Local Shark Scientists Explain Why There Are So Many Shark Attacks in Hawai‘i

Hawai‘i’s shark researchers are some of the world’s best. Here’s what they say about our recent spike of shark attacks.

February 3, 2016

Don Wallace,

Early in his research for this month’s story on shark attacks (available on newsstands now), HONOLULU Magazine senior editor Don Wallace contacted Kim Holland and Carl Meyer, Hawai‘i’s go-to shark scientists. The resulting back and forth moved Holland, of the Hawai‘i Institute of Marine Biology, and Meyer, of the University of Hawai‘i at Mānoa, to prepare this exclusive executive summary and Q&A, portions of which were excerpted in the main article. (The questions have been abridged, but not the answers, which were also copyedited for style.) Among the surprising findings: The rise of the Dow-Jones Industrial Average closely parallels attacks in the U.S. (and if science says it’s so, you know it’s true). Of perhaps greater interest is their observation that sharks have memories—and very good ones when it comes to where and when to find the best food sources. In other words, they’re a lot like us.

HONOLULU: Why do we see an increasing number of shark bites over time in Hawai‘i?

Holland/Meyer: The annual number of shark bites in Hawai‘i has increased over time in concert with increases in the human population of the Hawaiian Islands. Not only are there now more people living in Hawai‘i than previously, but we also have more visitors to the Islands every year. These increases have resulted in ever larger numbers of people using the ocean for recreational purposes. Over time, we are also inventing new types of ocean recreation, many of which take us further offshore into shark habitat than was previously the case. Long-term, ongoing increases in the numbers of people using the ocean, and changes in the types of recreational activity, could easily explain increases in shark bites over time in Hawai‘i, and similar trends have been seen in other locations around the world. Although speculation persists about other environmental factors contributing to an upward trend in shark bites, there is currently no scientific evidence to support these claims.

Hawai‘i human population versus shark bite incidents.

Source: International Shark Attack File

It is also important to understand that, although we may see correlations between the number of shark bites and other phenomena, these relationships are not necessarily causal. For example, there is a strong (95 percent), long-term correlation between the Dow Jones Industrial Average and the number of shark bites in Hawai‘i. No-one would reasonably expect share values to drive shark bites, as this is obviously absurd, but other apparently more plausible correlations may be equally coincidental, so we should be cautious when interpreting trends, especially when we lack empirical data to prove a causal link.

Dow Jones Industrial Average (10-year average) versus total number of Hawai‘i shark bites per decade (1900–2015).

We should also note the very uneven year-to-year pattern in shark bite frequency. For instance, in 2011 there were three unprovoked shark bites, whereas in 2013 there were 13. Certainly nothing about the biology of sharks, the number of sharks or the number of people in the water changes that fast—so there appears to be a large “chance” factor in play. Similarly, there are as many sharks in Hawai‘i on the 359 days of the year when there are no shark bites as there are on the six days a year (e.g., 2014) when they do occur. In the past decade, the average number of unprovoked shark bites averages about 5.5 per year—very different from the 13 that occurred in 2013.

HONOLULU: Do more turtles equal more shark bites?

H/M: Much speculation surrounds a putative link between the recovery of the Hawaiian green sea turtle population and an increasing number of human-shark interactions. Some people claim that more sea turtles has allowed for tiger shark population growth in our coastal waters, and also that the large number of turtles present in shallow areas has drawn tiger sharks inshore to forage where they are more likely to encounter people. The reality is that no one knows for sure whether the recovering sea turtle population has had a significant influence on shark bite numbers, and some known facts potentially confound this speculative relationship.

For example, the only published scientific study of tiger shark diet in Hawai‘i found turtles to be a relatively minor component of the diets of both medium (7-10 feet total length, 7 percent occurrence of turtles in stomachs) and large (greater than 10 feet total length, 15 percent occurrence of turtles in stomachs) tiger sharks. The stomachs of large tiger sharks more often contained sharks and rays (42 percent), fishes (40 percent), crabs and lobsters (35 percent) and birds (25 percent) than turtles. However, the data for this study were derived from the shark culling programs of the late 1950s, 1960s and 1970s, when Hawaiian green sea turtle populations were seriously depleted.

It is certainly possible that tiger sharks have shifted toward eating more turtles over the past 40 years, but we lack empirical data needed to determine whether this is in fact the case. Again, it is important to remember the uneven inter-annual pattern of shark bites—neither the turtle population nor the shark population varies this quickly.

