Should beachgoers be worried about year-to-year changes in shark attack numbers?
The University of Florida’s International Shark Attack File released its 2012 findings this past Monday, and some major media outlets were quick to report that the 53 “unprovoked” shark attacks in the U.S. last year ties the “record number” of annual attacks reported in the U.S. (the same number of attacks was reported in 2000). While the data seems pretty straight-forward, what does a record number of shark attacks actually mean from a scientific and statistical standpoint?
As the ISAF 2012 summary is quick to point out, short-term trends should be viewed with caution as they are not necessarily indicative of any kind of statistical significance. When viewing annual counts of attacks, each annual number of attacks is a single data point making the possibility of any one year’s count being an outlier, and the ISAF summary notes that year-to-year variability is high.
The report goes on to state that the number of “unprovoked attacks” worldwide has consistently increased each decade since 1900, and while no statistical analysis of this data is discussed in the summary, it seems that a significant positive trend in the number of reported attacks exists. But, does an increase in the number of reported attacks translate to an increased risk of being attacked by a shark? Are individual beachgoers across the U.S. (and the world) less safe from shark attacks now than they were in the past? To answer these kinds of questions, one must consider more data than simply the number of reported attacks.
The 2012 summary discusses other factors which can affect the number of reported attacks. The report states that the number of shark-human interactions is directly correlated with the number of hours humans spend in waters where sharks are found. The report goes on to say that an increase in attacks should be expected over time as an “upsurge” in aquatic recreational activities continues. In addition to more people being in the water, the ISAF also has improved its efficiency in recording attacks over the past few decades, which logically would result in an increase in reported attacks, even if the number of actual attacks remained consistent during this time period, given the assumption that not all attacks were recorded.
The gist of this past year’s ISAF summary seems to be that while the data might indicate a “record number” of attacks in the U.S. and a slight increase in worldwide attacks from 2011 to 2012, the relative risk of an individual human being involved in a shark attack has not necessarily increased over the past year.