NOWRAMP 2002

Counting Sharks
Posted by Mark Heckman, Educator, Waikiki Aquarium/ University of Hawai'i - Manoa
Underwater Photography by Jim Watt

One of the main differences between the marine environment of the Northwestern Hawaiian Islands and that of the main Hawaiian Islands has to do with the number of predators (like sharks) you find - big top predators (also called 'apex' predators). Off O`ahu or the Island of Hawai`i, it is rare to see a shark close up. In fact, it is rare to see a shark at all. The top predators are just not around.

Here in the Northwestern Hawaiian Islands, things are a bit different. Our dive crews see sharks routinely - one shark, two sharks, five, ten, fifteen sharks and more (1). This is fast becoming one of the last places on Earth to find reef sharks. Elsewhere in the Pacific they are getting rapidly fished out for the shark fin market. Currently, divers and snorkelers come back from travels to even the most remote islands in the Pacific and report that the reef sharks are disappearing fast. With the advent of inexpensive power boats, local (and not so local) fishing crews can get just about anywhere.

So let's take a look at some of the sharks we are encountering in this last bit of protected ocean wilderness.

Whitetip Reef Shark
Triaenodon obesus (Ruppell, 1837)
Hawaiian name: mano lalakea

This shark does not need to swim to breathe; it can pump water over the gills while resting on the bottom. Note the white tips on the dorsal (top) and caudal (tail) fins. Quiet during the day, these sharks are seen singly or in pairs in caves, under ledges or in quiet spots. If they do rise up and swim during daylight hours, the swimming is typically slow and relaxed. These sharks are not considered a threat to humans, although this can lead to problems as well. Swimmers and divers now sometimes crowd around and even poke (3) at these sharks in the wild - surely not a good choice.

At night, however, this shark is a different animal. Sinuous and fast, they pursue reef fish, octopus and lobsters on the reef. Whitetip reef sharks can reach nearly 6 feet in length, but 4 - 5 feet is more common. They may return to the same resting spot each day for weeks or even months if not disturbed.

Gray Reef Shark
Carcharhinus amblyrhynchus (Bleeker, 1856)
Hawaiian Name: mano

We began encountering the grey reef sharks on our first dive at Nihoa Island. These sharks swim constantly, patrolling the reef. Maximum size is about six feet, but swimmers and divers often report that they seem much larger (4). Gray reef sharks are famous for their visible threat posture (unfortunately, not always used). Grays have been observed dropping the pectoral fins down and arching the back when their "personal" space is invaded.

Gray reef sharks are curious about changes in their environment. New features or activity in their environment may mean food (not the divers, but what ever the divers scare up). Our divers are used to having a shark or three keeping an eye on them as they go about their work.

Galapagos Shark
Carcharhinus glapagensis (Snodgrass and Heller, 1905)
Hawaiian Name: mano

Galapagos sharks start appearing at Gardner Pinnacles. They will become the more common shark as we move up the chain. Larger than the Gray reef shark, maximum size is to over eleven feet. When swimmers and divers encounter these sharks and report that it was a big shark, it really could have been a large animal. Galapagos sharks lack the dark black edge (5) to the tail fin found on grey sharks and have a slight ridge (5) between the dorsal (top) and caudal (tail) fin.

Galapagos sharks, like the Gray reef sharks, sometimes accompany our divers as they go about their business. Numbers can range from one, to five, to more than fifteen sharks slowly circling, especially as the divers ascend at the end of the dive. Expedition divers are all experienced and keep an eye on these animals. Our crew are all instructed to abort a dive if conditions warrant. Galapagos sharks have been known to be aggressive, bumping or charging divers; but are typically not an issue.

Tiger Shark
Galeocerdo cuvier
Hawaiian Name: niuhi

Tiger sharks are found throughout the Main and Northwestern Hawaiian Island chain. These are the sharks most often implicated in attacks (6) on humans in Hawaiian waters. Tiger sharks are the main predator in the Hawaiian Islands large enough to feed on turtles, seals, and on other sharks. They are also the clean up crew. Tiger sharks will take care of any dead animals washed down in storms, and have been seen feeding on floating bags of trash.

In the Northwestern Hawaiian Islands, Tiger sharks have been observed taking newly fledged albatross chicks that land on the water. It would seem that this would be an easy meal, but observations have shown that they often miss, especially in the beginning of the season. Sometimes observers of this scenario are split between those rooting for the bird and those rooting for the shark (they need to eat also). In reality, sharks eat less, pound for pound, than humans or other mammals.

White Shark
Carchcarodon charcarias
Hawaiian Name: niuhi

Current information on White sharks shows that they can be long distance travelers. Reliable records for the Main Hawaiian Islands exist, with only a very few individuals seen in any year. A recent study showed that tagged individuals leaving California tended to move deeper into cooler waters as they made way into the tropics.

References: Randall, John, 1996, Shore Fishes of Hawai'i, Crow, G.L. and Crites, J., 2002, Sharks and Rays of Hawai'i

(1) Count the sharks in this picture, how many can you find? Note how they just disappear into the blue. They sense us long before we can notice them. Click here (2) for the correct answer.

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(3) Not a good idea, even non-aggressive sharks may have lots of sharp teeth. With all of the shark shows on television, perhaps people are now feeling a false sense of security with sharks. Our advice: don't feed or pet the bears in the national parks; don't pet or poke the sharks on the reefs. As to those sharp teeth? Sharks need them to catch their food. Fish are slippery.
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(4) Our eyes and brain are adjusted for seeing objects through air not water. Light travels at a slower speed through water. This speed change, coupled with problems with reduced visibility, causes swimmers and divers to misjudge the sizes of objects in water. Objects in water look 25 percent larger than on land.
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(5) On the other hand, Galapagos can have black shading (versus a darker black edge) on the tail fin. Unless a diver is familiar with the differences between Gray and Galapagos sharks, it is easy to misidentify the two. And if you are close enough to see the slight ridge between dorsal and caudal fins - you are probably too close.
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(6) Despite local news coverage, shark attacks are not a high risk for most people. Shark attacks in Hawai`i continue to average at 2 - 3 a year. Given the millions of water entries by tourists and locals each year, the odds for any individual to have a negative encounter are extremely low. Driving to and from the beach is a much more risky affair. Still, sharks should always be treated with caution and respect.
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(2) How many sharks?

Answer: 35
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