An obvious problem with inferring tiger shark dietary preferences from anecdotal observations of turtle predation is that these events are highly surface biased (i.e., most people see sharks capturing turtles at or close to the surface), and thus are much more likely to be observed than predation events occurring at deeper depths beyond our view. Tiger sharks are vertically dynamic swimmers, constantly yo-yo diving between the surface and the seabed, or down to depths of 100 to 600 meters in open water, so conclusions about tiger shark diet based on a small number of observations at the ocean surface are likely to be inaccurate. Tiger sharks have an extremely broad diet consisting of prey occurring throughout the shark’s wide vertical range.

HONOLULU: Has there been an increase in tiger shark populations?

H/M: We have no information on tiger shark abundance in Hawai‘i because their movement patterns make it impossible to accurately estimate their abundance. Individuals move routinely (but unpredictably) between islands and far out into open ocean, which violates key assumptions (that immigration and emigration rates are known) underlying the “mark-recapture” methods used by biologists to estimate wild animal population sizes. At best, we could generate population size estimates with enormous margins of error. These would be useless for evaluating population size changes over time.

However, there are at least three reasons why tiger shark populations are probably fairly stable over time in Hawai‘i waters:

First of all, there are no commercial shark fisheries in Hawai‘i coastal waters, so this major source of mortality seen in other geographic locations simply doesn’t exist here. Similarly, putting an end to some previous (nonexistent) fishery has not caused the tiger shark population to “rebound” and expand.

Secondly, there have been no major changes in the fishing pressure on Hawai‘i’s coastal areas in recent years and so the amount of food available to sharks has probably remained more or less stable. Further, there is no evidence that tiger sharks have ever been food-limited in Hawai‘i waters. Tiger sharks are broad, generalist predators that eat a wide variety of prey, including many species that are toxic or unpalatable to other animals and humans. When you eat a lot of different things, you can easily switch prey if any particular species becomes hard to find. When most of your diet consists of non-fisheries species, then you are insulated from declines in abundance of fisheries species.

Thirdly, tiger shark populations may be naturally self-limiting because one of the most common prey types for large tiger sharks is other sharks, including other tiger sharks. As the number of large tiger sharks increases, the survival of juvenile tiger sharks may decline, because the big sharks eat the little sharks.

HONOLULU: What does tagging tell us?

H/M: Virtually everything we know about the movements and behavior of tiger sharks in Hawai‘i is a result of tagging and tracking experiments conducted over the past 30 years by research teams based at Hawai‘i Institute of Marine Biology. Tagging sharks also tells us about growth rates. Electronic tagging (i.e., “tracking”) can tell us how often tiger sharks visit coastal recreation sites and how much time they spend in these areas. One of our first discoveries was that tiger sharks are extremely mobile, moving easily and frequently between islands. Tracking can also determine how far afield tiger sharks range, and can identify seasonal migrations and other predictable patterns of movement. Understanding tiger shark movement patterns allows for fact-based decision making on shark bite response strategies. Making our shark satellite tracks publiclly available on the PACIOOS (Pacific Islands Ocean Observation System) website also allows the general public to be better informed about the presence and movement patterns of large tiger sharks in coastal waters.

To date, we have electronically tagged over 450 sharks in Hawai‘i waters, including 174 tiger sharks. The 14 individuals tagged around O‘ahu during 2014–2015 are part of a much larger, ongoing effort to quantify tiger shark spatial dynamics. (The tagging instruments include acoustic transmitters, satellite transmitters and triaxial accelerometer-magnetometer data-loggers.)

Conventional identification tagging tells us about movements and growth rates. To date we have conventionally tagged over 1,300 sharks in Hawai‘i waters, including 501 tiger sharks. Our longest distance record for a tiger shark tagged in Hawai‘i was from an identification tag recovered in Mexico. This shark was tagged off Honolulu and subsequently recaptured by Mexican fishermen off Isla Jacques Cousteau (formerly Isla Cerralvo, 24°13′17″N 109°52′14″W), in the southern Gulf of California (minimum distance between tag and recapture sites was approximately 5,000 kilometers), after 366 days at liberty.

Our conventional tagging efforts have also enabled us to calculate the growth rates of tiger sharks in Hawai‘i. We found that the average tiger shark in Hawai‘i reaches 12 feet in length by age 7, and the fastest growing individuals reach this size by age 4. On average, female tiger sharks in Hawai‘i reach reproductive size (11 feet) by age 5 and males (9.5 feet) by age 4, but faster growing individuals reach maturity when they are only 3 years old. These growth and reproduction rates are faster than we would have expected prior to conducting this research. The largest female tiger shark we have captured in Hawai‘i measured 15.2 feet in total length, and the largest male 14.3 feet. Tiger sharks larger than 14.5 feet total length are rare.

