Friday, September 13, 2013

Tiger shark attacks linked to migration patterns?

Aloha all,

This is Ben from Australia chiming in today on a topic I have been following since starting my interning here. Let me know what you think.

There has been a fair amount of media coverage recently over the number of shark attacks in Hawaii and the unfortunate death of a German tourist. Some of this coverage could plant the seed of uncertainty within the minds of many as to whether it is safe to swim at local beaches. However the reasons behind the shark attacks do not seem to me to be due to these individuals acquiring ‘a taste for human flesh,’ rather it could be better associated other factors, such as chance events affected by migration.
Tiger Shark Midway
Retrieved from: Meyer & Holland, (2009) Long Term Movement Patterns of Tiger Sharks in Hawaii
I have been reading recently about a study carried out by the University of Hawaii’s Carl Meyer and the University of Florida’s Yannis Papastamatiou in regards to migration patterns during pupping season. This study has revealed that a quarter of female shark individuals migrate to the Main Hawaiian Islands from the Northwestern Islands once a year. Coincidentally, this period occurs during the fall, which is when shark attacks seem to be more common.

There have been previous studies in regards to breeding migrations carried out in different areas of the world. Hueter et al. (2004) carried out a study exploring evidence for philopatry (meaning to return to, or stay in home range). They concluded that ‘the search for evidence of philopatry in sharks is still in its early stages, but there exists enough behavioral, genetic and fisheries data to conclude that at least some sharks are strongly philopatric for portions of their ranges, especially nursery areas, and many other sharks may be at least moderately philopatric for nurseries, mating areas, feeding areas or other localities.’ While this paper explored migration patterns of sharks in general, it doesn't focus on specific shark species which could exhibit wildly varying migration patterns or lack of any pattern. However, it could be said that the work carried out by Meyer and Papastamatiou is assisting in building a greater knowledge base in this area.

The findings from Meyer and Papastamatiou may ultimately also be related to traditional Hawaiian beliefs regarding the blooming of the Wiliwili tree.  It is said that the wiliwili blooms during the mating season of sharks, and that is when they will "bite".” It seemed the Native Hawaiians had a pretty good idea of what was going on. I also like how they extended this concept. According to the Hawaiian Ethnolbotany Online Database , “When the wiliwili blooms, the sharks bite; when a beautiful woman blossoms the law bites. A beautiful woman attracts young men, "sharks", who fight over her. The law steps in to keep the rivalry from getting out of hand (the law "bites"). 

Wiliwili (Erythrina sandwicensis) is a species of flowering tree that is endemic to the Hawaiian Islands. It is the only species of Erythrina that naturally occurs there. It is typically found in dry forests on leeward island slopes up to an elevation of 600 m (2,000 ft). The Hawaiian name, wiliwili, means repeatedly twisted and refers to the seed pods which twist open to reveal the seeds.
Flower of an Endemic Wiliwili Tree. Retrieved from:
Meyer mentioned in recent news articles (such as Red Orbit News) that people should not jump to conclusions about these attacks being directly related to the migration of female sharks. There could easily be a number of other factors determining the rates of attacks. From my personal speculation, other factors could include the number of water entries in Hawaii per year and/or the availability of resources. In regards to number of water entries, this could be related back to the chance events mentioned earlier. If there is a higher amount of people in the water around Hawaii in 2013 than 2012, then the probability of a shark attack would also be higher. While it’s hard to quantify the number of water entries per year, this number should be related to the number of tourists who come to the islands. According to the July News Release by the Hawaii Tourism Authority, it determined that there was a 4.6 percent increase in the number of tourists visiting the Hawaiian Islands from 2012 to 2013 . In a news article by Pacific Business News, it discusses the record year for visitor arrivals of 7.9 million people. The previous record was 7.6 in 2006. This means so far Hawaii has been having the highest number of visitor arrivals ever! Shark bites could simply be related to the number of water entries. I realize I may be pulling the string of my bow far on this, but this is merely just food for thought.

In regards to availability of resources, this could be another factor contributing to the number of shark attacks. As many may already know, unsustainable fishing practices have severely reduced populations of many species of fish, which may also be a component of a shark’s diet. Sharks typically go where resources are more abundant, which could be shallow water where many people may be present.  In most cases involving a shark attack, it is probably the result of the shark mistaking the person for being prey such as a seal or a turtle, or just curious to what the large unwieldy thing in the water is.   

However, at the end of the day, it’s important to consider perspective. One of our favorite fast facts which we use in our community education program when discussing sharks is you are more likely to meet your demise from  riding in a car than from a shark.  In Hawaii, there are approximately 70 deaths on the road each year. Yet each day most people fearlessly face the peril of hopping into their car without a whimper of fear. Just some more food for thought.



Bishop Museum. (2013). wiliwili. Retrieved from Hawaiian Ethnobotany Online Database:
Hawaii Tourism Authority. (2013). Hawaii Tourism Authority: July News Release. Retrieved from
Hueter, R. E., Heupel, M., Heist, E., & Keeney, D. (2005). Evidence of Philopatry in Sharks and Implications for the Management of Shark Fisheries. J. Northw. Atl. Fish. Sci., Vol. 35, 239–247.
Red Orbit. (2013). Tiger Shark Attacks In Hawaii More Prominent During Migration. Retrieved from Red Orbit:
Silverstein, S. (2013). Hawaii tourism sees record year in 2012 for visitor arrivals. Retrieved from Pacific Business News:

+Torrent, D. (2013). UF: Newly discovered tiger shark migration pattern might explain attacks near Hawaii. Retrieved from University of Florida News:

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