September 6, 2013
Tiger Shark Attacks In Hawaii More Prominent During Migration
[ Watch the Video: Migrating Sharks May Be To Blame For Increased Attacks ]
Brett Smith for redOrbit.com - Your Universe Online
Based on a seven-year study of tiger shark movement, marine biologists from the University of Hawaii and University of Florida discovered that about 25 percent of the mature female tiger sharks that normally patrol the waters around the Northwestern Hawaiian Islands embark for the Main Hawaiian Islands about 1,500 miles away in the late summer and fall.
The study researchers noted that the annual migration conveniently precedes the tiger shark birth season in September to early November – also the months with highest shark bite risk.
“We have previously analyzed data to see which sharks are hanging around shark tours with cage divers on Oahu, and one of the things we noticed was that you’d get a spike in how many tiger sharks are seen in October, which would match our predicted model that you’re having an influx of big, pregnant females coming from the northwestern Hawaiian Islands,” said study author Yannis Papastamatiou, a UF marine biologist. “There even tends to be a spike in the number of shark bites that occur during that season.”
“Both the timing of this migration and tiger shark pupping season coincide with Hawaiian oral traditions suggesting that late summer and fall, when the wiliwili tree blooms, are a period of increased risk of shark bites,” added co-author Carl Meyer of the University of Hawaii.
Meyer cautioned against prematurely concluding that the female shark migration is directly related to recent shark attacks around Maui, Oahu, and the Big Island. A multitude of factors influence shark behavior in a wide range of ways that could lead to more frequent and violent encounters with people, the researchers said.
The team emphasized that their newly published study examined shark migration patterns – not human-shark interactions. According to data from UF’s International Shark Attack File, Hawaii had 10 reported attacks in 2012. Eight attacks have been reported so far this year and an August fatality was the state’s first since 2004.
“We knew tiger sharks had fairly complicated movement patterns and it seemed to be sort of a free-for-all,” said Papastamatiou. “Once we looked at data for the full seven years and used the right analysis, everything started to make sense. Now we have a much better understanding of the migration patterns of these sharks.”
In the study, marine biologists tagged over 100 tiger sharks starting in 2004. Each transmitter tag emitted a unique high frequency sound code that was picked up when the sharks swam close to one of more than 140 underwater receiving stations that were placed throughout the Hawaiian Archipelago. These stations recorded the time, date, and the identity of the shark whenever they received a signal over the three-year life span of each individual transmitter.
“We believe approximately one-quarter of mature females swim from French Frigate Shoals atoll to the main Hawaiian Islands in the fall, potentially to give birth,” Papastamatiou said. “However, other individual sharks will also swim to other islands, perhaps because they are trying to find a more appropriate thermal environment, or because there may be more food at that island. So, what you see is this complex pattern of partial migration that can be explained by somewhat fixed factors, like a pregnant female migrating to give birth in a particular area, and more flexible factors such as finding food.”
The Hawaiian researcher warned against culling sharks as a way to resolve the recent spate of attacks, despite call from the public for a course of action.
“The one thing I hope they don’t do is try to initiate a cull as was done in the 60s and 70s. I don’t think it works. There is no measurable reduction in attacks after a cull,” said Papastamatiou.