Overall, electronic and conventional tagging have shown us that tiger sharks in Hawai‘i have large home ranges that typically include several islands, but each individual shark is most frequently found around a “core” island within its broader home range. Movements between islands, and more extensive movements of hundreds of miles along the chain, or out into open ocean, are relatively common. Adult females make seasonal migrations from the remote Northwestern Hawaiian Islands to the main Hawaiian Islands during the fall pupping season. The recapture from Mexican waters shows connectivity exists between tiger sharks in Hawai‘i and those in the Eastern Pacific.

HONOLULU: What effect does culling sharks have?

H/M: Historical shark culling efforts in Hawai‘i did not stop shark bites, nor is there evidence of a reduction in shark bites associated with culling efforts. For example, the 1967–69 Shark Control Program killed 1,727 sharks (including 280 tiger sharks), yet 1969 saw more shark bites (two total) than any other year during the 1960s. Licius Lee was bitten while surfing at Mākaha (March 9, 1969), and a few months later (Nov. 11, 1969) D. McGinnis was injured by a shark while diving for lobsters off Barbers Point. The latter incident is particularly demonstrative of the ineffectiveness of culling programs, because 33 tiger sharks were removed from waters off Barbers Point alone during the 1967–69 program, yet a shark bite still occurred at that location in late 1969. The reason for this is likely simple—tiger sharks are highly mobile and the culled sharks were soon replaced by other animals moving into the area.

Given what our tracking and growth rate studies have shown, we can state that, in order for any culling operation to have any chance of success at reducing shark bite frequency, it would require a huge, costly and continuous fishing effort that (even if feasible) would result in an unacceptable perturbation of our marine ecosystem. And, as pointed out above, even this would not guarantee that shark bites will not occur.

HONOLULU: As a scientist, what would you like to see measured or quantified to bring clarity here?

H/M: Detailed information on the volumes and types of ocean recreation activities occurring over time around the coastlines of each island would allow much more detailed analysis of shark bite data. It would be useful to reassess the importance of turtles to the diets of tiger sharks—but this would require nonlethal ways of doing so, perhaps by using ultrasound to see inside the stomachs of sharks captured during research activities.

HONOLULU: Do sharks have memories?

H/M: We have good evidence that sharks have memories. Long-term, reciprocal movements between distant locations suggest tiger sharks possess detailed cognitive maps of resource availability. The precise, seasonal arrival of certain tiger sharks at French Frigate Shoals atoll in time for albatross fledging season (which occurs during only three to four weeks each summer) indicates these sharks may also use internal clocks to guide their movements. Not only do these results demonstrate “memory” but also a very sophisticated ability to navigate over large distances of ocean.

Repeated sightings of sharks at particular locations may arise because the same individuals return to these sites, and also simply because these locations may simply be good habitat for sharks.

HONOLULU: Tigers are perpetrators of severe and fatal attacks on humans, along with a possible great white shark visitor. Do Galapagos, sandbar, reef and hammerhead sharks ever cause fatalities, or can we call them biters but not eaters?

H/M: Based on victim descriptions, a very small number of unprovoked shark bites in Hawai‘i are likely due to small sharks other than tiger sharks, but identifying the specific species responsible has not been possible in most cases. A lack of clear species identification is common in shark bite incidents. Victims are concerned with survival and escape rather than species identification. However, tiger sharks are large, very distinctive in appearance and are also the only large shark species routinely found in waters used by humans for ocean recreation in Hawai‘i. Available evidence suggests tiger sharks are responsible for most shark bite incidents in Hawai‘i waters. All of the other species of sharks that you mention have been implicated in attacks somewhere around the world at some time or other—for instance, during ship disasters at sea (e.g., during war). In Hawai‘i there are no confirmed attacks from Galapagos sharks, although they are certainly big enough to cause damage. Our research indicates that this species very rarely comes close enough to shore to interact with swimmer or surfers. Worldwide, only one fatality has been conclusively linked to a Galapagos shark (LTJG John W. Gibson, age 25, April 20, 1963, Magens Bay, Saint Thomas, U.S. Virgin Islands).

Three species of shark (white, tiger, bull) account for the vast majority of severe shark bites worldwide.

