell from 2.9 in 1986 to 0.64 in 1995” [5]. What this graph pictured above shows is the reported catch per unit effort data from 1986 to 1995. Catch per unit effort is a measurement that shows how many organisms, in this case sharks, are caught per trial. Or simply how much effort do they have to put into catching these animals. This number has gone down dramatically showing that the same amount of effort now yields significantly fewer sharks than in the past, because of the declining population.
Populations for hammerhead sharks in the northwest Atlantic have declined more than 80 percent over the last two decades. DNA analysis of great hammerhead fins sold in the Asian shark fin market has shown that a large majority of the sharks came from Atlantic waters. Due to these findings one recent development has been made in an attempt to save these creatures. A ban has been placed on fishing Hammerhead Sharks in the waters of the coast of Florida, one of the sharks’ nursery habitats. “Research shows that Florida waters are important pupping and feeding grounds for sharks, providing them with the critical habitat required for these slow-to-mature ocean animals to survive.” These new measures went into effect Jan. 1, 2012 and prohibit the harvest, possession, sale and exchange of tiger sharks and great, scalloped and smooth
hammerhead sharks harvested from state waters.
“The USA data for the scalloped hammerhead is often combined with two other species, the great and smooth hammerheads. The scalloped hammerhead is by far the most abundant of the three species, so the data probably reflects best the condition of scalloped hammerhead stocks. Cramer reported that the CPUE (catch per unit effort) index for hammerheads in the US Atlantic fisheries fell from 2.9 in 1986 to 0.64 in 1995” [5]. What this graph pictured above shows is the reported catch per unit effort data from 1986 to 1995. Catch per unit effort is a measurement that shows how many organisms, in this case sharks, are caught per trial. Or simply how much effort do they have to put into catching these animals. This number has gone down dramatically showing that the same amount of effort now yields significantly fewer sharks than in the past, because of the declining population.
Populations for hammerhead sharks in the northwest Atlantic have declined more than 80 percent over the last two decades. DNA analysis of great hammerhead fins sold in the Asian shark fin market has shown that a large majority of the sharks came from Atlantic waters. Due to these findings one recent development has been made in an attempt to save these creatures. A ban has been placed on fishing Hammerhead Sharks in the waters of the coast of Florida, one of the sharks’ nursery habitats. “Research shows that Florida waters are important pupping and feeding grounds for sharks, providing them with the critical habitat required for these slow-to-mature ocean animals to survive.” These new measures went into effect Jan. 1, 2012 and prohibit the harvest, possession, sale and exchange of tiger sharks and great, scalloped and smooth
hammerhead sharks harvested from state waters.
We can only hypothesize what the future holds for these animals. However, if bans on fishing or limits are not imposed on the fishing of these sharks the species will soon be so decimated a comeback is impossible. These animals are very slow to reproduce and mature so it will take a long time for the population to attempt to rebound from the damage humans have
done. Even if limits are placed on the fishing of Scalloped Hammerheads, there is no guarantee that these limits will be effective. There will always be people who are willing to break the law to make more money; and the fin market is a very lucrative market.
done. Even if limits are placed on the fishing of Scalloped Hammerheads, there is no guarantee that these limits will be effective. There will always be people who are willing to break the law to make more money; and the fin market is a very lucrative market.
1. Florida Museum of Natural History: Department of Ichthyology
www.flmnh.ufl.edu/fish/Gallery/Descript/ScHammer/ScallopedHammerhead.html
2. National Geographic
www.kids.nationalgeographic.com/kids/animals/creaturefeature/hammerhead-shark/
3. New South Whales Government: Department of Primary Industries
www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/401838/Scalloped-Hammerhead-proposed-determination.pdf
Smith, S.E., Au, D.W. and Show, C. (1998) Intrinsic rebound potentials of 26 species of Pacific sharks. Marine and
Freshwater Research 49: 663 - 678.
4. New South Whales Government: Department of Primary Industries
www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/401838/Scalloped-Hammerhead-proposed-determination.pdf
Baum, J. et. al. (2007) Sphyrna lewini. In IUCN 2010. IUCN Red List of Threatened Species. Version 2010.
5. Food and Agriculture Organization of the United Nations
www.fao.org/docrep/003/X2352E/x2352e07.htm#b32-4.2.30%20Hammerhead%20sharks,%20Family%20Sphyrnidae
6. University of Miami
www.rsmas.miami.edu/news-events/press-releases/2011/scientists-applaud-florida-new-shark-fishing-ban
www.flmnh.ufl.edu/fish/Gallery/Descript/ScHammer/ScallopedHammerhead.html
2. National Geographic
www.kids.nationalgeographic.com/kids/animals/creaturefeature/hammerhead-shark/
3. New South Whales Government: Department of Primary Industries
www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/401838/Scalloped-Hammerhead-proposed-determination.pdf
Smith, S.E., Au, D.W. and Show, C. (1998) Intrinsic rebound potentials of 26 species of Pacific sharks. Marine and
Freshwater Research 49: 663 - 678.
4. New South Whales Government: Department of Primary Industries
www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/401838/Scalloped-Hammerhead-proposed-determination.pdf
Baum, J. et. al. (2007) Sphyrna lewini. In IUCN 2010. IUCN Red List of Threatened Species. Version 2010.
5. Food and Agriculture Organization of the United Nations
www.fao.org/docrep/003/X2352E/x2352e07.htm#b32-4.2.30%20Hammerhead%20sharks,%20Family%20Sphyrnidae
6. University of Miami
www.rsmas.miami.edu/news-events/press-releases/2011/scientists-applaud-florida-new-shark-fishing-ban