Severe or fatal bites by other shark species are extremely rare. Sandbar sharks have only been implicated in five nonfatal bites worldwide (none in Hawai‘i). Seventeen nonfatal shark bites are attributed to the genus Sphyrna (hammerhead), but most of these are likely due to great hammerheads (rare in Hawai‘i).

The scalloped hammerhead, commonly found in Hawai‘i, is a much smaller species than the great hammerhead. Three Hawai‘i shark bites have been attributed to hammerheads. Although the specific hammerhead species was not identified, two of the incident locations are known nursery areas for scalloped hammerheads:

Sept. 2, 1953, Waiau, Pearl Harbor, O‘ahu, Daniel Gonsalves, bitten on leg and foot by 5-foot hammerhead shark while crabbing;
June 3, 1984, Kāne‘ohe Bay, O‘ahu, Susan Buecher, bitten on the foot while towing her sister on a plastic ski board. Incident happened at 5 p.m. in water 5 feet deep, about 400 yards from shore. Surgery and lower leg cast required to repair damaged tendons. Four- to 5-foot hammerhead shark believed to have been responsible; and,
Jan. 20, 1989, Waialua Beach, Moloka‘i, Earl Dunnam, 10-year-old boy bitten on the foot by a 6- to 8-foot hammerhead shark while riding a body board 200 feet from shore. Wound required 8 stitches. Bite occurred to a naked foot, and not to the foot wearing a swim fin.

These incidents reflect the fact that hammerhead sharks have quite small mouths and their teeth are not designed for taking large chunks out of large prey.

We do have the oceanic blacktip (Carcharhinus limbatus) in Hawai‘i coastal waters (the “oceanic” part of its common name is a misnomer—it is a coastal shark). This species has been implicated in 29 bites worldwide and is thought to be responsible for less-severe bites along the U.S. eastern seaboard, but definitive species identifications are lacking in most incidents.

The severity of damage from shark bites is really a matter of (bad) luck—even a “small” bite from a small shark on an artery can be very serious if no help is at hand. This is why the single best safety tip is to never be in the ocean alone.

HONOLULU: Colin Cook’s rescuer, Keoni Bowthorpe, said the tiger he shoved off was very skinny for such a large shark. Would this indicate a very hungry female post-pupping? The “ravenous gravid shark” theory gets a lot of play. Your thoughts?

H/M: There are natural cycles in shark biology that may help to explain seasonal patterns of shark bites. For example, in Hawai‘i, we typically see more shark bites during the fall pupping season for tiger sharks.

However, although we can identify these broad, seasonal patterns, it is important to remember that shark bites occur in all months of the year, and not all shark bites are caused by pregnant female tiger sharks.

We also need to understand that any single shark bite is a unique event resulting from a confluence of factors that bring a person and shark together. This makes individual shark bites hard to decipher because we invariably know almost nothing about the particular shark involved in any given incident.

We should not over-emphasize the “hungry gravid female” concept. I am not familiar with the circumstances of the rescue of Colin Cook but I do know that the shape of things (like sharks) can be very deceptive when viewed from above water: I don’t know how good a look the rescuer got at the shark. Plus, there is no reason why a shark that has recently pupped should look “skinny”—it should just go back to looking like a regular, non-pregnant shark.

HONOLULU: What do we make of a situation like that on the island of Reunion, where seven fatalities since 2011 have actually resulted in a ban on swimming? What would you do if a similar spike were to occur here? Is there an emergency protocol in the state?

H/M: The incidents on Reunion Island were a combination of attacks from tiger sharks and bull sharks. Fortunately for us, bull sharks are not found in Hawai‘i. Bull sharks are bad news because they occur in shallow coastal waters (and can swim hundreds of miles up into freshwater rivers and lakes) and have evolved to hunt in shallow water. Therefore they overlap with swimming/wading/surfing humans on their hunting grounds. [As for banning swimming,] Reunion is a French protectorate—the French can be much more autocratic about these things than some other nations. I doubt that a total ban on swimming or surfing would get very far in Hawai‘i—and may not even have legal status.

HONOLULU: Any other considerations?

H/M: One of the outcomes of the occurrence of shark attacks in Hawai‘i is that it has led to local and federal funding of shark-related research. This has put Hawai‘i in the forefront of this field and especially in the development and deployment of increasingly sophisticated tracking devices that are now allowing us to not only track the sharks but also learn about their feeding habits, social interactions and the oceanographic properties of the water through which they are swimming.

For more about sharks, pick up the latest issue available on newsstands now or on the online store.